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Discussion Starter #5
As I have written before, try torquing the head bolts. It has worked for me 2 out of 3 times to stop base gasket leaks. Do them one at a time. Clean and lube well before tightening. Good luck.
I'll try torquing the bolts first, but with my luck I'll be replacing the gaskets.
 

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Discussion Starter #6
Great post and information! I just have one question, why take the piston off with the cylinder? I have a ring compressor to reinstall the piston in the cylinder.
Thanks!
 

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If you reload ammo or know someone that does use some case lubricant -- keeps the cases from getting stuck in the resizing dies -- on the head bolt head and washer.

As for leaving the piston in cylinder when removing it is so the rings don't have to re seat or wear in. When I tired to do it the piston slipped out anyway. When I reassembled things it all worked OK so I am not convinced of any substantial advantage in doing it that way.

You don't need a ring compressor. There is a bevel around the bottom the cylinder and it is pretty easy just to "wiggle" things and it goes back together.
 

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Yamaha did change the part # for the base gasket at some point, from what I've heard success is mixed with swapping to the updated part. The root cause isn't the gasket, it's that the two halves of the case weren't machined quite evenly, so there's a small "ridge" halfway across the case mouth, leading to leaks. If you decide to pull the cylinder, examine this surface. If you can feel a ridge with your fingernail, stuff a rag in there and sand it smooth. Can also use a very light smear of case sealant (Yamabond #4, or Permantex 2 Non-Hardening) on the gasket to help fill in any small gaps that lead to the nagging minor leaks. Don't glob it on there, all the excess will squish out and end up in the engine, no bueno.

As said re-torquing the head bolts can often do the trick as well. There's also no real reason to keep the piston in the cylinder- there's no harm in removing and reinstalling it.
 
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