How does the Cylinder Machining Service Work?

CMC Philosophy

It's a sad fact that many of the key components of vintage motorcycles are no longer produced and will never be produced again. To keep these old bikes in service and preserve their existing parts as long as possible, we here at Common Motor Collective believe in doing only what is necessary to mechanically restore these parts to OEM Honda specifications. It’s sort of like sharpening a pencil. Sure you can sharpen the pencil all the way down to the eraser, but the pencil is going to be hard to hold and write with. What happens when the pencil needs to be sharpened again? At that point no pencil is left, and it must be discarded and replaced with a new pencil. Since there is no “new pencil” when it comes to these old Hondas, we want to leave room for these parts to be “sharpened” again in the future. All engine parts sent to us will be thoroughly evaluated by our professional staff in order to determine the best course of action for service.

Why your cylinders need service

After years of use, or neglect, cylinders wear out or suffer other physical damage. This wear or damage can push cylinders past their OEM service specs. Even in the best case scenario, the pistons have moved up and down inside the cylinders millions of times, wearing the engine out and reducing the bike's performance. Typical of Honda’s engineering, tight tolerances must be maintained for a proper running engine. A tired, worn out engine will be low on power, run inconsistently, and may leak or smoke. All of these conditions are a good indication that your engine’s cylinders need to be serviced.

Rust, corrosion, or mechanical damage are other reasons you might need your cylinders serviced. The cylinders of a bike that’s sat neglected for years or decades, as many of these old Hondas have, can rust or become otherwise stuck and corroded. This is especially true of bikes that have been stored outside for long periods. Mechanical damage, caused by a broken piston ring or a small foreign object sucked in through the intake, for example, can scratch, dent, or score the cylinders. The pits, scratches, and scores left behind by both rust and damage are often so deep or severe that boring the cylinders out to a larger size is necessary. 

What do we do when evaluating cylinders

To figure out exactly what a cylinder needs to be mechanically restored, it must first be thoroughly inspected. This inspection is done both visually and with the use of precision measurement tools. First, the entire cylinder jug must be cleaned. 

If you are sending in parts for service, don’t worry about the grime. We will clean your parts and remove all gasket material before evaluation and service. Even after thorough cleaning and de-greasing there is usually some stubborn crust or corrosion on the cooling fins. Additionally, there is always carbon buildup in the combustion areas. As part of our machining service, we knock away all of that carbon buildup and clean up the cooling fins in our media blaster.

Once the cylinder bank is clean we evaluate each cylinder’s health. First off, we check for any physical damage. Common damage that can be seen without specialty tools includes pits from rust, along with scoring, deep scratches, or even chips on the cylinder walls caused by catastrophic engine failure. We also check the condition of all of the threads in the cylinder bank and make note of any damage on the exterior of the cylinder jug. While most exterior damage won’t affect overall performance, if too many of the cooling fins are broken the engine will likely run at a higher than normal temperature. If the physical damage is too severe, the best option is to find another cylinder jug in better condition and put efforts into servicing it instead. 

The next step is to measure the cylinder to determine its size and shape. By using a dial bore gauge and outside micrometer, we can accurately determine a cylinder’s current condition. The first measurement is the overall diameter of the cylinder bore. Since the cylinder is not brand new, some parts will be larger than others, so we check the diameter at multiple depths within the cylinder. If at any point we find an area that is larger than what is specified in the Honda factory service manual, the cylinder must be bored oversize to fit a larger piston.

Once we determine the bore diameter, we then check for taper. Because of how the piston moves inside the cylinder, it is completely normal to measure a certain amount of taper and out of round. Normally the upper portion of a cylinder will be larger than the lower portion, and the cylinder is rarely perfectly round. If any of these measurements have a variation outside of .002”, the cylinder will need to be bored oversize to correct its shape.

What is involved in cylinder service

The most common way to restore a cylinder to spec is enlarging the cylinder by boring it out. This is accomplished by using a machine with a cutting bit to remove material from the cylinder walls. By doing this, any taper and out of round condition is corrected. It may seem counterintuitive to make a worn out cylinder larger, but it allows us to match the size of the cylinder to a new larger piston. When we do this, the clearance between the piston and cylinder walls will be exactly what Honda determined is the best for daily riding performance. 

While the boring machine does most of the work when oversizing a cylinder, we have to slowly approach the target diameter with a process called “honing”. By using sanding stones in a rotating motion, we can slowly remove material .0001” at a time. It’s important to make the cylinder size exactly what it should be, especially since factory tolerances are kept at or under .0002”. The honing process also creates the surface finish we are looking for. A proper cylinder finish will have light scratches in a cross-hatched pattern, this allows oil to cling to the cylinder walls during operation so the piston and cylinder remain lubricated. If the surface is too smooth the oil struggles to do its job, and if the surface is too rough the additional friction creates excess heat which can lead to mechanical damage to the piston and/or cylinder.

After the cylinders are properly serviced, we turn our attention to the head gasket mating surface. Over time and many, many heat cycles, the head gasket surface may warp slightly. Also, we find that there are usually small pits in the gasket surface right next to the cylinders and combustion areas. If these problems aren’t addressed, your engine may have a difficult time sealing properly. When an engine’s combustion chamber isn’t properly sealed, performance suffers. Machines are able to remove material quickly and make a perfectly level surface, but we’ve found these methods to be a bit aggressive and don’t leave much material left in case the cylinder jug needs to be serviced again in the future. 

Using a perfectly flat and level surface, we sand the surface with a fine grit until the gasket surface has been reconditioned. Not only is this an effective way to create a clean sealing surface, approaching the service in a more conservative fashion extends the overall service life of the cylinder jug.

Conclusion

An engine’s cylinder health directly correlates to overall performance and reliability. That's why this service should not be neglected on these vintage machines. While every cylinder won’t require the same attention, by using precision measurement tools and a patient eye the proper approach to mechanical restoration can be determined. You can rest assured that our experts in the CMC service department are trained and equipped to handle this job for you, and your engine will be able to remain in service for decades to come!

Glossary

Cylinder Jug - The cylinder jug refers to the entire block that the cylinders sit inside. On these vintage Honda motorcycles, the cylinder jug is made of cast aluminum and complete with cooling fins all around

Cylinder - The cylinder sits inside of the cylinder jug, and is the area where the pistons move up and down to create combustion. Honda twins have an iron cylinder which is pressed into the aluminum cylinder jug.

Cylinder Bore - Bore refers to the hole that is the cylinder. This hole is a predetermined diameter based on the size of the piston. The shape and size of the bore will change over the course of an engine’s life.

Cylinder Wall - The cylinder wall refers tso the inside surface of the cylinder, which the piston rings slide against while the piston moves up and down inside the cylinder. 

Service Limit - This term is used to describe the allowable amount of wear in mechanical components. These limits are determined by referencing your factory Honda service manual. When a part exceeds its service limit, it is time for repair or mechanical restoration.

Boring - In reference to an engine, this is a process used to enlarge a cylinder’s overall diameter. This is done with specialized machinery commonly found in machine shops.

Honing - This is a process used after boring a cylinder oversize. By using sanding stones in a rotating motion, it is possible to achieve proper piston to cylinder clearance and create the desired surface finish on the cylinder walls. Additionally, it is used to clean the surface of cylinder walls before reassembly when a cylinder isn’t required to be bored oversize.