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The Complete Reassembly of The Honda CB350 / CL350 / SL350 Twin Cylinder Engine

Caleb Foster
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350 cam chain tensioner engine drawing.png

Welcome to the arguably more important half of our vintage Honda CB350 twin cylinder engine rebuild series, the reassembly of the engine piece by piece. This assembly or "rebuild" documentation applies to the Honda CB350 twin cylinder engine and its CL350 & SL350 variants; it is the step by step process necessary make a pile of parts into a functioning internal combustion engine. This video series is filmed in "real time" and is to act as a supplemental demonstration in use with the Honda factory service manual. Link below:

 

Honda CB350 | CL350 | SL350 Factory Service Manuals

 

Notes on Service Limits, Inspecting & Measuring your Engine Components Prior to Reassembly

 

There is more to an engine rebuild than simply taking everything apart and swapping in new pieces and then putting it back together. The inspection and precision measurement of your engine's critical internal wear components is the most important step between the disassembly and the reassembly of any engine. When an engine is fully disassembled, many of the critical wear components must be inspected and have precision measurements taken on them. These measurements must not exceed published values from Honda known as "service limits." Think of it like measuring how much tread is left on a tire, after a certain amount of wear it must be replaced.

Many parts inside the engine rely on small margins and tolerances that cannot be seen by human eyes. The only way to know if a part is good enough to re-use is by measuring it. The Honda factory service manual is the best and only guide on how to inspect each crucial component in the engine; the published "service limits" are the exact measurements used to determine if a part is reusable or must be re-machined or replaced. 

🔧 Some parts like the cylinders and cylinder head require highly specific measuring tools and experience to properly determine if a component is worn beyond service limits. Common Motor is happy to offer comprehensive cylinder and head measurement / inspection & rebuild services to bring your engine components back to factory specification performance or at least determine if they are well within service limits.  Learn more about our machining services here: 

Cylinder Boring and Honing Service

Cylinder Head Rebuild Service

 

Notes on the Engine Used For This Video Series

Every engine wears differently, and we must maximize the useful life of a given part (aka they are not making new ones, so re-use what you can). With the particular engine used to film this series, it was found to be in remarkably good shape upon tear down. It probably really didn't need to be taken apart for a rebuild, but this engine was picked at random without prior inspection, which is the same reality of any engine in a vintage bike. You get what you find and work from there. Regardless, we did not know exactly the condition of the critical wear parts until we clean and measured everything. In this example engine, the cylinder bores were in great shape, and only required a light de-glaze hone. This means new piston rings and we reused the original pistons. The head was also in remarkable shape, which only required a quick valve seat lapping; all the other head parts were able to be used again. This illustrates the point that it required measurement of the parts to determine what the exact course of action to be taken for the rebuild. It would have been foolish to assume that everything was out of service limits, and all parts must be replaced and machined (this is often the mindset of engine rebuilding in the automotive world). This particular engine should be viewed as more of an exception to what one would normally expect, however one must never assume an engine's true condition rebuild scope without measuring parts first. 

 

Honda Motorcycle Models That Use This Engine Architecture

CB350 K0 / K1 / K2 / K3 / K4 / K5 / G

CL350 K0 / K1 / K2 / K3 / K4 / K5

SL350 K0 / K1* / K2*

 

*SL350 K1 - K2 uses a slightly different architecture on the lower half of the engine. Cases were redesigned to eliminate the electric starter and have similar looking, but uniquely different physical castings and gaskets; these are not interchangeable with the CB & CL engines. While the casting and a few parts changed, notably the kick starter mechanism, the internal design and upper engine assembly is almost identical to the CB & CL.

 

Lubricating the Engine during Assembly

When assembling an engine it is recommended to use engine assembly lube in addition to motor oil. Assembly lube is thicker and stickier than engine oil and is designed to cling to metal parts and slowly be washed away by engine oil as you break the engine in. Assembly lube protects the many oil dependent critical parts in the engine in the first minute or so of the initial start up of a rebuilt engine. This critical first minute of running allows the oil pump to prime and begin to circulate motor oil at full pressure throughout the engine. 

 

Parts Needed For The Rebuild

Overhaul Gasket Kit

Oil Seal Kit

Upper Engine Allen Bolt Kit

Lower Engine Allen Bolt Kit

SL350 K1 / SL350 K2 Lower Engine Allen Bolt Kit

Cam Chain

Cam Chain Tensioner Components

Clutch Rebuild Kit

Piston Rings

Valves

Pistons

Wrist Pin Clips

Wrist Pins

Electric Starter Rebuild

 

Part 1: The Bottom End

 

 

Oil Seals

Before assembling the bottom end, install as many oil seals as possible. Some seals are sandwiched between the crankcases and must be installed later, but others—like the neutral switch oil seal and shift shaft seal—can be installed now.

Factory vs. CMC Method:

  • Factory method: Installed dry.

  • CMC method: Apply a thin coat of Hondabond 4 or equivalent gasket sealer to the outer circumference. This:

    • Provides extra sealing insurance.

    • Helps the seal slide into place more easily.

Most seals can be pressed in by hand, but a few may still require a socket and hammer to seat properly.

Kickstarter Shaft

If you’ve followed along with our disassembly process, reassembly will feel like second nature—it’s essentially the same steps in reverse. The first key component to reinstall is the kickstarter shaft. While our rebuild video provides a visual walk through, there are some important details to highlight.

Differences in Kickstarter Shaft Design

Before diving in, keep in mind that not all kickstarter shafts are the same. Variations exist between different 350 engine generations, particularly with later SL350 models. To avoid confusion during reassembly, it’s a good practice to take detailed photos of the kickstarter shaft part sequence as you remove it. This will make it much easier to reference later.

Key Design Features

The version shown in our rebuild video has a few notable features that make assembly smoother:

  • The cast sprag that connects to the shaft includes a small raised wedge.
  • As the shaft turns, this wedge pulls into a small finger bracket, which in turn pulls the sprag away from the idler gear.
  • This design effectively holds the entire mechanism together, preventing unwanted movement during installation.

Assembly Tips

When reassembling the kickstarter shaft, this mechanism proves extremely helpful. It allows you to:

  1. Align and position the smaller components properly.
  2. Hold the entire assembly together securely.
  3. Install the snap ring on the end of the shaft without worrying about parts flying out of place.

Preparing for the Next Steps

With the kickstarter shaft installed, the next step is to begin assembling the rest of the upper crankcase components. To make this easier:

  • Flip the upper crankcase upside down on your workbench. This will cradle the internal components and keep them stable.
  • Transmission shafts and other internal parts will sit loosely in the crankcase at this stage.
  • These parts will be fully secured when the two crankcase halves are sandwiched together later in the process.

Shift Drum & Shift Forks

The shift drum slides into the upper crankcase, and each shift fork must be installed in the correct order before the shift drum is fully in place.

Installation Steps:

  1. Slide the shift drum into the crankcase while you align each shift fork onto the drum in the correct orientation. Refer to our video if you have lost track during disassembly.

  2. Secure shift forks with pins and spring clips:

    • Each fork moves along a specific track to engage gears properly.

    • A small machined pin anchors the fork into its track.

    • A spring clip secures the pin to prevent it from coming loose.

📌 Pro Tips:

  • Use grease or assembly lube to hold pins in place.

  • Be extremely careful when installing these clips—they can easily fling across the shop if not secured properly! If lost, replacements can only be salvaged from another engine.

Crankshaft

Before proceeding, inspect and measure the crankshaft against the service limits outlined in the Honda service manual. Ensure all bearings and surfaces are in good condition before installation.

Crankshaft Bearings Overview

The crankshaft in a 350 engine has four bearings:

  • Two center bearings in the center
  • One bearing on each end
  • Early 350 engines: The left-side bearing behind the rotor is a caged roller bearing and uses an alignment pin in the crankcase.
  • Later 350 engines: (as seen in our video): The left-side bearing is a larger ball bearing without a pin.

Installation

  • Apply assembly lube and engine oil to the bearing races for smooth installation.
  • Use grease to hold the alignment pins in place:
    • Place the two center alignment pins into their bearings.
    • Place the right-side alignment pin directly into the crankcase.
  • Carefully lower the crankshaft into the crankcase.
  • Guide the connecting rods through their openings.
  • Rotate the end bearings until they click into the alignment pins and bearings seat cleanly in the case.

📌 Cam Chain Consideration:

  • If using an endless cam chain, it must be installed before the crankshaft is fully in place.

  • If using a open loop cam chain with master link, installation can be done later.

Crankshaft Bearing Retainer

  • The retainer has two distinct ends:
    • One end has a groove running its full length—this side faces the front of the engine.
    • The other end has a shorter groove with a flat spot—this faces the rear of the engine.
    • Apply motor oil to the threads and underside of the heads on the four retaining bolts.

  • Torque four bolts in a crisscross pattern to 11.6 ft-lb - 15.2 ft-lb (139.2 in-lb - 182.4 in-lb).

Shift Drum Indexing Wheel

Installing the shift drum indexing wheel allows for transmission functional testing prior to lower case assembly.

Installation Steps:

  1. Slightly lift the shift drum.

  2. Slide the indexing wheel fork into the shift drum edge.

  3. Install the special washer and bolt to secure it.

  4. Install the spring:

    • The pigtail end goes into the small hole in the crankcase.

Neutral Ball Detent & Neutral Switch

Neutral Ball Detent Installation:

  1. Drop the ball bearing into the crankcase.

  2. Install the spring.

  3. Install the new O-ring, lock washer, and cap.

  4. Tighten the cap and bend up the lock washer to secure it.

Neutral Switch Installation:

  1. Install the cam by aligning the pin with the notch in the shift drum.

  2. Secure with a bolt.

  3. Bolt-on the neutral switch next to it.

Transmission Gears

If you followed our disassembly guide, the transmission gear shafts should still be fully assembled.

⚠️ If the transmission was disassembled, refer to the Honda service manual for proper reassembly before proceeding.

 

Pre-Install Alignment Components

Before placing the gear shafts, install in the case:

  • Bearing alignment pin for the needle bearing.
  • Half-circle clip (resembles half of a piston ring) for the ball bearing.

💡 Tip: Use a small amount of grease to hold these components in place.

Steps to Install the Output Shaft

The output shaft is the first to be installed. This is the shaft that connects to the drive sprocket.

  1. Turn the shift drum using a JIS screwdriver so the shift forks point upward.
  2. Align the two sliding gearsets with the outer shift forks.
  3. Lower the output shaft into place, ensuring:
    • The shift forks sit in their correct grooves.
    • The end bearings rest on the correct alignment clip and pin.

Steps to Install the Input Shaft

  1. Align the input shaft with the center shift fork.
  2. Lower it into place, ensuring:
    • The shift fork sits in the correct groove.
    • The bearings are fully seated in the crankcase.
    • The alignment pin and clip are properly engaged.

Final Check

Once both shafts are installed:
✅ Ensure all shift forks are properly seated in their gear grooves.
✅ Check that the bearings sit flush with the crankcase.
✅ Confirm the alignment pins and clips are engaged.

 

Testing the Transmission

Before closing the crankcases, test gear engagement:

  1. Use a JIS screwdriver to rotate the shift drum.

  2. Shift through all gears while spinning the input shaft:

    • 1st → Neutral → 2nd → 3rd → 4th → 5th.

    • Then back down to 1st.

  3. Repeat the test several times.

Final Lubrication:

  • Coat gears & crankshaft with assembly lube & engine oil for startup protection.

Bolting the Crankcases Together

Prepping the Gasket Surfaces:

  • Clean surfaces with brake cleaner or acetone.

  • Use the proper latex-acetone-based sealer (ie. Hondabond 4, Motoseal, or Threebond).

  • Do not use RTV silicone sealants—they can clog oil passages.

  • Output shaft and clutch push rod seal should be installed now with a thin coat of gasket sealer on the circumfrance. Lube the inside of the output shaft seal. 

  • Apply a thin, even layer of gasket sealer on both faces of the crankcase, error on the conservative side of application.

  • Install alignment dowels if removed.

Bolting Together the Crankcases:

  1. Position the lower crankcase onto the upper crankcase with the alignment dowels and seat the two halves together. It should drop together easily with minimal resistance. 

  2. Install bolts in the correct locations (refer to Pg. 44, Fig 3-88 of the Honda manual if needed).

  3. Install the bolts with a small amount of oil on the threads and a drop on the bottom side of the head.

  4. Torque bolts in two stages:

    • M6 bolts: 6-7.5 ft-lb (72-90 in-lb).

    • M8 bolts: 15-16 ft-lb (180-192 in-lb).

    • Use a crisscross pattern.

🔧 If you need a torque wrench, you can purchase a KT Pro Torque Wrench right here.🔧

 

✅ Your bottom end is now fully assembled!

 

Part 2: The Clutch, Oil Pump, and Cam Chain Tensioner System

 

Now that the bottom end cases are assembled, it’s time to finalize the lower engine assembly by installing everything contained within the left and right engine covers.

Kickstarter Spring

During the kickstarter shaft assembly, we left the spring unattached because it needs to rest on the upper case ledge when properly tensioned.

How to Install the Kickstarter Spring:

  1. Locate the curl at the end of the kickstarter spring.
  2. Use a flathead screwdriver to grab the curl and pry it up and over the ledge in the upper crankcase.
  3. Ensure that the spring is securely seated against the case, providing proper tension.

Shift Shaft Mechanism

The shift shaft is a critical part of the gear shifting system, as it:

  • Transfers movement from the shift lever (operated by your left foot) to the shift drum wheel.

  • Allows for precise gear changes by pushing and pulling to rotate the drum.

  • If shifting is stiff or unresponsive, wear or damage to the shift shaft might be your issue. 

How to Install the Shift Shaft:

  1. Slide the shift shaft into the engine from the right side.
  2. As the shaft nears full installation, tilt the fork down to fit under the shift drum into the notch.
  3. Release the fork into place so it sits correctly on the drum.
  4. Check the return spring:
    • The spring arms should rest on both sides of the black metal pin that protrudes from the crankcase.
    • This ensures the shift shaft returns to center after each shift.

 

Clutch Basket and Oil Pump Assembly

Rear Crankshaft Drive Gear

  1. Slide on the crankshaft spacer washer.
  2. Install the first crankshaft drive gear, ensuring:
    • The machined flat side faces inward toward the crankshaft bearing.
    • The stepped side faces outward.

💡 Tip on Crankshaft Drive Gears: These gears develop wear patterns over time together. If installing new ones or reversing them, expect possible excess gear noise as they wear into their new positions. When possible, install them to their original positions.

Oil Pump Piston

  1. Attach the piston to the drive arm located on the rear of the clutch basket by inserting the pin.
  2. Apply assembly lube to coat the pin and piston, ensuring smooth operation.
  3. Slide the oil pump over the piston to complete the assembly.

Clutch Basket and Oil Pump

  1. Position the clutch basket and oil pump assembly into the engine. Make sure the oil pump gasket is in place.
  2. Secure the oil pump with three bolts:
    • Hand-tighten all bolts before fully securing them.
    • The two bolts captured in the locking washer strip should be snugged down slightly past tight.
    • Tighten the third bolt to the same level.
  3. Lock the bolts in place:
    • Use a punch or flat head screwdriver to bend up the edges of the lock washer strip, preventing movement.

Front Crankshaft Drive Gear

  1. Install the second crankshaft drive gear:
    • The stepped side should face inward.
    • The smooth machined side should face outward.

Oil Filter Rotor

  1. Install the oil filter rotor:
    • Rotor body first then, slide on the lock washer (ensure the "OUTSIDE" marking faces away from the engine).
    • Place the lock tab washer on the end of the crankshaft, align the internal notches to the notches on the inner circumference of the rotor casting.
  2. Use the #9005 oil filter rotor nut tool to thread the special nut onto the crankshaft, beveled side in, flat side out.
  3. Lock the engine in place to prevent rotation:
    • Insert a penny between the bottom offset gears to safely stop movement (the CMC favorite gear wedge tool).
  4. Torque the nut to 33-40 ft. lbs.
  5. Secure the nut by bending up a tab on the lock washer into one of the notches to prevent it from backing out.
  6. Install the oil filter rotor cover:
    • Use a fresh O-ring.
    • Align one of the tabs on the cover with the double-raised grooves inside the oil filter rotor.
    • Secure the cover with the snap ring, ensure the snap ring is fully seated in the groove.

Clutch Pack and Outer Cover

  1. Slide the inner clutch basket onto the input shaft and secure it with the snap ring.
  2. Install the clutch friction discs and steel plates:
    • Pre-soak the friction discs in oil for 10-15 minutes before installation.
    • The first steel plate should already be locked into place in the back of the basket with a wire circlip.
    • Stack the friction discs and steel plates in alternating order.
  3. Place the clutch lifter inside the input shaft.
    • This component engages with the clutch push rod to open and close the clutch when pulling the clutch lever.
  4. Install the outer clutch basket cover, springs, and bolts with washers.
  5. Tighten the bolts properly:
    • Turn them just until they bottom out and are snug.
    • ⚠️ Do not overtighten—this can crack the threaded towers on the clutch basket.

 

Cam Chain

You can install the cam chain at various stages of your engine rebuild, depending on the extent of disassembly. 

  1. During Crankshaft Installation (Ideal Time)

    • Why: This is the easiest method, as it’s how Honda originally assembled the engines.
    • How: Attach the master link to the cam chain first, then slip the closed loop chain over the crankshaft, and install the crankshaft to the upper crankcase. 

  2. Before Cylinders and Pistons (Top-End Rebuild)

    • Why: If you’re only working on the top end, you can install the cam chain without tearing into the bottom end.
    • How: Lower the broken cam chain into the crankcase, use a magnet or hook to fish it out, and then install the master link.

  3. During Camshaft and Rocker Arm Replacement

    • Why: If you’re just replacing the camshaft and rocker arms, this method avoids disassembling the entire engine.
    • How: Break the old cam chain, attach an end of the new cam chain with safety wire to an end of the old one, and pull it through the engine using the old chain. Install the master link once the chain is in place. 

Important: Always be careful not to drop any pieces of the cam chain or master link into the engine. Use towels to cover open holes during installation. See our installation article for instructions on installing a press-style master link included with our Common Motor cam chains for your CB/CL350 and SL350.

 

Cam Chain Tensioner System

    • Insert the inner sleeve into the large roller with lubricaton.
    • Slide the small roller tensioner bracket over the cam chain and align the notch on the small roller bracket with the sleeve on the large roller and the cam chain running through it.
    • Slide the second inner pin through the large roller and place the entire system into the crankcase cutouts.
    • Stuff any holes with towels to prevent the cushions from falling into the engine.
    • Insert the cushions into the openings on each side of the large roller pin: the rounded side faces down, and the square side faces up, flush with the crankcase.

Stator Coil

While it’s not essential to install the stator coil right now, it helps with alignment for the upcoming top-end assembly. 

  1. Install the Stator:

    • Attach the stator coil the cover by installing the bolts.
    • Index Mark: On CB/CL350 and SL350 engines, the stator coil's index mark should point to about 11 o’clock when looking at the left side cover, level with the ground.

  2. Seal the Grommet:

    • Pro Tip: Apply a small amount of Hondabond 4 around the edge of the dust grommet. These grommets can harden over time and don’t seal as effectively as they once did. Since a good replacement isn’t readily available, adding some sealant will help ensure dust doesn’t get in and oil doesn’t leak out.

Rotor & Starter Clutch

Rebuilding the Starter Clutch

  • Begin with the starter clutch plate already attached to the back of the rotor. If you need to rebuild the starter clutch assembly, see our starter clutch rebuild video.
  • During disassembly, the rollers, springs, and caps may have fallen out. Reinstall these parts into the starter clutch housing.

Starter Clutch Gear

  • After installing the rollers, springs, and caps, place the starter clutch gear back into the center.
  • The gear should lock in one direction and spin freely in the other.
  • A Note on the Starter Clutch Gear: Over time, the starter clutch gear can become out of round. If there’s significant wear on the edge where the rollers grip, the gear may develop a more triangular shape, causing the starter clutch to slip even with fresh components. If this happens, replace the gear or machined back to a perfectly round shape to prevent issues

Installing the Rotor Assembly Onto the Crankshaft

    • Install the half moon key into the end of the crankshaft and apply assembly lube to the inner bushing of the starter clutch gear.
    • Carefully slide the starter clutch and rotor assembly onto the crankshaft, ensuring the half moon key fits into the groove inside the rotor.
    • Install the specialized OEM Honda bolt into the end of the crankshaft to secure the rotor in place. This bolt must be OEM; no substitutes.
    • Torque the Rotor bolt to 24-25 ft. lbs.
    • Use a penny or a similar tool to hold the gears in place while torquing the rotor bolt, just like when you kept the engine from turning during the oil filter rotor nut installation.

Temporarily install the left side engine cover for use during the top end assembly.

 

Part 3: The Pistons and Cylinder

 

Another Note on Service Limits

Before diving into the specifics of piston and cylinder installation, it’s vital to emphasize the importance of service limits. Components like the cylinders, pistons, and wrist pins can be reused, but you must check measurements to confirm they are within specifications. And even when they fall within the service limits, there is still work to be done on many parts before assembly. For example, while the cylinder bore in our engine was in great shape, it was honed it to create fresh oil-retaining cross-hatching for new piston rings.

 

Pistons and Piston Rings

Check and Install Piston Rings

  • Piston Ring End Gap: For each piston and cylinder, check the piston ring end gap using the procedure outlined in our video. This ensures a proper fit and function of the rings.
  • Piston Ring Groove Clearance: If you are reusing the original pistons its important to check every aspect of the service limits. There is plenty of talk about piston diameter, skirt, and wrist pin bore but things like piston ring groove clearance (to avoid piston ring flutter) need to also be in spec. We outline this procedure in our video and specifications can be found in the Honda service manual on page 30. 
  • Ring Installation: For this rebuild, we are using our new Common Motor piston rings, which feature a 3-piece oil ring design (a modern upgrade over the OEM single-piece oil ring). This design is not only easier to install but also performs better.
    • Be sure to install the rings slowly and carefully to avoid damaging them.
    • Make sure the rings are oriented correctly before installation and ensure that the correct ring in in the correct groove in the piston. Most piston rings in general have a specific side that should face up indicated by a marking on the ring itself. Our CMC rings are symmetrical and can be installed either orientation. However the silver faced ring goes in the top groove, carbon gray on in the second.
    • For even more information, check out our piston ring installation video in this article.

Install the Inner Wrist Pin Clips

  • Tip: Install one of the wrist pin clips in each piston now while on the bench. The clip needs to be installed to the side of each piston which face the cam chain center-line of the engine. This will make installation of the pistons to rods with the wrist pins and second clip easier in the next steps.
  • Position each wrist pin clip inside the piston’s groove, with the open end facing 180 degrees away from the groove’s notched opening. Double check that the clip is fully seated in the groove. 

Install Pistons onto Connecting Rods

  • Apply assembly lube to the connecting rod and piston wrist pin bore.
  • Align the piston with the correct cylinder, ensuring the directional arrow on the piston crown points toward the front of the engine. If there’s no arrow, align the larger intake valve relief facing the rear of the engine.
  • Install the wrist pin through the piston and connecting rod. Secure it by installing the outer wrist pin clip, with the open end facing 180 degrees away from the groove’s notch.
  • Tip: Use rags to block any openings in the engine to prevent the wrist pin clip from falling into the bottom end.

Prepping the Cylinders for Installation

Wash the Cylinders Before Assembly Begins!

  • Wash the cylinders in hot soapy water to remove any grit or oil residue. Once clean, dry them thoroughly. Inspect by wiping a clean white rag in the cylinder bores. If there is residue on the rag, rewash, dry and inspect again. Once no residue is present on a clean rag wipe, it's ready for installation. 
  • Apply a coat of engine oil to the iron sleeves of the cylinders to prevent flash rusting.

Install Cylinder Base O-Rings

  • Installing cylinder base o-rings can be tricky, as they tend to be tight in their grooves. Use a dull flat tool like a putty knife to carefully press the o-ring into place.
  • Apply pressure evenly, crisscrossing around the groove like torquing bolts.

Add Extra Sealing Protection

  • To prevent oil leaks from the cylinder base, we like to add Hondabond 4 around the oil passages on the rear outer studs of the engine. This adds extra insurance against leaks that could otherwise come from the engine studs at the base gasket. 
  • We also use Gasgicinch gasket sealer to secure the cylinder base gasket in place. It’s an easily removed, old-school gasket adhesive that helps create a reliable seal for extra insurance.

Installing the Cylinders

Align the Piston Ring End Gaps

  • Before placing the cylinders on the engine studs, make sure the piston ring end gaps are properly aligned:
    • The top two piston ring end gaps should face 180 degrees away from each other.
    • The 3-piece oil ring should have its gaps spaced 120 degrees apart.
    • If using a single piece OEM style oil ring, all three rings should have the gaps spaced 180 degrees apart.

Lower the Cylinders on

  • Tip: Give the cylinder bores and pistons a thorough coating of oil before installation.
  • Slide the cylinders onto the engine studs, aligning the cam chain tensioner upward and carefully lower toward the first piston to be installed.
  • Gently wiggle the piston under the cylinder taper until the piston rings are aligned and slide smoothly into the cylinder. Do not force anything — this should happen smoothly with gentle pressure. Visually the piston should be perfectly inline with the sleeve when viewing from the side. If the piston is cocked forward or backward, gently straighten it up and continue to slide the cylinder down over it.  Repeat for the second cylinder and piston. 
  • Tip: If needed, use your fingers or small dull tool to compress the piston ring slightly, aiding the process.

Secure the Cylinder

  • Before fully seating the cylinder, clean the gasket surface on the crankcase and apply a small amount of Hondabond 4 around the oil passages and Gasgicinch on the rest of the surface.
  • Slide the cylinder down the engine studs, pressing it onto the crankcase. It should fit with minimal effort, but you may need a few light taps to seat it properly.
  • Tip: Be careful not to rotate the engine until the cylinder head is installed in the next section.

 

Part 4: The Cylinder Head, Camshaft, and Rocker Arms

 

 

Cylinder Head Preparation

We thoroughly disassembled the cylinder head for cleaning, inspected it for any wear, lapped the valves, decked the head gasket surfaces, and reassembled it to insures the head is fully prepped and ready for installation.

 

Need a Cylinder Head Inspection?
If you need your head checked for wear or potential repairs, Common Motor offers a Cylinder Head Repair and Inspection service to ensure your engine is in optimal condition.

 

Head Gasket Installation

Cleaning

  • Thoroughly clean the head gasket surfaces of any oil and grease. Any residue left over from installing the cylinders on the pistons could interfere with the gasket’s seal.

Alignment Dowels

  • Install the alignment dowels on the front two outside engine studs before proceeding. These help maintain proper gasket alignment during installation.

Gasket Alignment

  • Ensure the gasket sits flat on the cylinders, with the fire rings properly centered over the cylinders.
  • Sometimes the holes in the gasket can be slightly off when punched at the factory, causing it to bow upwards. If needed, gently file or sand the hole edges for proper clearance so that the gasket lays flat.

Hondabond 4 & Copper Spray

  • Apply a small amount of Hondabond 4 around the oil passages of the cylinder and cylinder head and spray the head gasket with a few coats of copper head gasket spray. The Hondabond 4 seals the oil passages, and the copper spray aids in sealing the head gasket to any irregularties in the casting surfaces for extra insurance against future leaks. Copper coating head gasket is not a must, but rather our preference installing head gaskets.

Cylinder Head

Positioning the Head

  • Align the head casting with the case studs and gently lower it onto the head gasket.

Torquing the Bolts

  • Install the two M6 bolts next to the spark plugs holes, torquing them to 6-7 ft-lbs (72-84 inch-lbs).

Camshaft

Cam Box Installation & Cam Sprocket Types

  • There are early and late versions of the camshaft sprocket.
    • The early version uses a "flat spot" or "line" on the rubber molding to show the alignment to the top end of the cam box during positioning with the crankshaft. This sprocket is typically used with the "fat" cam or the 286 cam
    • The late version uses a "line" in the rubber molding to show this alignment. Typically used with the "skinny" cam or know as the 312 cam
  • Cam sprocket will also have the letter "L" imprinted on black rubber side near the aforementioned "line" or "flat spot". This indicates the sprocket faces the left side of the engine for installation.
  • Insert the camshaft sprocket into the cam chain and install the cam box, with the lower gasket.

Crankshaft to Camshaft Alignment Position / Synchronization

  • Before turning over the engine, the cam box, cylinder head, and cylinders must be secured in place from moving as the engine gets turned over. We prefer to stack up the top engine stud washers on an engine stud and secure a single top acorn nut. This prevents the cylinder base or head gasket from becoming separated as the crankshaft is rotated. 
  • Rotate the engine and position the rotor's LT mark to the index mark on the stator coil.
  • Adjust the cam sprocket so that the alignment "line" is level with the top of the cam box gasket surface as viewed from the left side of the engine .
  • Slide the camshaft into the engine from the right side and place the cam gear onto the camshaft. If everything is properly aligned, it will slide in smoothly.

Camshaft Sprocket Bolt Installation

  • NOTE: Each bolt on the camshaft is unique:
    • One is fully threaded.
    • The other has a smooth, thicker shank just under the bolt head before the threads start.
      Ensure you install them in their correct positions to avoid damaging the camshaft. Examine the threads on the camshaft for the cam sprocket bolts, one will be fully threaded and the other will have a few MM of smooth bore before the threads start. These match the cam bolts. Reversing these can crack the camshaft!
  • Insert and tighten down the first camshaft sprocket bolt using a 10mm offset box wrench. Use blue Loctite on the threads to prevent them from loosening over time.
  • Put pressure on the cam chain tensioner roller through the hole in the cylinder (using your thumb) to prevent the cam from loosing synchronization position in relation to the crankshaft as you rotate the engine counter clockwise just enough to install the second camshaft sprocket bolt. Use loctite on the threads. 
  • Once both bolts are installed, temporarily install both camshaft bearing end covers with gaskets to prevent the alignment from being lost. At this point fully tighten the camshaft sprocket bolts before the loctite sets. 

Camshaft End Play

End play refers to the amount of movement (or clearance) the camshaft has in between its bearing end covers. From the factory, every camshaft had its end play carefully measured and adjusted and you will find a few shims on either end of most camshafts to regulate this back-and-forth movement. 

Why Check the Camshaft End Play?

During reassembly, even though you’re reusing many of the original shims, several factors can cause the end play to be off:

  • New gaskets: The thickness of the new gaskets between the bearing covers and the cam box could alter the clearance.
  • Camshaft or End Cover replacement: If the camshaft or end covers were swapped at some point, it’s possible the end play is out of spec.

To make sure everything is within spec, you’ll need to check the end play with a feeler gauge.

How to Check Camshaft End Play

    1. Keep the End Bearing Covers Temporarily Installed with at least 2 bolts each.
      You’ll want to leave the bearing covers on temporarily during this step. This way, you can measure the clearance between the camshaft and the end bearing cover.

    2. Measure the End Play
      Push the camshaft and towards on the the end covers and then use feeler gauges to measure the maximum clearance between the camshaft and the bearing cover on the opposite side. The acceptable range for end play is 0.002" to 0.014".

    3. Adjust if Necessary
      If the end play is out of spec, you’ll need to adjust it by adding or removing shims. It’s important to use the correct shims for your engine. If you find that you’re missing shims or need more, we offer camshaft shims to help you get the correct fit.

Rocker Arm Assembly

Camshaft Alignment

The most important thing when installing the rocker arms is to preserve the camshaft to crankshaft position synchronization that you set earlier in the rebuild. If you lose this alignment, the timing could be thrown off, causing potential damage when the engine is started.

    • NOTE 1: To prevent misalignment, only remove one camshaft bearing end cover at a time during the rocker arm installation process. This ensures that tension remains on both the camshaft and cam chain throughout the rocker arm installation process. 
    • NOTE 2: It is imperative that assembly lube is used liberally on all the following part assembles to ensure no damage occurs during initial start up. The camshaft and rockers and supporting bearings are prone to oil starvation leading to part failure (a weak point of the 350 engine design). While they are the parts that need the oil the most for survival, they are the last parts to receive oil upon each engine start up (this is due to how the oil system routes lubrication through the engine). Thus the assembly lube provides short term lubrication of these parts until the oil system can fully pressurize and deliver regular oil flow to the top of the engine. 

Installing the Rocker Arms

Rotate the engine counter clockwise until the ignition advance alignment pin  (on the snout of the camshaft) is pointing approximately at the 4:30 - 5 o'clock position when viewing the end of the camshaft from the left side. This is the position where the camshaft lobes will be pointing downward or level, which allows the rocker arms to be installed without pressure from the valve springs. 

 

Right Side Bearing Cover

    • Take out the screws from the right camshaft bearing end cover (which houses the tachometer drive).
    • Reinstall the tachometer drive gear with a healthy amount of assembly lube. There’s a small washer followed by the drive gear shaft and then a larger washer. After these parts are in place, make sure the oil seal is also installed into the drive.
    • With the end cover off, install the right exhaust and right intake rocker arms into the engine. Be sure to apply assembly lube to the rocker arm pads, tips and the rocker shaft bore. Camshaft lobes, tachometer drive gear and cam bearing surfaces should also have ample assembly lube applied. 
    • Slide the rocker arm shafts through the cam box and into the rocker arms. Rotate the reference tick marks on each rocker shaft to point up at 12 o'clock. Don’t forget to install the sealing O-ring at the end of each rocker arm shaft, put a drop of oil on them and reinstall the right-side cam bearing cover. This cover should be installed permanently with all four bolts. Note the orientation of the gasket in relation to the oil passages for the tachometer drive. 

Left Side Bearing Cover

  • For the left-side engine cover, follow the same steps. Lubricate the equivalent spots on the left side of the camshaft and rocker arms during installation with ample assembly lube.
  • Be sure to install the camshaft oil seal just as the other seals have been installed. Honda Bond on the outer circumference and lubricate the inner seal and lubrication) before reattaching the cover. As you install the rocker arms on the left side, check that everything is lubricated properly and that the rocker arm shafts are securely in place. 
  • Install rocker shaft o-rings, cam cover gasket and permanent bolt into place.

Final Alignment Check

With the top cam box and rocker arms fully installed, take a moment to rotate the engine counter clockwise (keep some tension on the cam chain) recheck your alignment. The camshaft sprocket alignment line should be level with the top of the cam box casting, and the crankshaft should be at the LT mark. 

If alignment is off, remove the camshaft sprocket bolts, adjust the sprocket by one tooth, and reinstall to recheck. Alignment must be spot on—Being a single tooth off on the camshaft sprocket in its alignment to the crankshaft can lead to valve and timing issues that could cause serious engine damage when started.

 

Part 5: Torquing the Engine and Final Assembly

 

Now that your engine assembly is almost complete, you're past the trickier steps. It's time to put the finishing touches on your engine to ensure it's ready for installation. 

Cam Chain Tensioner

The cam chain tensioner operates using a plunger mechanism that presses a small wheel against the cam chain. A properly functioning plunger should move smoothly in and out of the tensioner casting.

 

Hydraulic vs. Spring Tensioner

  • Interestingly, early CB350 engines used a hydraulic pressure-driven tensioner which is documented in the factory service manual. This particular design is very rare and Honda quickly switched to the more reliable spring-based version for the bulk of the 350 engine production. If your engine still has the hydraulic style, you can easily retrofit it with the spring version.

Assembling the Tensioner

    1. Slide the plunger through the large spring and into the tensioner casting.
    2. Ensure the flat side of the plunger rod faces up towards the lock screw hole on the casting.
    3. While holding the plunger all the way in, install the special screw, lock nut, and new O-ring.
    4. Tighten the screw until it just presses against the flat part of the plunger and locks it in place. The plunger should be locked in position as far in the casting as possible.

Final Installation

  • Once the tensioner is assembled, install it onto the engine with a fresh gasket. Apply grease to both sides of the gasket for easy removal during future service.

Torqueing the Engine Studs

Lubricate the cam chain and valve springs with motor oil then Install the gasket on the cam box, ensuring the passage hole is aligned on the front right side hole in the casting. Then, install the metal baffle plate, also making sure the hole aligns properly. Follow this with the upper gasket and top engine cover. We like to continue in aiding the engine stud oil passages by applying a small amount of gasgicinch applied to the oil passage corners ensures a proper seal.

Unique Washers and Nuts

  • The washers and acorn nuts on top of the engine are specific parts and should not be replaced with off-the-shelf alternatives. Apply a small amount of motor oil to the stud threads and bottom of the nuts.

Torqueing the Studs

  • Refer to the Honda service manual (page 21, Fig 3-26) for the correct torque sequence. Torque the engine studs to 13 ft-lbs (156 inch-lbs). We recommend doing this in two stages: First, torque to 80 inch-lbs, then finish with the full torque. Repeat the final torque stage until there’s no movement on the acorn nuts.

Left Side Cover and Electric Starter

  • With the top end of the engine fully assembled, it's time to install the electric starter. Start by installing a fresh O-ring on the starter and sliding it into the engine casting. Ensure the battery cable runs behind the starter, connecting to the positive terminal.
  • Wrap the starter chain around the starter clutch sprocket, and slide the small splined sprocket onto the starter as you push it into place. Secure the starter with two bolts.
  • Lubricate the parts with motor oil and Install the left-side cover gasket with grease applied to both sides, which helps remove it easily later on. Then, route the stator wiring, under the engine and through the holding clamp near the neutral switch and connect the neutral light bullet to the switch. Install the cover bolts using some anti-seize on the threads. Consider using the Lower Engine Allen Bolt Kit.

Clutch Pushrod

  • Lubricate and Insert the clutch pushrod into the engine through the pushrod seal.
  • NOTE: The rounded end goes into the engine and the flat end faces outward.

Right Side Engine Cover

Preparation

  • Coat the outside of the kick start shaft oil seal with Hondabond 4 and press into the cover. Grease the inside of the seal lightly to ensure the shaft slides through without damaging the seal.
  • Install the washer on the kickstarter shaft and insert two dowel pins in the case.
  • Grease the gasket on this side as well for easier future servicing.

Assembly

  • Apply assembly lube to the main clutch basket and crankshaft drive gears. Install the cover, apply  some anti-seize on the threads of the cover bolts and tighten the bolts in a crisscross pattern. Ensure all bolts are snug. We suggest using the Lower Engine Allen Bolt Kit.

Oil Filter Access Cover

  • Install the oil filter access cover with fresh O-rings. Check the alignment of the oil passage covers before pressing the cover into place and securing it with three M5 bolts.

The Final Pieces

Rocker Arm Shafts and Tachometer Drive

  • Install the washers and lock nuts on the rocker arm shafts. Attach the right-side tachometer drive cover.

Points Plate and Cover

  • Install the ignition advance, points plate, run the wires through the grommet. Then, install the points cover with a fresh gasket.

Valve Covers and Rotor Cover

  • Install the valve covers with fresh o-rings, then install the rotor cover with a new gasket and three bolts with new o-rings.

Final Touches

  • Apply anti-seize to the kickstarter shaft splines and tighten the lock bolt. Don’t forget to install the oil drain plug with a new o-ring.

 

Final Steps Before Starting Your Engine

While your engine is rebuilt, there are still a few critical tasks before firing it up:

 

1. Add Oil and Zinc Additive
Make sure to add the correct amount of engine oil and a zinc additive to protect your engine components during break-in.

How to change your engine oil on the Honda CB350 / CL350 / SL350

 

2. Service Adjustments
Perform the necessary service adjustments in the proper order:

 

3. Break-In Procedure
We recommend our CMC Preferred Break-In Method:

    • Gently kick the engine over 20-30 times with the kick starter before starting to prime the oil pump and oil passages. Removing the spark plugs makes this a LOT EASIER!
    • Start the engine on the CENTER STAND! This keeps the oil level in the pan and fuel level in the carbs. Have a fan handy to keep some cool air passing over the cooling fins during this initial run.
    • Upon initial start, in this first 30 seconds or so, touch the header pipes and ensure that they are both getting hot, got to do this quickly before they get too hot to touch!
    • Loosen (but do not remove) the oil flow weep bolt next to the points cover to check oil pressure. If oil seeps out within the first minute, you’re good to tighten it.  
    • Also upon initial start, just let the bike run at low RPM, 1500 to 2000. We know it is tempting to want to rev the engine but just let it run for a few minutes and visually inspect for oil leaks or listen for any odd sounds. 
    • Let the bike run for 10 minutes or so then turn it off. Double check  your oil level once the bike has cooled off.
    • Ride gently for the first 100 miles. Avoid hard acceleration and extended high RPM bursts. 
    • After 100 miles, check oil and adjustments. Ride another 400 miles at a moderate pace.
    • Once you hit 500 miles, change the oil and reset adjustments and enjoy.

 

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