Modern Vespa

Hello Vespa fam, I am rebuilding a Vespa 300 GTS, the crankshaft is busted and ordered a Category 1 crank, then upon cleaning the case I realized there is a number 2 written on the case. Reading and watching builds emphasized a matching numbers on both crank and case. Question is: what is the difference between Cat 1 from 2?

Asking for advices. Thank you.

When the crankcases come off the line, they are measured, and the ones with larger crankshaft holes are Cat 2. Cat 1 has smaller crankshaft holes. People have used one in the other with mixed results. The Cat 2 crankshaft is likely a better bet if your cases have a lot of miles, because the crank bearings are cast into the crankcase and can't be changed.

The size of the "journals" where it rides in the case bearings.

The crank has to match the bearings for clearance purposes.

Oiling and tolerance are the machined specs. You can run a 1 in a 2 but it will be loud and likely last long at all. You can't really fit a 2 into a 1, at least not without a hammer.

Thank you for the insight. Ride safe to all

Makes me wonder if all cases start out with the smaller bearing size. Perhaps the larger size crankshaft simply allows Piaggio to rework any minor bearing defects found during engine case production?

I was going to mention roller bearing cranks until I discovered that Hondas "300" and "350" engines use thin wall bearing cranks with A,B,C,D,E,F and G designations.

The technical term isn't "crankshaft holes?"

perfectly cromulent for the audience.

I'm sure there's some grandiose technical definition (and it is most assuredly in German and doesn't translate well), but due to the architecture of the motor I think "crank holes" will suffice nicely.

(immediately gets pinged with an e-mail notification indicating forfeiture of all ASE certifications)