Piston ring end gap?

Paul LeClair

Paul LeClair

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Calgary, Alberta, Canada
I just spent the better part of an hour using Google to search info re calculating piston ring end gaps for an air cooled motorcycle engine with cast aluminum pistons running in an iron liner bore, freshly honed.... Street use naturally aspirated.

most of the info i found are in Imperial measurements, with a range suggested for top compression ring per inch of bore as variously 0.003 inch, 0.0035 inch, 0.004 inch, and 0.0045 inch

I am dealing with a measured 2.46 inch bore (roughly 62.5 mm bore)

so the suggested range seems to be 2.46 x 0.003 inch = 0.00738 inch gap (0.17 mm) which maybe seems tight, to the other end of 2.46 x 0.0045 inch = 0.01107 (0.27 mm) which maybe seems generous.

I am thinking maybe something in the middle of the range, say .008 inch (.203 mm) gap for the top compression ring, then a little bit larger gap for the second ring? This is for a Kawasaki 4 cylinder 650 engine using new old stock Kawasaki rings that did not come with any gapping info and were really difficult to source.

any thoughts? Do I go tighter or looser? I am thinking the ring gap formula would also be relevant to similar Laverda engines, air cooled, aluminum pistons, iron bores.........?;):rolleyes::):unsure:
 
Top ring .007in 2nd ring .009in If the oil ring is a one piece, set it at .011in

Those figures will work fine. Built a few of those. Any questions just ask.
 
There's a cheats way to assemble those plain bearing engines too. Pistons (with rings)on rods inserted into the barrel. Barrel inverted on the bench and top half of the case put on it - don't forget the base gasket. Put crank in place - with primary chain and camchain - and assemble the rods onto it. All properly lubed of course.
Doing it that way means you don't have to lower the barrel onto the pistons and rings.
 
thanks so much, Greg!!

I did a full bare frame bare engine cases build on this 78 650 SR some years ago. New crank bearings, new endless cam chain etc., etc..

at the time I had a heck of a time sourcing piston rings for the new old stock genuine Kawasaki 1 mm over pistons. The aftermarket rings I used have never seated properly and the engine smokes.

over the past couple of years whenever I saw a genuine Kawasaki 1 mm over piston ring set appear on eBay I have bought them, typically $40 - $80 per piston ring set. Recently I found and bought the last of the 4 sets required.

also, on the head I previously had rebuilt, there were problems with valve guides on two of the four cylinders, contributing to the smoking.

I recently bought another two complete spare KZ 650 engine ( really cheaply), and had the best head rebuilt. Also found 4 sets of the genuine Japanese made carb rebuild sets.

The rest of the bike is done to the absolutely highest standard. It has been the lightly smoking engine lettings it down. Now that I have acquired all the necessary bits I am finally remedying that.

The back story is that in 1978 I bought a brand new Kawasaki Z 1000 ZR-1 to drag race, and a brand new KZ 650 SR (D1) to street ride. I put a ton of miles on the KZ 650 SR, then had to sell it to assist funding for law school. When I got out of law school I bought another identical 1978 KZ 650 SR, then ultimately had to sell that to assist funding of the setting up of my first law office. Many years later I found and bought not one, not two, but three KZ 650 SR all in wretched condition, and built one of the best in the world from the three of them and a ton of new parts from Z1 Enterprises. Then the bloody fully rebuilt motor wouldn't quit smoking, so the bike has been languishing in the back of my garage for years haunting me by my failure......

now I am hopefully rectifying the engine smoking with the proper new parts I have slowly accumulated. Completely stupid project, as many of my projects are. I have triple or quadruple the funds into the bike than it will likely ever be worth, and hundreds of hours, and I am not likely to ever ride it much. But, hopefully it will quit haunting me if I at least know it no longer smokes if this motor top end rebuild works the way it should...... :rolleyes:

as an aside, many of the Kawasaki KZ parts are hard to get and stupid expensive compared to Laverda bits. I have recently seen on eBay offered for sale KZ 650 SR battered up used fuel tanks, asking prices between $500 USD and over $1,000 USD. I guess they have become a popular choice for custom bobber builds. No idea if anyone is paying that ask, but if they are my huge piles of battered KZ 650 parts, including a couple of those SR fuel tanks, is likely worth way more than my completed bike, bloody hell.
 
They're a surprisingly capable bike. The last one I built was for local pre '82 class racing and had a 750 top end grafted onto it. The cases need opening out but the 750 rods drop onto the 650 crank - with the bonus of 1mm bigger rod bolts. It's proved long lasting - onto it's third owner now since i built it - and competitive. Ist owner regularly ran with a pair of TZ350's and had great fun.

While it's apart, look at the clutch basket cush. The OE rubbers shrink and you finish up with a noisy idle. Classic fix is simply to turn up the idle speed,,,, I've in the past, both replaced the rubbers with urethane and made sleeves to go over the rubbers and compress them enough to take up the slack. Never even looked to see if new are available.
 
GregT said:
They're a surprisingly capable bike. The last one I built was for local pre '82 class racing and had a 750 top end grafted onto it. The cases need opening out but the 750 rods drop onto the 650 crank - with the bonus of 1mm bigger rod bolts. It's proved long lasting - onto it's third owner now since i built it - and competitive. Ist owner regularly ran with a pair of TZ350's and had great fun.

While it's apart, look at the clutch basket cush. The OE rubbers shrink and you finish up with a noisy idle. Classic fix is simply to turn up the idle speed,,,, I've in the past, both replaced the rubbers with urethane and made sleeves to go over the rubbers and compress them enough to take up the slack. Never even looked to see if new are available.
Click to expand...
Hi Greg

I have an old thread on a KZ 650 forum of how I dealt with the clutch basket cush rubbers when i first rebuilt the engine The rubbers are not available new as far as I know. Ground the rivets off the clutch basket, replaced the rubbers with fabricated replacements, then welded the basket back together with plug weld on the end of the ground off rivets. Worked really well.

thanks for the help with ring end gap clearances and general KZ info. Wide range of interests and skills here on this forum for sure.
 
There's an engineering study somewhere that says ring gap is of little importance. It can be 1/16" and not make any difference to blow-by or oil consumption. The study also said that installing rings with staggered gaps isn't necessary, as the rings work their way around the piston anyway. I'll see if I can find the article. Don't go away.

... some time later ...

Here it is. From the Institute of Diagnostic Engineers in UK. It's a respected organisation.
 
Hi Cam

it says that larger ring gap on fresh piston rings of full radial thickness is not a problem. Note the writers caveat that each set of the rings with bigger and bigger gaps were freshly made and still had full radial thickness. It appears ultimately it is radial wear on the rings that lessen the seal on the cylinder wall bore while also resulting in bigger gaps.....

it will still make me feel better to start with new rings with both full radial thickness and smallest possible acceptable gap.

interesting article, thanks!
 
Slightly off topic, but still about piston rings: I know one is not supposed to align the ring gaps, but since the rings rotate during use does it really matter? Or do all three rings rotate at the same rpm and in the same direction? I remember reading that they traveled around the piston at 3 to 5 rpm while running….. Surely any alignment is only momentary and will reoccur occasionally at random intervals?

Or am I completely off-base?
 
I think it does matter, I watched a rebuild of a BMW car engine with low compression on one cylinder and the alignment of the rings of that cylinder was the only reason they could find for the problem.
 
If you're going to quote BMW's I'd point out the change made in early production of the flat fours. To cure an oil smoke problem at startup, they introduced pegged rings - with the peg at the top. Apparently that cured the problem of oil accumulating in the heads while not in use.
An exception to all the rules I think.
 
Alzero said:
I know one is not supposed to align the ring gaps, but since the rings rotate during use does it really matter?
Click to expand...
See the link I posted a couple of replies back (post #7).
Basically, it doesn't matter. The rings are free to rotate around the piston and will do so. Like plantets, sometimes they'll align, sometimes they won't. When assembling an engine, you can stagger the ring gaps or not. It makes no difference, although it seems to make people feel better to stagger them.
 
I have never dismantled a bike engine and found that the ring gaps were aligned, I couldn't count how many I have worked on. The discussion about ring gaps was that some newer models recommended having the gap at the thrust faces front and back, as distinct form the established practice of having them about 30 deg off and staggered. I won't be assembling any engine with them aligned.
 
Further reading indicates that rotation is induced by the cylinder crosshatching which implies that once that is worn away the rings will no longer rotate. If so, then if the ring gaps are aligned due to random rotation at that point they will stay there with the negative results already mentioned. I cannot imagine that a BMW engine with a factory ring installation would be built with aligned rings. Ditto with any aftermarket mechanic doing a rebuild.

BTW, piston port two strokes have pinned rings to keep them from catching on the port edges at the ends if they turned.
 
ordered a cheap ring gap grinding tool from an Amazon vendor, arrived in a day using Prime. Came with a hand crank handle but I chucked the crank in a drill to drive it. Nice square easy accurate cuts. I liked using it a lot better than hand filing rings, which I have always disliked. Rings are now accurately gapped and installed on the pistons. Just waiting on the engine gasket set coming from Z1 Enterprises in the USA. Shipping tracking shows they have been held by Canadian Customs since September 26, buggers.....(n)(n):poop: Now turning my attention back to the 79 1200 Laverda wiring issues while I wait for customs clearance.
 

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Ross Pistons have a very good instruction sheet on there web site to work out ring gap for different engines and applications. Air cooled water cooled blown normally aspirated. Or by pass all that and go to there gapless rings
 
on the topic of egg decapitators and piston ring gap grinders, I sourced local to me very cheaply an authentic Kawasaki 1970's dealer tool for compressing the rings on all 4 pistons at once. Very slick....

dropped the barrel on the four pistons, got all excited at how easy it was, pulled out the ring compressor rack from under the barrel, slapped on the head and torqued the head down, then looked across at the work bench to see the new head gasket still in its package, bloody hell......(n):oops:
 

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