What could cause vibration when releasing the clutch?

Question:

When I installed the engine in my 1949 Hudson Commodore 6, I immediately noticed a severe vibration when engaging the clutch in low gear. I replaced the rebuilt clutch with another rebuilt unit, but that didn’t solve the problem. The engine also seems to have lost some of its power. A compression test showed 70-75 psi in each cylinder. Any advice?

Answer:

Engine vibrations can be caused by many things, but since you notice the vibration when engaging the clutch, you most likely have a clutch related problem. Take your flywheel and pressure plate to a machinist who has a flywheel refacer (not a brake lathe) and can balance the flywheel and pressure plate as a rotating assembly. The machinist will mark the flywheel and pressure plate so you can align them for optimum balance when installing them.

Another possible cause is a defective clutch or pressure plate. Contaminated clutch friction surfaces, weak clutch or pressure plate springs, or a misaligned pressure plate can lead to vibrations. I avoid using rebuilt clutch components when possible, but sometimes new parts are unavailable. If your unit continues to vibrate after balancing, you may want to try another clutch rebuilder.

The compression you reported is somewhat low. If your pistons or connecting rods are too short or if your cylinder head is not correct, low compression can result. However, it’s more likely a result of leakage due to one or more of the following: excessive piston ring gap clearance, piston ring wear, cylinder wall wear, out-of-round cylinder walls or bad valve sealing. A bad head gasket or a cracked head, block or piston can result in low compression, but since you have it in all six cylinders they’re probably not the cause.