Old pistons in new cylinders?

Hi folks,

Not been here in awhile.

My efforts on the XS are a bit sporadic as I keep striking issues with getting it going.

Now, I happen to have a set of NOS standard size barrels for my 79 XS400F. I was hoping to find new std pistons but they don't seem to exist in the 'A' variant I need.

If my old pistons are in spec, would fitting them to the new cylinders be advisable? I have new std rings to fit.

I have the top end apart now trying to sort out a compression issue (I think).

Bike wouldn't start with 120psi per side dry (180 wet). The rings were new and the bore honed. Valve and ignition timing was as good as I could get it. I had spark. Only question mark is fuel as one carb had no fuel in the bowl when I drained it and I subsequently found the little strainer blocked. (These are carbs I bought as a set).

But I have kind of homed in on the compression, possibly incorrectly. Reading other posts seems bike ought to at least run even badly at 120psi on both.

Appreciate any feedback.

Hey mate, I would assume the pistons will be fine, its usually the rings that wear. So if you have new rings to match the size of the barrels you should be golden. For piece of mind you could spec the rings in the barrels first. But if both the barrels and rings are NOS you shouldn't need to It sounds like you got a good thing going!

It seems like you couldve gone another troubleshooting route first before tearing the engine apart..

I dont mean to hijack the thread, but i am in the midst of a topend rebuild, and i am at a loss for getting the cylinders back on the pistons, do i need a piston ring compressor to slide the cylinders onto the pistons? (I left the engine in frame as i did want to get too involved before the bike has even started)

I see that the cylinders are tapered so i thought i should be able to just slide the cylinders on, but it seems like doing so might cause some damage to either the pistons, the rings, the cylinders or all three?

Any sdvice?

squeeze in the rings by hand and eaze the barrels on bit by bit. Its better to have two people, one to squeeze the rings and one to hold the barrels

Dont be afraid to use a ton of oil too. When i did this, the jugs almost fell over the pistons as i guided the rings with one hand