You'll need to split the cases.

I wouldn't call that damage "common" but I've seen it a few times.  

Check the small kick start gear on the back of the clutch.  That's the most common one to let go and it could take the idler shaft with it.  

My guess is 20 and 21 are outside.

^^^ Thanks Mark!

Yep!  I'm reluctantly committed to opening the cases

When you've seen broken shafts like this, did the cases survive?

I wondered about the kicker gear on the back of the clutch.  So much so that I took a picture of it because half of each tooth appeared to be ground off.  Then I saw some similar pics of supposedly good clutch hubs and all the teeth looked the same as mine...

Maybe not perfect but surely serviceable to my untrained eye.

The idler gear looked much worse...

I have another on the way.

My guess was 20 & 21 as well.
I appreciate your reply!  ZT

When the starter gear on the clutch goes, generally it explodes.  Which may have happened and has been replaced?  Doesn't matter though.

The unknown is the needle bearing that supports the output shaft...  Since you've been riding it so much as a kid, hopefully that's a sign that it's okay.  If the needle bearing grenades or seizes it sometimes ruins the hole in the case.

 

@MarkT said, "The unknown is the needle bearing that supports the output shaft...  Since you've been riding it so much as a kid, hopefully that's a sign that it's okay.  If the needle bearing grenades or seizes it sometimes ruins the hole in the case."

I've wondered about that needle bearing after seeing it in the tranny cross section diagram... because from the broken end, it looks like just a simple bushing that's exposed.  Is there a separate bushing outboard of this Drive Axle needle bearing?

I can't find a needle bearing or a bushing on either of those tranny cross-section or parts diagrams.  I couldn't find either on the case parts diagram either.  Can anyone tell me where I can find and identify part numbers for the needle bearing and/or bushing?

Thanks1  ZT
 

I don't see either of the trans needle bearings in the parts diagrams either.  (Input shaft should have one too)

It's possible Yamaha used bushings but I don't know enough about the little GT to know for sure and the engineering drawings do show needle bearings.  

I will look around some more and see if I can find anything.  If not, a bearing supply should be able to come up with one with the ID, OD, and length.  Yamaha tended to use standard sized bearings.  

Thanks Mark!
I'll know soon enough what's in there.  And I might wind up being the go-to-guy for GT80 info by the time I'm done.
ZT
 

Kevin aka "Mini Enduro Man" is an expert on the little bikes.  I haven't seen him here for a while but you might try contacting him if you have any questions

yamaha-enduros.com/index.php/user-contro...ni+Enduro+Man?back=1

Thanks Mark!
I just may do that.
ZT

Bush shows here on the inside in the link just scroll the page.
www.thumpertalk.com/forums/topic/1353989...ine-parts-reference/

Nice find RT325!!