[nalvarez]

I recently acquired a 1967 Pontiac Firebird with a freshly built 428 in it. I managed to track down the engine builder much later and get a copy of the build sheet. The builder used Keith Black KB304-030 pistons. My issue is that the build sheet stated that the top rings were gapped to .022. Based on what I have been reading, they should ATLEAST be gapped to .026

How concerned should I be? Should I pull the engine and gap the rings? I really don't want to have to worry about my engine grenading every time I go out for a cruise.


Thanks!

[Tim Corcoran]

My machinist told me that Hyperutechnic pistons are good pistons and stronger than a forged piston. There have been many problems associated with them over the years, mostly with the top ring gap. Don't run nitros oxide with these pistons. Run the ring gap that is recommended by Keith Black, if you don't you might be throwing away your engine and starting over. Ring gap is everything with these pistons. I don't know what Keith Black recommends for these pistons but you need to find out and if it's more than .022 then you better tear it down and adjust the gap. Contac Keith Black for your answer.

[Dragncar]

I would run whatever gap KB says to run. If you do not, they have been known to rip the top of the piston off.
Stronger than a forged ? Maybe a TRW but they were stronger than a Ross, JE, Diamond ect they would be in use for high HP builds.
I have always known them to be somewhere between a good cast piston and a forging, but you can get away with less piston to wall than a forging.

[Tim Corcoran]

Originally Posted by Dragncar

I would run whatever gap KB says to run. If you do not, they have been known to rip the top of the piston off.
Stronger than a forged ? Maybe a TRW but they were stronger than a Ross, JE, Diamond ect they would be in use for high HP builds.
I have always known them to be somewhere between a good cast piston and a forging, but you can get away with less piston to wall than a forging.

My machinist has been building racing engines for a very long time, over 40 years anyhow. I was surprised to hear him say that. Anyhow I personally don't know other than what I hear from others.

[steve25]

With KB pistons it’s the second ring that needs more gap then the top ring.

I forget the details, maybe the top ring was .005” per inch of bore and the second ring was .006” per inch.

I am not sure about that, but I am sure the second ring needs a greater gap.

With rings it’s better to be .00005” too big then too small!

[nalvarez]

Here are the instructions:
https://uemimages.blob.core.windows....structions.pdf

They say bore X .0065 for top ring gap. In my case that would be 4.120 X .0065 which is 0.02678. My question is how imperative is it that I tear the engine down asap? It's been this way for probably 1,000 miles.

[steve25]

Here’s the KB chart for you.

I had it backwards, the top ring needs the bigger gap.

Yes I would pull the motor out and get the gaps to where they need to be.

While your at it check the piston to wall clearance as that might be too tight also.

[Formulas]

"Modern piston design locates the top ring higher for improved
performance. A high top ring operates at higher temperatures and
requires a larger top ring end gap. To find the proper ring end gap,
multiply your bore size by the ring end gap factor listed on the chart (i.e.,
Street Naturally Aspirated 4.000” bore x .0065” gap factor = .026” total
top ring end gap).
Your hypereutectic performance piston will expand less than typical cast
or forged pistons. Because of this and the wear characteristics of the
hypereutectic alloy, you can run tight piston-to-wall clearances.

Final piston clearance should be based solely on the demands of your application.
Factors such as fuel type, altitude, outside temp., humidity, tune up, and many others factors need to be taken into account for your final clearance. "

;
;
Heat is the key here extended high RPM runs Lean AF ratios over heating from a marginal cooling system AVOID all that for sure run a 160 stat.. you havnt broken it yet grandma it till it wears in a bit then dont abuse it keep it cool might be fine, but if you cant sleep at night then pull it..

What will happen if things go south the rings butt then its to much effort for the piston to move them then the top of the pistons can destruct then you have large chunks of pistons in places they are not supposed to be

.

[Tim Corcoran]

Originally Posted by nalvarez

Here are the instructions:
https://uemimages.blob.core.windows....structions.pdf

They say bore X .0065 for top ring gap. In my case that would be 4.120 X .0065 which is 0.02678. My question is how imperative is it that I tear the engine down asap? It's been this way for probably 1,000 miles.

If you don't set the correct ring gap on that top ring you could butt the ring, tear up your bore and destroy your piston. It's up to you if you want to take that risk. If it were my engine I wouldn't run it another minute until I fixed it. I have no idea what the engine builder was thinking but it sounds like he had his head in a dark place. I would set them at .027 and drive it like you stole it with no fear.

[steve25]

Running a 160 degree stat in no way guarantees that the cars cooling system can keep the temps that low, besides timing and jetting have a far larger effect on cylinder temperatures.

[chiphead]

How concerned should we be for .004"? If you don't want to tear it down, you should take it easy on the engine. No setting the tune on kill and no forced induction/nitrous. You'd definitely want to run it rich and keep the timing down. Extended high speed interstate runs should be done with caution. The high levels of silicon are great for dimensional stability, but they don't conduct heat well to the second ring or the skirt. So more heat goes into the top ring. If the ring material tends to expand with heat, it'll butt.

Is this 1/16" moly cast ring, a ductile iron ring, a AP steel, what is it?

I've seen threads where the top of a KB piston got torn off. In those cases, I believe the builder gapped the top ring like a N/A moly ring on a cast piston, like .016". That would cause me night sweats for sure.

[Holeshot71]

How is the motor set up? What compression, heads and cam are you running?

[chiphead]

https://forums.maxperformanceinc.com...d.php?t=499347

https://www.hotrodders.com/threads/k...problem.51322/

If you look at the above failures, they happened when making dragstrip passes or otherwise pushing the engine hard. If you can limit the use to the car show/dairy queen/easy burnout routine, you may be fine. But taking it easy on a performance engine is anti-climatic, and one pass at a T&T is all it takes to pop the weasel.

[nalvarez]

Originally Posted by Holeshot71

How is the motor set up? What compression, heads and cam are you running?

Edelbrock D port Aluminum Heads, Compression ratio 11:1, Dougs headers, Holley 750 double pumper, and cold case radiator with electric fans.

Here is the cam that's in it. https://www.compcams.com/xtreme-ener...5-455-cpg.html

[nalvarez]

Originally Posted by chiphead

How concerned should we be for .004"? If you don't want to tear it down, you should take it easy on the engine. No setting the tune on kill and no forced induction/nitrous. You'd definitely want to run it rich and keep the timing down. Extended high speed interstate runs should be done with caution. The high levels of silicon are great for dimensional stability, but they don't conduct heat well to the second ring or the skirt. So more heat goes into the top ring. If the ring material tends to expand with heat, it'll butt.

Is this 1/16" moly cast ring, a ductile iron ring, a AP steel, what is it?

I've seen threads where the top of a KB piston got torn off. In those cases, I believe the builder gapped the top ring like a N/A moly ring on a cast piston, like .016". That would cause me night sweats for sure.

I believe these are the rings the builder used, the build sheet is hard to read.

https://www.summitracing.com/parts/slp-e-299x30

[Half-Inch Stud]

.022" at BDC is fine. Maybe he referenced the min gap t bdc? Such a .022" gap at the top sounds like real trouble at WOT. Awaiting the rings to wear might bring .004" more gap, but when?

[steve25]

If indeed these are the rings used in your motor then this is yet another big reason to take the motor apart.

These plain cast iron rings are not even up to the quality of what the motor came fitted with stock, which where double Moly type face rings .

[chiphead]

A 5/64" high tension cast iron ring with marginal gap in a KB hyper piston? XE roller cam at 11:1 static on pump gas? I'm starting to sweat just thinking about it. It can certainly make enough cylinder pressure hurt itself. If this was a low compression, low RPM 455 in a Grand Prix I'd be more inclined to think the cast ring/KB piston would live.

Time to pull it and turn those pistons into ashtrays. I'd use a set of Autotec 4032 pistons with LS ring pack in that application.

[nalvarez]

Originally Posted by steve25

If indeed these are the rings used in your motor then this is yet another big reason to take the motor apart.

These plain cast iron rings are not even up to the quality of what the motor came fitted with stock, which where double Moly type face rings .

Not positive those are the rings. Only thing I can make out for certain on the build sheet is Sealed Power E-299 followed by maybe an "X" or a "K" and a number.

[steve25]

The main issue is that top ring.
If the top ring was atleast a ductile iron Moly face ring I could live with that.