Right side seal

What would cause a new seal to go bad,engine was rebuilt right side, viton seal.

improper installation, worn crank stub

Vinton does not really mean much to me. I think in terms of double lipped. At least that one can be changed without splitting the cases. It resides in an aluminum piece held on with four small machine screws behind the I guess it is an alternator. (for the stuff I have, no year or model in this thread that I see)

As to possible reasons to fail.

Sat on shelf for many years
Spring that goes around the inner lip escaped
Heat damage from positioning the bearing

Maybe the lower end filled with liquid perhaps race gas containing dangerous chemicals. Only seal I had go bad was the big one on the other side and that engine had some strange corrosion on the connecting rod. (and it was not new) It was an auto so not magnesium case.

Fran

Back when I was racing I had a brand new ignition side crank seal fail immediately on a fresh rebuild... Honda two stroke... cost me a race and a piston.

Honda gave me a new seal after a factory inspection revealed a manufacturing defect. (I didn't see anything wrong with it, nor did the local dealer, dealer sent it to Honda). I knew it was a bad seal because I learned to pressure test a failed engine before teardown.

That "bad brand new seal" experience taught me to always pressure (and vacuum) test a two stroke even when new parts are used. You just never know.

(A few years ago I had a brand new YZ85 from the dealer with a fairly significant intake manifold leak... I pressure tested it before it was ridden for the first time... )