Most frustrating stories start with, “I bought it on ebay and it looked so nice”….
But really, I bought this (cosmetically) nice cylinder and head on ebay for my CT2 restoration project. The guy said, “it’s in great shape and will take a standard piston”. Well, it's pretty worn out so I was all set to buy a 2nd over Yamaha piston and have it bored (yes, it’s that worn).

So, I’ve got my rebuilt bottom end on my freshly powder coated frame and I put the top end on to do a leak down test. I’ve got everything blocked off, spray it with soapy water, pump 6 psi into it and discover bubbles coming from the head gasket (granted it’s a used gasket). I take the cylinder and head to my machinist and he tells the head looks fine but suggests that I lap the top of the cylinder.

I get it home and wet the back of some 320 grit sand paper, tape it to my piece of glass and slowly polish away. Then comes 400 grit, then 600. I put it back on the bike and it’s still bubbles, using 3 different head gaskets (that I've annealed) and two different heads. So, I take it off and do some more lapping. Now the top surface of the cylinder looks better than NOS (like glass).

I put it back the cylinder back on the bike and it still bubbles. Now I’m getting frustrated, so I grab my old, crappy, petrified with who knows what, deeply gouged cylinder off the shelf and try that one and it does not leak (WTF!) I chose not use my new head gasket on the ebay cylinder because I didn't want to bugger it (and the crappy old cylinder did not leak with any of the used ones I put on it).

Now I’m thinking of looking for another cylinder on ebay (oh no, not again) and remember someone had mentioned greasing a head gasket before you put it on. I had mentioned this to my machinist and he looked at me funny.

Well, long story short, I installed the “nice” looking ebay cylinder with a freshly greased head gasket and, “voila” the bike held 8psi for an hour without some much as a single bubble.

But greasing a copper head gasket seems counterintuitive. Grease liquifies when hot and a running engine gets hot therefore, when I run the bike the leak will return.

So, before I go out and buy a 2nd over piston and pay to have this cylinder bored out I’d like to clear this up. Should I or shouldn’t I assume this cylinder will work?

Have you tried the copper spray they sell for spraying on copper gaskets and mating surfaces?

Isn't that like putting a bandaid on a bigger problem though?

Had the same thing on my CT1-C recently.
www.yamaha-enduros.com/index.php/forums/...restoration?start=70
MarkT (he knows a thing or two) advised that I anneal the gasket, which I see you've done and use the copper spray.
I checked both cylinder and head for flatness and low spots by coating the mating surfaces and then rubbing them on my dead flat quartz worktop.
Even though I was confident that the flattening of the head and cylinder I had carried out was good I still used the spray.
It seems like a good product made for this application so it's not like you're bodging it up with a load of sealant.

I'll pick up a can and see if it solves the problem.

I assume you were sanding the cylinder and head on your piece of glass in a figure eight motion like the service manual shows? If not, then even best intentions could just be making it worse.

Yep, always grease all my gaskets unless there's a prior leaking issue in clutch cover or base gasket but always grease head gasket. Goes back to the very first CB350 in about "69-"70 when i replaced the head gasket. Not sure why i had the head off but anyway. Went to start it & it squeaked & leaked everywhere just turning on the starter. Workshop foreman "old fella" said did ya grease the gasket. Said No as had a sort oil film on it that looked like it was meant to bond it. So in desperation i loosened all the many head nuts & a couple of short bolts & put oil in both plug holes. Plugs back in & either kicked or starter can't remember. Oil shot out through the gasket. Bolted it up tight & was perfect, sealed up & no leaks of either compression or oil.
Anyway--woffling on--i did wonder if you had a porous cylinder in how the cast in liner is mating to the alloy. A mate of mine did a pressure test to an RT1 & oozed out everywhere up through the fins. Never did hear the outcome of that as the bike had a bad habit of melting pistons.