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Discussion Starter · #1 ·
Hi guys, I’m new to the forum and have a few questions to start off with. I have a 1972 Yamaha R5 350. My dad was the original owner and this was the bike I learned how to ride on. I have a lot of 2 stroke experience but stopped riding when I stopped road racing back in the early 90s so I need my memory to be jarred. I just took the heads off my R5 expecting to find .25, .50, .75 or 1.00 stamped on top of the pistons. What I found was .040. Can anyone tell me what size this is? 1st, 2nd, 3rd over? And what ring size I need to re-ring them? The engine is free and cylinders have very minimal scoring. Also, if I remember correctly, RD 350 cylinders will directly slip onto the R5 bottom end. CAN somebody confirm this for me. I appreciate any help I can get with this.
 

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From my experience, its not the ring size that is important on a 2 stroke, its the piston/cylinder clearance that is. The tighter the better. Piston heat is lost through conduction with the cylinder wall. If it is loose, the piston heats up and the oil breaks down and you get a seize. You can quite happily run with one ring for racing. For commuter, 2 rings just add a bit more bottom end.

Piston Seizures.
and
Gordon Jennings
Top End rebuild

I think most bikes need rebores when worn and the increments depend on the manufacturer. Some scooters, on the other hand, have pistons that go up in very small sizes so often, just a hone brings it back to the new size.
 

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Discussion Starter · #4 ·
From my experience, its not the ring size that is important on a 2 stroke, its the piston/cylinder clearance that is. The tighter the better. Piston heat is lost through conduction with the cylinder wall. If it is loose, the piston heats up and the oil breaks down and you get a seize. You can quite happily run with one ring for racing. For commuter, 2 rings just add a bit more bottom end.

Piston Seizures.
and
Gordon Jennings
Top End rebuild

I think most bikes need rebores when worn and the increments depend on the manufacturer. Some scooters, on the other hand, have pistons that go up in very small sizes so often, just a hone brings it back to the new size.
Is my memory correct in thinking that I am at the end of my cylinders life span?
 
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