Quote RT325: "Bush shows here on the inside in the link just scroll the page.
www.thumpertalk.com/forums/topic/1353989...ine-parts-reference/ "

Thanks RT325,  I appreciate your help and the link you provided.
ZT

If it's your kickstart gear that is buggered I might have one in a little low end I have had out in my garage for years now that is a gt80. Even if yours comes back as being a GTMX 80 the parts could be compatible I can look into that for you if you'd like. I hope you can get the little thing running I love those little bikes they're so fun

QUOTE: @Yamahottie: "If it's your kickstart gear that is buggered I might have one in a little low end I have had out in my garage for years now that is a gt80. Even if yours comes back as being a GTMX 80 the parts could be compatible I can look into that for you if you'd like. I hope you can get the little thing running I love those little bikes they're so fun"

Thanks Yamahottie!  I appreciate the offer but I've already acquired a new idler gear and tranny countershaft,

I've now split the engine cases (with significant difficulty, I might add; and I'll post on that here in a bit) but to my untrained eye, other than new bearings and seals... (as a matter of course?), the only component I think I'll need to replace is the gear shifter shaft.  The pedal splines on my shift shaft are pretty much shot and the shaft itself was bent just enough to present some difficulty on removal.

So if you get to digging around the garage have a look at your GT80's shifter shaft.  If both covers have been removed from the case, the shifter shaft should slide right out the right side after removing the pedal, circlip and washer from the left side

EDIT:  You may need to loosen or remove the shift mechanism, to free the pin on the shift shaft's bell crank.

Please let me know how your shaft looks and what you might take for it.
Thanks again!  I appreciate the help.
ZT

Well, with some difficulty I finally split the cases... a little at a time, using wood spacers and shims. It was not an easy task. In addition to the slightly bent shifter shaft I mentioned above and eventually removed, I soon discovered the main impediment to separating the cases, was the crankshaft being firmly stuck in the crankshaft bearings on both sides. Eventually, the nearside (right-hand) case half came up and off the crank; but the crankshaft was still firmly stuck in the bearing on the left side.

Supporting the case on blocks, and with a few good taps with a mallet and wooden block, the crankshaft popped out of the left case half but, unfortunately, the left side main bearing came out of the case still quite stuck to the crank.



I finally got the stuck bearing off the left side of the crank with the following setup and copious amount of Mouse Milk, PB Blaster, and a heat gun. Nothing damaged that I can see.



For anyone familiar with these little GT80/MX 80cc motors, I have a few questions...

1) Is it common for a 2-stroke crank to get stuck in the bearings like this, I mean so tight that it's tighter than the bearing is pressed into the case?

2) Upon assembly, should the crank slide freely into and out of the inner race of the crankshaft bearings? Or should this be a close tolerance or perhaps even an interference fit?

3) My replacement countershaft came as part of a complete transmission component set that included both shafts with gears, shift drum, and shift forks. Assuming no visible damage to the replacement set; nor any visible damage to the remaining components of my existing tranny components, would it be advisable to replace only the broken counter shaft (along with the gears that came on the replacement shaft? Or would it be better to pitch all the existing tranny components and replace all the components with the complete set of new (used) tranny components, simply because the gears and shafts had previously run and "worn-in" together?

Comments, advice and suggestions are welcome.
ZT

All yamaha cranks are designed to be very tight for the magneto side. Other side is originaly a sliding fit on the primary side held over & tight by the crank gear & nut. I purposely didn't mention left & right partly because yamaha reversed the motors on some motors so ended up with mag on the right but thats on IT175's & similar. Hope yours goes together good. They're a neat little bike.

Very interesting... and somewhat comforting.

Thanks RT325!

There is a manual in the Tech Library.  Crank should be pressed out with case splitting tool (aka two bolt puller) and then pulled back in.  It's pretty easy to knock crank out of true with a hammer.  

Not uncommon for bearing to stick on mag side on crank.  Interference fit on both ID and OD.  Clutch side has gear and nut to "clamp" the inner race of bearing so generally the bearing ID is a light slip fit and OD is a press fit into case.

A good matched set of gears is best if you have them.  But it's not going to matter much with so little power and it's not like it's going to be ridden 10,000 miles a year.  

Thanks Mark! I appreciate your thoughtful reply and addressing my concerns.

I'll be sure to take a good look at the crank and try to figure out a way to check for any runout. And unless I notice anything not up to snuff, will likely install the complete replacement tranny set I acquired.

I hope to be checking cylinder wear and piston fit later today.
ZT