Standard size piston rings

I'm looking to redo the top end on my 1980 XJ650 to fix its low compression problems, but can't seem to find any standard size piston rings for it.

I can find lots available in the various oversizes but don't really want to go to larger pistons due to the cost. The pistons in the bike are still within spec and the condition of the bores is good enough that that they could be run through with a hone be good to go. (the bike only has 20,000kms on it)

I don't know if these are even available, or if it is standard practice to go to oversize pistons whenever you do the top end on a bike. I'm somewhat new to this; I've had tons of experience on car engines but never really on bikes.

Thanks in advance

 

Bike Bandit lists them
www.cheapcycleparts.com lists them cheaper.

 

Did you check your valve clearances? 20k seems awfully soon for rings to wear out.

 

No i never bothered to check the valves, they're not ticking or anything.

Yeah i know that it's odd for the rings to be worn out at such low millage. The bike sat for about 7 or 8 years. I got it running and it initially had poor compression, presumably because the rings were just gummed up from sitting. I just kept doing frequent oil changes and riding it and it got better over time.

Now after about a year it has no compression again and it has enough blow-by to force oil out of the PCV port and in to the airbox. So from this i'm assuming its the rings.

Its strange cause it's almost intermittent. Some days i have perfect power and loads of engine braking and some days it just revs but makes almost no power with no compression braking at all when you let off the throttle.

Would there be anything else that could cause this???

 

In the tech manual there is a procedure for measuring ring wear. Basicly you put a ring in the cylinder without the piston and measure the end gap with a feeler guage. That will tell you if your rings actually are worn beyond spec. It sounds to me though you just need to get in there and clean everything up real good. Carbon after it sets can do nasty things. It will start flaking in the combustion chamber, and get caught in the valve seats, causing valves not to close all the way. It also mixes with oil in the ring grooves and turns into a tarry like substance that doesn't let the rings float in the groove and seal like it should. So, I would suggest measuring end gap, and if it checks out, clean the piston ring grooves, the combustion chambers, give the valves a polish with some lapping compound, and replace the valve seals. Careful on the lapping though. The valves are are only coated in stellite, and the material underneath is fairly soft. So, if you try to grind them like a Briggs and Straton or SB Chevy you'll grind right through the coating and ruin the valve. A gentle hand lap just to clean it up should be fine though. If you do end up needing rings they are still available through a Yamaha dealer. They run anywhere from $32 to $44 a piston depending on which dealer you go through. Hope that helped.

 

Thanks civitas, and everyone else, for the reply. You're probably right, it is simply more in need of a good cleaning than anything, it is pretty gummy in there.

 

I checked with bikebandit.com for rings. About $56 per piston. cheapcycleparts.com has them for about $172 for a set of 4. Gaskets would be close to another $100. Man, this is obscene. In order to do this you would have to really like the bike because you could never come out on it by trying to sell it. You would have twice as much in it as it would be worth.

Ed