70 RT1
Bike was running fine (start of every good story). I replaced the cylinder base gasket because I thought it was leaking. It turned out the leak was from the oil feed line in the side of the cylinder. I saw some scoring on the cylinder and piston but the bike had been running fine so I didn't sweat it. I put it back together torqued it all up and bled and primed the oil line up to the cylinder.

The bike did not start but it did seize. I took the head off, put some 2 stroke oil in the piston and got the piston free by turning the engine backwards with the rear wheel in 1st gear. Piston was then free and everything is moving nicely. I crank it with the head off until im tired and it seems good. I put it all back together again and it promptly seizes back up. I take it apart AGAIN and now the going backwards trick is not working.

What did I do wrong? Is there some trick to putting the cylinder back on that I missed?

Sounds like maybe you didn't get the rings positioned correctly when you slid the cylinder on. You saw the little tits in the ring grooves of the piston, yes? The opening in the rings has to be positioned at those tits when you slide the cylinder on. Critically important so you don't bind up and break a ring and gouge the cylinder. Slide it back off and verify. No other reason I can think of since it was running. You didn't remove the piston and re-install front to back? Don't know just guessing now. Post what you figure out.

Did you take the piston off of the rod when you took the cylinder off? I have seen pistons put in backwards.

I'll chime in and say yes, I think a ring is sitting on top of one of the retaining "tits" and you now have a ring popped out of the ring land & hung up on a port.

Thank you guys. I did not pay attention to the "tits" (first time I've ever said that!). I'll have to see if I can get it unstuck, pull the cylinder off and reinstall. Any tips on getting those rings in properly?

...and no I did not take the piston off the connecting rod.

There was a sticker I saw when I was a teen on a 56 VW - said "if it has tits or wheels it will give you problems." This thing apparently has both.....now what......

Good luck and let us know the prognosis once apart.

Ok then that confirms your problem. Slide it back off...could be tough if "stuck" like you say but it has to come off. Worse thing probably would be to break a ring and have it fall into the case. Be aware of that possibility. After apart....Look for damage to rings, piston and cylinder from being bound up. I'm a little anal about those things because you can ruin a lot of parts if there is damage and your spinning the assembly at 6000 or 7000 RPMs!

When going back together...position the rings where they go then carefully lower the cylinder with one hand while compressing the rings with the other. First time is always a little tricky. If there is no damage the rings will stay in their correct spot if you pay attention to that while you compress them and slide the cylinder on. Pretty critical to get it right!

BTW there is a top and bottom to the rings...make sure you have them on right and not upside down. You'll see the ends are angled and will only compress correctly if in right side up. Look at the rings when in the piston grooves and make sure you can fully compress them. You should see what I mean if you look at those "tits" and how they are constructed along with the angle of the ends of the rings. A lot of words there but got to say tits again.

Mystery solved! Boy I am a terrible mechanic. Pulled the cylinder off. no problem. The piston rings were lined up good on the pins on the piston. The engine would still only move about 1/2 inch. I decided "F%&^ it" and turned the engine with the flywheel nut. By the grace of the Yamaha God a crushed exhaust pipe spring comes up with the connecting rod!!!!! Holy crap...the one part I was missing after putting her back together. So when I pulled the cylinder off the first time I left the exhaust spring hanging on the cylinder and it jumped off into my crankcase. Then I carefully taped up my crankcase so nothing would get in there. LOL This is a safe place right? You guys wouldn't laugh at me would you???

Now I just have to order more gaskets and try it again.