What is a cylinder head, and why does it matter? An engine’s cylinder head is a large chunk of milled metal that caps off the engine’s cylinders. It contains the combustion chambers where power is made, and controls the airflow into and out of the engine via the valve train—a series of valves and springs installed in the head. It also influences engine compression, and overall performance. During an engine rebuild, focus often will fall on the cylinders and pistons, while the cylinder head is often neglected. If this vital component is ignored when rebuilding an engine, there is a good chance that engine will need to come apart again sooner rather than later.
Why you need service
Much like a piston inside a cylinder, valves move up and down inside the head thousands of times per minute. Over time, the sealing surfaces steadily wear away causing engine performance to slowly degrade. When it’s time for a cylinder head to be serviced, your engine is probably running at a low compression, you may see smoke venting from the crankcase, and overall power is probably pretty low. Although an engine can continue to run like this, neglected service can lead to bent or broken valves which in turn destroy an engine from the inside out. To prevent this type of catastrophic damage, it is important to give your cylinder head its due attention during an engine rebuild.
How we evaluate
Every good rebuild starts with a thorough evaluation, and that’s exactly how things happen with our machine services. To do this, the cylinder head must be entirely disassembled and cleaned. Your cylinder head can be sent in just as it comes off of your engine (minus camshafts and camshaft accessories). After we disassemble your head, we will clean it in a multi-stage process to remove nearly all of the crust and grime.
Once the cylinder head is clean and disassembled, we give it a good visual inspection and check all of the threads and surfaces. There’s not much we can do about broken and bent cooling fins, but as long as the damage isn’t excessive it won’t affect overall performance. We commonly find damaged threads during this stage, and most of them can be repaired. If chasing the threads with a repair tap doesn’t do the trick, threaded inserts can usually be installed into the cast aluminum. This includes damaged spark plug holes, though they can be a bit more difficult to repair.
All of your valves will be replaced as a part of our rebuild service, so we don’t focus too much on inspecting the original valves. A brief inspection of the sealing surface and a quick concentricity test can give us an idea of how the valve guides and valve seats will have worn over time. Concentricity refers to the alignment of the valve face to the valve seat, if they are off-center this will cause irregular wear over time. Much like an engine’s cylinders, valve guides wear out from the action of the valves moving through them. The overall size and shape changes from this, and must be checked against factory specs. If a valve guide is heavily worn or misshapen, it must be repaired before installing new valves. In the event of a guide being broken, it is possible to replace the guide all together.
Valve seats are another component which takes abuse, and the sealing surfaces must be restored when rebuilding a cylinder head. The valve seats are pressed into the head itself, and it is the surface that the valve contacts when sealing off the combustion chamber. The seats become deformed over time from being beaten by the face of the valve repeatedly. Also if the bike has sat unused for decades, the valve seats often become rough and pitted from corrosion.
The last components that must be checked are the valve springs. From heavy use, springs of all kinds become fatigued over time. If the valves in your cylinder head wear out, they won’t offer sufficient pressure to return the valves properly. If ignored, these springs could lead to “valve float” and bend or break the valves at high RPM as mentioned previously. By measuring spring pressure and spring height, we can determine if your springs are healthy enough for continued use.
How we service
The main focus of our cylinder head rebuilding service is on the valve seats and guides. By replacing the factory valves with the improved CMC Airlock valves, we are able to eliminate a major variable from the equation. To work properly, the valve seats must be reconditioned to a smooth surface that seals tightly up against the valve, and the new valve must fit inside the guide without excessive free play or any tight spots.
Every valve seat is reconditioned by cutting material from the surface to create a tight fitting seal for the valve. By using a three angle cutting technique on the valve seats, we accurately locate and size the sealing surface for the valves. In most cases, we are able to remove pits and corrosion during the cutting process as well.
To repair worn out valve guides, we have found that installing a bronze sleeve is an effective way to restore a valve guide to Honda specs. After installing the bronze sleeve, we slowly size the guide to perfectly fit the new valve. In the event that a valve guide is missing or broken, it is possible to replace the guide all together.
There are a couple of ways to remedy worn out valve springs. Depending on your engine, there may be new valve springs available. In that case, the worn springs can be replaced. If not available, or if there are only a couple of tired springs, shims can be placed beneath the springs to increase spring pressure. By increasing the spring rate, the valves will remain under control during high engine RPMs. Do note that CB450 torsion bar style springs cannot be shimmed.
Conclusion
The health of a cylinder head directly correlates to engine performance, and should never be overlooked when doing an overhaul. Even with new pistons and freshly surfaced cylinders, an engine will never behave normally if cylinder head service is ignored. Send in your cylinder heads to us for evaluation, and rest assured we will give it the attention it needs. A healthy cylinder head makes an engine happy, and a happy engine makes for thousands of miles of smiles!
Glossary
Cylinder Head - This is the top cover on the engine that seals off the combustion chamber. It contains the valves which are operated by the camshaft. Honda twins use a cast aluminum body with iron valve seats and valve guides installed.
Valves - The up and down movement of the valves control the flow of air and fuel through the cylinder head, as well as sealing off the combustion chamber. The valves are mechanically controlled by the camshaft to operate at the proper time.
Valve Guide - The valve guide is essentially a tube pressed into the cylinder head. The valves move up and down inside of the guide, which offers support to the valve stem. The guide keeps the valves straight and concentric to the valve seat during operation.
Valve Seat - This is the surface that the valve seals against. On Honda twins the valve seat is made of iron and is pressed into the aluminum cylinder head. The valve seat has specific angles cut into it to mirror the shape of the valve face.
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.