Most piston ring manufacturers recommend a minimum end gap of .004 inches times the bore diameter for the top piston compression ring. So for a 4 inch bore, the standard end gap would be .016 inches.

For the 2nd compression ring, the standard end gap recommendation for most stock engines is typically .005 in. x the bore diameter. So for a 4 inch bore, the minimum end gap on the 2nd ring would be .020 inches.

For a modified street performance engine that generates more horsepower and heat, the end gap should be opened up a bit to compensate for increased thermal expansion. The recommendation would be a minimum top compression piston ring end gap of.0045 to .005 inches times the bore diameter. For a 4 inch bore, the ring end gap on the top ring should be increased to .018 to .020 inches.

For the 2nd compression ring on a modified street performance engine, the recommendation is typically open up the end gap to .0055 inches times the bore diameter. For a 4 inch bore, the 2nd ring would be gapped to .022 inches.

For a nitrous or blown racing engine, the top ring end gap sould be opened up to as much as .006 or .007 inches times the bore diameter. Now we're looking at a 2nd ring end gap of .024 to .028 inches in an engine with 4 inch bores.

For the 2nd ring on a nitrous or blown motor, the recommended ring end gap is even wider: .0063 to .0073 inchestimes the bore ( or 0.025 to 0.029 inches with a 4 inch bore).

The recommended ring end gap for oil rings regardless of the engine application is typically .015 inches.



Hastings did an R&D and found that engines running above 10,000 RPM, ran the same with and without rings.............. go figure

 

Hastings did an R&D and found that engines running above 10,000 RPM, ran the same with and without rings.............. go figure

Really? That is very interesting! Does hastings explain his theory on why this is?

 

Thanks for some good info...GA

 

I have keith black hypereutectic pistons in my 347 and I just tore it down due to a wiped cam lobe and I made sure my machine shop used the recommendations from the keith black website for the top ring gap and my pistons are fine. I didn't have any issues with compression either. But I have read about the KB Hypers that if you don't do the top ring gap like they recommend then it will eventually collapse the gap and break the top ring gland. I'd go off the info ki posted.

 

I agree with my machinist and used .020 on the top ring gap and .018 on the 2nd ring. I used SRP pistons made by Je pistons. and Je file fit plasma moly rings.....

Good luck on your build.........

 

My experience and research is spot on with what kim posted. I am currently building a 308 road race motor and gapping my top ring to 0.019" (~0.0047"/bore dia) and 0.021-0.022" 2nd ring.

 

KB hyper pistons move the top ring up closer to the crown. That is why they recommend such large clearences for the top ring. They run hotter needing more gap to prevent binding.

 

I can speak from experience with Keith Black Hyper pistons. Their top ring land is close to the top of the piston. Use their gap specs for their pistons. I gapped mine differently and I paid the price. My engine rebuild book said different from the Keith Black specs, obviously you should follow the specs of the piston manufacturer.

My top piston ring on one piston expanded and the ring ends butted and obviously pushed up due to the less material and being so close to the top of the piston. This blew a chunk out of the piston top. The chunk got wedged and pushed into the side of the cylinder and cracked the block. Other pieces got mushed and went out the exhaust, bending the exhaust valve to #####.

 

yes, i am speaking of forged je pistons with a distance of about 0.25" from the top of the ring to the top of the piston. i am not familiar with KB hyper. agree, use their recommendations.

 

I read somewhere - maybe on the Hastings site - that most blow by is between the rings and cylinder wall, not through the end gap. I'm going .024" and .028" on my turbo motor. I may regret it, but that's how I gapped them.

 

I saw another ring test, don't remember which ring manufacturer, where they progressively opened the first and second ring gaps and ran dyno tests. There was no additional blowby or power loss when the gaps were anywhere from .020" to .060".

 

open that gap up a little. it wont hurt a thing. i set mine up to about .028-.030 if i remember corectly cause i had planned to spray the bag out of it. and i did spray the bag out of it. 5 years later, and it still has less than 3% leakdown. the end gap is not a big deal as long as its big enough

 

GARY ALP,

?I still do not see where YOU have posted that YOU are using the KB piston...
...) you only referred to as having spoken to the person at the company.)...

ARE YOU ARE ARE YOU NOT...???

if you ARE using the KB piston, then ...
just as many have posted in reply, you MUST use the larger ring end gaps.

IF you ARE NOT, and thus using another brand of piston, then you can use the tighter / smaller GENERALLY accepted common ring end gaps.

some of the issues with the KB is ring placement, and the amount of material BEHIND the ring groove, the mix of alloy in the aluminum itself, the overall shape in / of the piston ETC.......they are a P.O.S. piston.

 

He posted this almost a year ago...it's probably too late.

 

I agree with Kato - a material this brittle should not have a cross-section this thin. This is my failed piston BTW - 4 others had cracks at the eyebrow including a redesigned version where they scalloped the area near the thinnest part of the piston. If you want a good inexpensive piston, get a set of Probe forged pistons.

 

I agree with Kato - a material this brittle should not have a cross-section this thin. This is my failed piston BTW - 4 others had cracks at the eyebrow including a redesigned version where they scalloped the area near the thinnest part of the piston. If you want a good inexpensive piston, get a set of Probe forged pistons.

Just curious
Did you speak to KB about this? What was their reaction? What wwas the ring gap? Any sign of ring butting? What part number?

Thanks
Rusty

 

I had a set of KB's in my 393 that lasted all of about 50 miles before lifting the ring lands . We gapped the rings at .028 and never even had a chance to spray it .

Needless to say , its the last set of KB's I will ever buy or suggest to anyone .

 

I had the same thing happen. I was 17, and the guy that built the motor didn't gap the rings properly. Here's what happened...

 

wow...ive been spraying 150-250 to mine for years

 

I ended up with the Probe Sportsman 10682 pistons. I gapped them quite large at 0.026" I think and see no oil usage. I would not worry about saving a couple thousandths on the gap as you will not see any reduction of power or oil usage. I'd go with the 0.032" gap.

 

for sure....040 gap wouldnt even show any difference. this is one thing thats better too loose than too tight

 

 

Ya, KB pistons are crap. I had an engine built for me many moons ago and after a couple thousand miles, all the ring lands lifted on every piston. It was just a N/A 308 with P heads, small comp cam, and an RPM air gap manifold, nothing special. I was pissed because I was young and spent decent money that I worked very hard for. Lesson learned though, I build my own engines now and stay very far away from KB pistons and its worked out very well. I used DSS pistons on my last motor. Did a .026 gap up top and a .028 on the 2nd ring, motor went though hell (used with a vortech and nitrous [at different times]) and lasted 4 years/52,000 miles. Engine finally let go due to my Romac balancer going fubar.

 

ive been using K.B. pistons on my engine for about 1 1/2 yrs w/ no prob.I gapped my top ring even more than they reccommended.It seems all prob w/ K.B. pistons are the top lands which is why I added extra gap.

 

yeh seems like people cant read feele guages to me

 

when I talked to a guy at K.B. he enlightened me to using enough gap.When you think about it its not the whole width of the ring that is being exposed to crankcase press its pretty much just the amount thats sticking out past the piston when its in the cyl.Its extremely small.So I def used enough gap,more than the reccommended.