I am attempting to find the source of a leak in my 1970 RT1. The soap bubbles show that the leak is at the left rear of the head. I'm pretty sure that the head gasket is not leaking. When I first saw the leak I thought it must be the head gasket because I had replaced it with a new one when I did the timing and did not anneal it first. I pulled the head and checked it for flatness with a sheet of 600p emery on a flat blanchard ground surface and found it was cupped a little, so I sanded it flat. Then I annealed the gasket and sprayed it with Permatex copper spray-a-gasket and let it dry overnight. Reassembled and carefully torqued all the nuts and it is still bubbling in the same spot! It looses about 5psi in 5 min. I have a suspicion that it is coming from the rear left cylinder stud or the cylinder base gasket is leaking by in that area and coming up through the stud hole. I am going to take the head to a shop that can double check flatness tomorrow, but I really think it is that stud. Has anyone had a similar problem? If I pull the cylinder and I don't see a problem with the gasket, could the stud be leaking at the case? Is that a common problem? How do I check for a leak there? How would I seal a leak there?
I guess that"s enough questions, hope someone can reply to one or two. Thanks to everyone out there!

What size piston do you have? Is it the original or oversize? I ask because if it is on the third or fourth oversize the cylinder liner may be too thin and become separated from the cylinder. Happened to me on a '73 RT3.

Excellent point, I had not thought of that. I just went out to measure the bore and it's 3.150", 80mm on the money so its original. I suppose it could still be leaking there though....

Is the cylinder base gasket Yamaha OEM? After market ones can be too thick and cause leaks, happened to me.

Don't know I haven't pulled the cylinder yet. I'll do that tomorrow.

Do you use a sealant on that gasket?

Mate had that on a piston port RT1 & was just oozing out porous casting of the alloy. As far as i know its still running ok & think he wished he'd not tested it as only did it to be nosey. I have two RT2 cylinders out to 82mm & all good for years so in theory there's plenty of meat in the liner but that's a seperate issue to having it leak due to i guess bad sealing of how the liner is cast in place. I'm certain its not a straight liner & is cast out into the trasfers a bit. Be interesting if i had a beyond use cylinder to put in the log fire & see whats left in the morning. Did that with a 4 stroke head once to burn the crap off it & pulled out two guides & two seats--Where the $%^^ is my head, mate. Send me a new head, this ones vanished lol.

On one of my many chainsaws i was blowing air through the vacuum small pipe as suspected the base gasket was blocking it. Anyway it spun it over flat out Brrrm. I'm certain i still had the plug in & all. Thought blimey those seals are putting up with about 100-120 psi & not complaining.

OK, I didn't realize that the threaded holes for the studs were blind holes. Duh! No need for sealant there.

Sealant in a blind hole may hydro-lock and split the case!

That's why the studs are interference fit to the cases.