piston rings

I have a set of rings that are 1mm over for my kz650. I was told that this slight variation will help compresion. My question is as to how true that statement is. It makes sense, but i have heard arguments either way. Will this damage my engine in any way?

id be worryed about it not fitting in the stock bore

1mm over means that you need to bore cylinders 1mm over (larger) and 4 NEW 1mm larger pistons. I believe it's considered the first overbore. Sorry....

I believe first overbore is .5mm (.020"), second (and largest for stock parts) is 1mm (.040").

You don't want to run 1mm oversize rings in a stock bore, serious mistake. After you file the ends to give you enough gap that they will fit, they will no longer be round when they are compressed far enough to fit in the stock bore. They will loose compression and wear your cylinders into an oval shape.
Plus cost you power due to increased friction because they will be applying more pressure on the cylinder walls.

Example:

( ) is fairly round
() is oval and what happens


Bad idea all around.

Post edited by: steell, at: 2008/02/24 13:20

Thanks for saving me from prolonged engine blunder. So, I need to find a set of standard 62mm rings. I didn't see any on Z1, anybody know where I can get a good deal on new rings?

Cycle-Parts.com is usually cheaper than buying directly through a Kawi dealership, and show the Standard pistons listed at $50.44 each (= $201.76 + shipping) or the 2nd over (.040) pistons listed at $53.74 each (= $214.96 + shipping). For the money, the larger pistons to match the rings you already have makes more sense IMO.
This is providing that either are even still available though.

I do see NOS (standard and 0.040) ring sets and piston sets pop up once in a while on eBay for a fair price.

But since you obviously have the motor torn down this far and are looking to get a little more "umph!" out of it (and if course if it fits your budget), you could always go with one of these and be done with it:

BIG BORE PISTON KIT KAWASAKI KZ650 1976-1983 700CC $576.95 including shipping is too pricey IMHO, plus you have to consider you'll still need to have the cylinders bored to match.

KAWASAKI KZ650 WISECO PISTON KIT 2MM OVER KZ 650 76-83
$469.99 is a slightly better deal, and this particular auction has the "best offer" feature added. So who knows how low the seller is actually willing to go on it, especially since it appears that he's had this listed in his eBay store since as early as April '07.

IIRC though, there's someone (DennisKirk.com? APE?) online that offers this kit for about $440.00 + shipping, so I'd say shop around if you decide to go this route.
I was fortunate (ok, lucky as all get out!) and scored this same kit 2 years ago on eBay for $220.00 including shipping. So diligent searching and playing the waiting game just might net you similar results.

Heck, you could even go the 750 cylinder bored to 810cc route like a few others (rstnick is one of them)here have.
I'm sure any one of them would be willing to give you the break-down on costs and work involved to do this swap, that way you can figure out which of the above suggestions fits your needs/desires and budget the best.

Have you checked the bore to make sure that standard rings will work?,

I talked to my mechanic/ gear head friend and he assured me that file fit rings were not only common place, but a perfectly fine application that would last me many miles.


I one day plan on going to an 810 kit, but next time i crack my motor open I am going to find my 4 best pistons (Mic them to see who who the least worn of the 4 top ends i have is) then file fit my 1mm oversize rings.

I don't plan on running it like that for 20 more years,I just want 5 years or so out of that top end wile i collect 810 parts.

yes oversize rings are common, but most of the time its only .005 over and you file the end gap to fit, the size you are trying to use is way too big , the rings are not going to fit right.
Check the bore to make sure its within spec then decide on the correct direction

Listen to steel and bskz650. They are right on the money. most of the ring seal is caused by compresion aplied behind the ring ,forcing it out.this compresion comes from the preasure created as the piston goes up.if u use toooo large a ring u will only add mechanical pressure wich realy wont do much but wear the cyl oval .in a short time u will lose power rather than gain power.if u NEED a cheep fix. u would be better off the mill the block to creat more compression preasure.it is the compresion #'s that will give u more power.if u find that your bore is not too big for the pistons due to wear just buy new rings maybee, .005 over , hone and go.or shop ebay for the next size over pistons and rebore. u may find rings cheap at Hastings rings.they make rings for most everyone in the buss.
what is the reason u think u need rings? did u do a compression test? leak down test? loss of power could be valves. did u check valve clearance?

Post edited by: mark1122, at: 2008/02/25 14:01

steell wrote:

I believe first overbore is .5mm (.020"), second (and largest for stock parts) is 1mm (.040").

You don't want to run 1mm oversize rings in a stock bore, serious mistake. After you file the ends to give you enough gap that they will fit, they will no longer be round when they are compressed far enough to fit in the stock bore. They will loose compression and wear your cylinders into an oval shape.
Plus cost you power due to increased friction because they will be applying more pressure on the cylinder walls.

Example:

( ) is fairly round
() is oval and what happens


Bad idea all around.<br><br>Post edited by: steell, at: 2008/02/24 13:20

Kawi considers +0.50mm to be "first over" but aftermarket rings are +0.25mm for first over (they go in increments of 0.25mm)

You can install these +1.0mm rings on the standard piston if you file enough off the ends of the rings to set the correct end gap for the cyliunder diameter. End gap should be roughly .003" - .004" PER INCH OF BORE DIAMETER.

As advised, you will be compressing the rings an additional 1mm (.040") smaller diameter than designed. Problem? Well, look at how much the ring is compressed from it's "free open" position anyway, not sure another .040" is horrible. I know a lot of rebuilds that have done it. If you are NOT boring the cylinder, the added pressure will help seat the new rings and you will probably get 20k to 30k service out of it. If you are planning to bore, get new pistons and rings the next size up.

If you go to a larger ring size, be sure to trim the ends properly to fit the bore. If you run with the end gap too small, the rings will expand as they heat up and get tight enough to score the cylinder and/or break.

BTW: the pros I read from say that most of the compression loss is not at the ring/cylinder wall interface, it is at the gap in the piston the ring seats into. That is a "steel-to-aluminum" contact and the pistons do wear there as the rings bang up and down on each stroke. Changing rings only and using the old pots won't help reduce the loss at the ring gap, so the effects will be a lot less than if you replace both pistons and rings. Smart money if you are keeping it is get the new pistons to go with the new +1.0 rings and have it bored.

Post edited by: bountyhunter, at: 2008/02/25 16:43

Scatdrat wrote:

Thanks for saving me from prolonged engine blunder. So, I need to find a set of standard 62mm rings. I didn't see any on Z1, anybody know where I can get a good deal on new rings?

www.cyclewareables.com/ had brand new NOS piston/ring sets for my 750 twin that were the amazing price of $36 each. Don't know if they have your size.