Lessons learned on piston ring gaps

David G · May 23, 2016

Seems like there has been a lot of talk on this lately, I will pipe in with what I learned, please add.

The gap needs to be small enough to essentially close up when the engine is at operating temperatures, but large enough that it does not bind. This measurement is based on the operating temperature, the bore and type of ring. Different metals expand slightly different. An engine that is operated at a lower temperature will have a smaller gap when cold.

The ring needs to be square in the bore to get an accurate reading, I push it down with an upside down piston.

I picked up a little tool for grinding the ends, it has a crank and an abrasive wheel. The grinding will produce a little burr on the end of the ring, make sure and clean this burr off when done grinding.

It is important that the ring grooves are CLEAN and square or they will not seal. It seems like most everyone replaces pistons now, but if not the groove may need to be machined. My dad did a lot of this in the 60s an 70s, he would turn the piston on the lathe, square up the grooves and put a spacer on the top of the groove above the ring.

rrlund · May 23, 2016

I'll just throw this in about where to place the gaps. My son builds high performance engines for a living,one of the best in the business. He said to stagger them of course,but line them up with the wrist pins,one on one side,then next on the other side. It's microscopic at first,but the piston slap will eventually cause the gaps to start to wear in to the cylinder wall if they're not lined up with the wrist pins.

David G · May 23, 2016

My dad always told me to do that, never knew why, thanks.

hanovertom · May 23, 2016

Thanks! I just learned something new

loko17 · May 23, 2016

Like they say, "You learn something new everyday." Thanks.

Super-H-Mike · May 23, 2016

For all my years setting up motors ,ring gap was always .004 per inch of bore,ex: 4 inch bore = .016 gap. The second ring was usually set up tighter and the thinking was it didn't see as much heat and expanded less . Speed Pro now says to open up the second ring gap to .020-.022 on a 4 inch bore. The thought is it allows gasses that get by the top gap to get out and not be trapped between the rings which causes flutter at high speeds and loads. Supposedly helps the top ring seal and even picks up a bit of power depending on application and conditions of course. I have an old school 327 SBC of my own I am putting together now and am going to try the bigger second gap on it.

wilson ind · May 23, 2016

racing? stock car as NASCAR or Indy cars? Bet he can make the ol oliver talk!!

David G · May 23, 2016

I did the math:

Diameter * 3.14 * temperature differential * .000006 = expansion

4" times all this = .011, so the .016 makes sense.

David G · May 23, 2016

I assumed 50 degree initial and 200 degrees final.

Ken(Ark) · May 23, 2016

It's microscopic at first,but the piston slap will eventually cause the gaps to start to wear in to the cylinder wall if they're not lined up with the wrist pins.

Piston rings are not fixed in place and can rotate microscopically . Cylinders do not wear evenly . An inch down the bore it may have wear for 120 degrees on the front and another inch down it may have wear 60 degrees on the rear . When I bore a block I can see odd wear patterns that vary from cylinder to cylinder . With the bore wearing erratically the rotation of the rings can vary .

Not saying anyone is right or wrong since there are many variables . Some motors live through 3 races and some live 300K miles . Some last 10K hours but still get used another 5K hours . Then you throw in low compression vs high compression and that changes the equation too .

sotxbill · May 23, 2016

especially true on the old y blocks.. pistons had LONG heavy skirts that added excessive mass and caused the piston to try to flip at the end of strokes.. so wrist pin ring timing was a must on those engines. Probably no so much true on the newer light mass, short skirt pistons used on gas engines today. The old yblocks ALWAYS had a "bell worn in the top and bottom of the cyl that required excessive boring to clean up.. and Locations of the bell was at the critical location of max compression or vacuum. These engines needed major work at 60,000 miles back in the day.

The bell meant that you had to measure ring gap in the bottom, middle and top of the cyl.. and often it would be beyond spec for rings to handle. Only soft cast iron rings would work if at that.

Ultradog MN · May 23, 2016

I bought a 3 cylinder 4000 a couple of years ago.
It had real bad blow by.
When I opened it up I discovered someone had had it bored and installed new pistons.
Then installed .020 over rings in a .030 over bore.
Rings had about 3/8" end gap.
You gotta wonder...

rrlund · May 23, 2016

Dirt track,street rods,drag boats,he's even done some stuff for Swiss Grand Prix. He built an engine for a street rod for one of the hurricane experts at the Weather Channel. A lot of aluminum stuff. Matching numbers for restorations. If it's fast and powerful,he builds it.
He was nominated for mechanic of the year by Michigan Motorsports Hall of Fame last year but didn't have time to fill out the resume.

rrlund · May 23, 2016

More risk of slap in the ring groove at the ends of the gap too.

dr sportster · May 23, 2016

Get smaller yes but close up the gap no . The rings rotate three time a minute when the engine is running so forget staggering the gaps. Lots of good reading at the Hastings website.

dhermesc · May 23, 2016

I know they rotate when installed in a freshly honed cylinder - but don't they eventually quit rotating after break in?

Dick L · May 23, 2016

My observations as well. Hard to explain to the newbe that has rebuilt a couple engines and never witnessed the hollow areas. A lot of opinions that are not based on facts. For ring gap I used the manufactures specs. It does not always come out the same as the formula even though the formula works fine.

dr sportster · May 23, 2016

Well according to everything on the web being true { but these guys are engineers}. Read :
Untitled URL Link

buickanddeere · May 23, 2016

(quoted from post at 10:55:59 05/23/16) For all my years setting up motors ,ring gap was always .004 per inch of bore,ex: 4 inch bore = .016 gap. The second ring was usually set up tighter and the thinking was it didn't see as much heat and expanded less . Speed Pro now says to open up the second ring gap to .020-.022 on a 4 inch bore. The thought is it allows gasses that get by the top gap to get out and not be trapped between the rings which causes flutter at high speeds and loads. Supposedly helps the top ring seal and even picks up a bit of power depending on application and conditions of course. I have an old school 327 SBC of my own I am putting together now and am going to try the bigger second gap on it.

Put a 400 crankshaft in there and some 6" rods.

Super-H-Mike · May 23, 2016

The 200 would be temp at the water, but at the top ring has to be a whole lot higher . I remember reading once about a test to see what gap would actually close up and butt. IIRC it was .008 on a naturally aspirated 4 inch bore. So the .016 gives a nice safety margin. I don't remember now how they determined the rings had butted, there was some evidence, evidently...

rustred · May 23, 2016

myself i dont think the ring end gap ever closes up completely. i think they allow for that in the spec's. installed in an engine the rings wear and the gap just keeps getting bigger. as the rings wear they loose tension or the pressure against the cyl. wall is reduced. i always install rings with the gaps towards the wrist pin sides. i make sure that the major thrust side of piston has full ring contact. i believe when you get an engine hot and keep operating it the piston skirts will gaul against the cyl. walls first before the ring gap will close.
and breaking in an engine is to load it for short periods to get the rings to expand fully outward and be forced into the cyl. walls to seat.

Ken(Ark) · May 23, 2016

(quoted from post at 11:56:54 05/23/16) My observations as well. Hard to explain to the newbe that has rebuilt a couple engines and never witnessed the hollow areas. A lot of opinions that are not based on facts. For ring gap I used the manufactures specs. It does not always come out the same as the formula even though the formula works fine.

Then you have Keith Black pistons which some of the performance pistons are made to run hotter and require an extra large ring gap on the top ring .

mite-help · May 23, 2016

Yes I sure remember the days of all too frequent re-ring jobs, & that piston shim. machine shop used to say ''you need to GI [groove insert] your pistons"" does anyone remember the ''Perfect circle rod bearing shim adjuster? tapered,1/2 as thick on the sides than bottom. allowed you to reuse your old insert-[its already worn to fit] the good old days!

Lessons learned on piston ring gaps

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