I'm building a new engine. A 355 designed to be run with a blower specifically. I was looking through for a new damper and found multiple sizes. 6 1/4 6 7/8 or 7. Is there really a difference between them, or could any be used. thanks.

Where are you looking? Most balancers come in 6 3/4" and 8". I guess it wouldn't matter much as long as it cleared and the timing mark was in the right place.

I was mainly looking in a jeg magazine. The 8" is external balance, 350s are internal. But still looms the question about both 6 " ones and the 7. Any info will help. Thanks

Geez. I'm almost afraid to be the one to say this, but...

Size doesn't matter. The torsional mass of the inertia ring is what does the dampening. The same effect can be achieved with a smaller diameter asembly if the inertia ring is constructed to have more mass, or even if the hub is loaded with a viscous liquid. The more inherently balanced the engine firing order is arranged, the lower the chance of harmful harmonics. Granted, each power pulse will create some harmonics, but a strong crankshaft and well designed engine will survive with minimal damping effect. Some engines don't even use a key to keep the damper in position. Early Chevys didn't use a bolt, either.

The more significant factor for most lower RPM (under 8,000) engines is whether the crank is balanced internally or not. An externally balanced setup may require a different damper as well as flywheel.