08-29-2014, 12:28 PM
Huh! You’re never too old to learn something. I never heard of bores getting smaller as the block heated up. I kinda still doubt it. As a block heats up it gets longer and wider. The cylinder bores get larger exactly proportionally. Aluminum expands more than steel or cast iron with the same increase in temperature. Typical hot rod lore includes the knowledge that forged aluminum pistons expand quite a bit and when given proper clearance, piston slap will happen until they heat up and take up proper clearance. Actually all engines have piston slap: The wrist pins are offset toward the thrust face so the piston (the largest and most critical bearing in the engine) rotate the skirt fully against the cylinder bore to take advantage of the full skirt for bearing area under the firing stroke. Conversely the pistons attempt to rock the opposite way on the intake stroke. This is very small movement and not heard on a good motor. Modern hypereutectic pistons have recipes in their casting materials, which minimize the difference between thermal expansion of the cylinder and pistons making piston slap loud enough to hear rare.
I believe motorcycle owners that aren’t gear heads worry overmuch about noises, especially in air-cooled motors. Large flat cast covers and fins have harmonics and can amplify noises. Honda is so aware of this I’m sure every cover, fin, every casting is engineered around this and many other parameters. They have done an amazing job and the casual rider has no concept of the depth of their research and knowledge over the years. There is violence going on inside these motors. Controlled explosions, valves opening and closing rapidly, pistons coming to the top only to be yanked back to the bottom at great rates. There will be noise, and we love all those noises. All motors have their signature noises. It is wonderful we can experience this. Try not to worry.
I haven’t got my shop manual from Helms yet but why are people calling the balancer an “engine and transmission balancer”? All inline fours vibrate in a vertical plain at twice the frequency of crankshaft rpm. Nothing in the transmission needs balancing.