I have two HT1 90 motors their serial numbers are close to each other but the kickstarter shaft on one is about 10mm round and the other one is 17 or 18 mm round does anybody know why this is. The one with the small shaft is in a matching numbers bike the other is for parts thanks for any help you can offer.

The LT2, LT3, HT,s have a smaller shaft and they are prone to loosening up and stripping the splines. So keep the kickstarter bolt tightened up. If it’s a larger shaft someone put a ct or at shaft in there and it can work but it’s incorrect and there may be some clearance issues.
The kick start shaft you need is scarce, pricey, and in demand. I just bought a new shaft for my LT3 as used ones are hard to find.
Common problem on the smaller bikes 90’s and 100’s.

I've always wondered if you could use a complete AT/CT cover and starter on an HT/LT.

Same gasket and same kick idler gear.

Thanks for your reply that answers that question. Now can you tell me why one motor has a clamp on style carb and the other has two bolts holding the carb on. Thanks again for your time

If both motors are truly HT1 motors the clamp on style carb was stock on the street legal HT1 models. The bolt on style carburetor was used on the HT1 MX as it used a larger carburetor. If the entire motor with the bolt on carb is an HT1 MX you should be able to determine that by looking to the VIN charts in the Tech Library. Otherwise, many have upgraded HT1 motors with the MX cylinder and carb.

Thanks for your help again, both motors are HT1-MX motors 015204 that matches the frame it's in but has the slip on carb with a GYT pipe on it the spare motor is 019186 so I guess someone changed the barrel and carb on the bike it has a yellow tank on it.