Cannondaleriders

I'm having shifter shaft seal issues...they keep blowing out. I didn't even get one ride with the last one I replaced. It seemed like it went in extremely easy, I expected it to have a tighter fit. Could it be possible that there's pressure building inside the tranny and the shifter seal is the weakest link?

If your tranny vent is plugged it can build up pressure. Cant say Ive ever seen a shifter seal leak much less pop out. You can use a little silicone on the seal to help hold it if for some reason it doesnt hold into the housing very well. You could be building pressure in the crank case from a missing TDC bolt, cracks, crank ,camshaft seals.

Where is the tranny vent?

Its right at the front top of the side cover . The frame covers it up so you have to look up there to see it. It should have a small rubber hose coming straight down by the starter. Some of the newer ones go to a 4 way tee connector but Id ditch that.