Ring end gap discussion [Archive] - Chevy Nova Forum

: Ring end gap discussion

Paul Wright
8th-August-2006, 08:12 PM
I'm finally getting around to working on a project that sat around in my shop for the last 8 years (last date in the build book was Feb, 1998). It's a 4.030" bore with an RHS balanced rotating assembly that came with KB hypereutectic pistons and Sealed power Moly rings (E251K)
KB tech page (http://kb-silvolite.com/article.php?action=read&A_id=32) states the top ring needs more clearance than normal. I'm glad I checked and read the instructions as these non file fit rings had the normal .016" clearance.
I'm filing them to .030" which is slightly smaller than the .008" conversion calls for. 4.030" x .008 = .032" gap.

Sealed power and other sites list .016" for the top ring gap so this is almost double the gap. The second ring gap is often larger on a normal ring set. (.022")
If I keep the second ring at .022"-.025" it would actually be smaller than the top ring gap.
The current trend is to use bigger gaps to relieve combustion pressure but
I can't imagine running proportionally that much bigger on the second ring (.045"?).

KB site has no spec for the second ring, just the top.

Federal Mogul/speed pro ring gap tech page (http://www.federal-mogul.com/cda/content/front/0,2194,2442_897063_8682,00.html)

Any input?

8th-August-2006, 08:46 PM
A friend of mine had a 351W built by a professonal engine builder who swore by the KB hypereutectics. He said those pistons initially got a bad rap from broken ring lands. Most if not all of these were a result of the engine builder not reading the instructions recommending the large upper ring gap. I understand the Hypers hold a lot more heat in the top of the piston and expand the top rings. I understand the second ring is unaffected.

The 351W was built about 8 years ago before larger second ring gaps were commonplace. The top ring gap was in the 0.030" range, but the second was standard for the time, which was probably between 0.012" and 0.014".

He ran that motor hard for years before selling the car. He road raced at Pacific Raceways in WA state when it was about a year old. He beat every 5.0 Mustang, even the forced induction ones. He beat several FE powered Cobras as well with his '67 fastback 4 spd. Point is, that motor was run very hard and never had any problems. Never used oil either.

I think if you stepped up to the 0.022" second ring gap commonly accepted today you'd be fine.

8th-August-2006, 08:58 PM
wouldn't "Total Seals" be a better choice in rings Paul ??? or are ya stickin' with the conventional style because ya have 'em already ???

just a thought... ;) :D :D :D

Paul Wright
8th-August-2006, 09:12 PM
Ron: That's good info! Thanks! When in doubt follow the instructions.

I normally run Total Seals in just about everything but these came with the kit.

.030" is a lot for a top ring but I'd rather have too much gap than rings butting when hot.
Right now I'm thinking of using .025" for the second ring gap for a little more venting...

Here's the 1st piston being mocked up. Lot's of valve clearance for a healthy roller. Deck height and squish seem right where I wanted it. I can bolt on a head and do an initial leak down check. I'm pretty sure it will be fine.

8th-August-2006, 09:20 PM
i hear also the hypereutectic pistons grow a bit more and require more ring gap... but as a wise man here has said many times... "seal the cylinder" or something to that effect... wouldn't the Total Seals be exactly what's needed ta properly seal the cylinders ???

"Sealed Power" ???

just my opinion, but i think you'd prolly find a use for the std style rings and ya should go with what ya know best... the Total Seals... and seal the cylinders...

just 2 pennies Paul ;)

Paul Wright
8th-August-2006, 09:37 PM
I think for this engine conventional rings will be fine. I had to re-read my notes to remember what I was doing back then. I feel confident in the piston to bore fit and ring gap at temp. It's a good idea to stick to "the plan" unless there is overwhelming evidence otherwise.
Besides, there are a ton of racing engines that use properly gapped and fitted rings with good results.
I'm pretty sure I'll find out fairly quickly if I made a mistake but I can always change them later. Now, that might be an interesting back to back test.

8th-August-2006, 09:49 PM
Would such a large gap cause a lot of blow-by until the pistons expand to close the gap? Or do these pistons expand quickly enough that it's not an issue?

Paul Wright
8th-August-2006, 10:16 PM
Read both the FM and KB end gap links in my first post. They have different specs for ring end gap but similar observations about end gap and it's effects.
Keep in mind the KB hypereutectic piston reacts differently to heating than a forged or cast piston. Ring lands play a major role in ring sealing. If ring to ring land clearance is too big, the rings will flutter and leak.

I think the main thing novice readers should come away with is not to simply install rings even if they aren't "file fit".

1. Read both the instructions that come with the pistons and the rings.
2. Make sure your machinist knows what rings you are using. Some rings require unique honing procedures.
3. Always check ring end gap and land (ring groove) fit. A piston that has been dropped may have a tight land that will bind when it gets hot.
4. Use a ring spreader tool to prevent premature cracking. I've seen many a rebuilt engine that has a broken ring.
5. Use a ring compressor to prevent ring chipping on installation. You can't see damage after it's in.
6. Properly lubricate your rings so they don't slide dry. Make sure the bore is clean and lubed.

8th-August-2006, 10:56 PM
Paul,in my experience tite second ring gaps or "gapless" second rings hold pressure above the second ring and minimizes the pressure differential between the top and second ring.
As you know this pressure differential has everything to do with the sealing ability of the top ring.
IMHO the modern low tension taper face (down pumping) or barrel faced top rings leak on initial cylinder pressure rise softening the combustion spike. These rings are much more efficient, react quicker to compression and seal better to intake vacumm, when the pressure from this initial surge is allowed to escape,,hence the use of "Vac-U-Pans" and lower crankcase vacumm pumps.
My dyno has a "blow-by gauge" to measure lower crankcase pressure.The engines will show less overall blow-by with a larger second ring gap given the top ring is properly prepaired.
We have tested;taper faced,barrel faced,cast iron rings and we have ground the OD of the rings to match the exact ID of the cylinder bores.We have changed the "face angle" (very important).The land clearence and flatness of the ring comes into play here.
A ground top ring fitted to a smooth cylinder with large plateaus needs more initial ring gap due to the fact there's less imperfections to "seat in".

Today with the common use of nitros,electronic controlled fuel injection,(read,programed high speed lean out)ultra high CR's and very thin engine oils, the ring and piston manufactures must error on the safe side.

:eek: I'm sorry I got carried away:o .I enjoy your post Paul,but seldom have anything to add.Best of luck.

RED67 (still learning):beer:

stock z/28
8th-August-2006, 11:09 PM

Paul I use a lot of the Kb pistons and a lot of the Speed Pro pistons. I have assembled 2 short blocks this week alone (its only tuesday night).

Its "must" to run the end gaps very large on the Kbs. The 412 I just assembled I set at .034" top and .018" second. My understanding is that the KBs level of silicon in the aluminum is what causes the need for for such excessive top gaps. I can guarantee you that if run a "normal" top gap the piston will destroy the top land as well as the cyl wall.

I don't really like the kbs for a street engine because of the large gap. My opinion is that until the the cyl temp (not coolant temp) reaches max (sever load) the gap will be excessive--that's why I use a normal second gap.

For street engines I would much rather use Speed Pros with normal gaps. I also use the speed pros in race type engines. The only problems I have had is that they tend to beat the keeper groves on floated style rods. Press fit are trouble free.

On last thing on the top rings. I refuse to use a top ring that is not ductile iron. I simply wont install it in a high perf application period.

As for the Gap less rings I will not use those at all with the exception of alcohol motors, where they do a great job of keeping moisture out of the oil. On every case I have seen on a competitive drag race engine they cost you power. In a lot of class engines I don't run a second ring at all (forged piston), it has less drag and the empty grove acts as an accumaltor so top ring flutter is greatly reduced.

Just my opinions.

Paul Wright
8th-August-2006, 11:36 PM
More good feedback. It sounds like I made the right choice. Would a gapless second ring might actually be counter-productive with a KB piston since the large top ring gap would increase the inter-ring pressure?
The KB's have an interesting wide radius groove between the top and second rings. Anybody know what it's function is?
I'll post a picture tomorrow.

8th-August-2006, 11:59 PM
I have KBP-KB168-030 w/22cc dish in my 406, and Sealed Power
E-243k30 rings. With my 4.155 bore it called for a .033 gap onthe top
ring. I called Speed-O-Motive where I got the rotating assembly,
and talked to a engine builder in thier shop concerning the ring gap.
He said they had no problems running them at .028 gap with my
combination, and to set the 2nd ring to .014-.016. Iv'e many miles
on this 4 yr old build with no problems, or oil usage. I can't rate thier
tech advice I'm no pro at this, but like to build my own engines. All
my machine work was done by a top rated shop. I was very carefull
gapping the rings to hit those specs, used a hand crank gap machine.

Many guy's around here running hipers, a friend with a 383 (.024-.028)
pulled his down after 2 yrs to go to go forged (Nitros) showed no hint
of cyl wall scuff or scoring.


9th-August-2006, 02:02 AM
The groove in the KB's between the top 2 rings is an accumulator groove for cylinder pressure that escapes past the top ring. It helps to prevent ring flutter and promote top ring sealing. I run KB's in my Pontiac 400's with top ring gaps a little big (.035") and gapless second rings set at .025". These semm to work very well in a street/strip engine. The hyper's don't expand as much as forged pistons, that's why you can run a tighter piston to wall clearance. The extra gap isn't for piston expabsion, it's for ring growth. The tops of the KB pistons are designed to retain heat and reflect it back into the combustion chamber. The top ring acts like a heat sink of sorts and grows more in this application than it would using a standard cast or forged piston, thus the need for a larger gap to prevent the ends of the rings from buttings and destroying the piston ring lands.

9th-August-2006, 12:17 PM
I heard several horror stories of cars that didn't get out of the driveway before killing a KB piston--in almost every instance they either used normal gaps or didn't check them. I ran them in a 391 I built and used .039 gaps; the engine was a cheapo build--Chinese crank, $130 rods, KB pistons; with a big roller and some AFR 220's it ran 10.20's at 131 in a 71 Nova. Even with the big gaps it ran clean and leaked down very well. I did spend a lot of time deburring and smoothing the entire top of the pistons to do away with any potential hot spots because there were a lot of sharp edges--I hear hypereutectics do not like to be rattled--they tend to break instead of bend, but I never had any trouble with them. Mine had about 13.6-1 compression and it usually was fed C-12.
I'd run em again.

9th-August-2006, 01:33 PM
I ran them in a 391 I built and used .039 gaps; the engine was a cheapo build--Chinese crank, $130 rods, KB pistons; with a big roller and some AFR 220's it ran 10.20's at 131 in a 71 Nova. Even with the big gaps it ran clean and leaked down very well. I did spend a lot of time deburring and smoothing the entire top of the pistons to do away with any potential hot spots because there were a lot of sharp edges--I hear hypereutectics do not like to be rattled--they tend to break instead of bend, but I never had any trouble with them. Mine had about 13.6-1 compression and it usually was fed C-12. I'd run em again.

Now THAT is a budget engine!!!! How long did you hold it together, Mike?


stock z/28
9th-August-2006, 01:57 PM
I agree 10.20s are very impressive in that heavy of a car.

I also agree on the detonation or "rattling" issue. The increased cyl temp with the KB can be a concern, especially with an iron head. I think KB recommends a reduced timing figure, but Im not sure what it is. I also always try and run the fuel mixture a bit on the rich side on circle track engines.

Kb has a line of very nice forged pistons now. I have only used a few sets but they appear to be a very good piston.


9th-August-2006, 08:44 PM
i had this same dillemma with my motor and the kb's....i actually ran a slightly larger than recommended top ring gap (around .038, just in case i ever wanna spray it) and i ran .016 for the second ring....i really never felt comfortable running such a large gap, plus you might be amazed to find out how many "reputable" engine builders don't know about the required gap increase.....

i believe the need for the gap has more to do with the raised top ring land being further up on the piston and hence hotter....i have been and may always be wanting some actual testing on these pistons and the ring gaps they require, the effect of the top ring on pressure and power with a consistent second ring and so forth....actually i would like to see dyno test with any pistons and varying top end gaps only....

and like you said, i too would rather have too much gap than not enough.....

stock z/28
9th-August-2006, 10:13 PM

I have heard that the higher location of the ring is the reason, but personally I think its mainly due to the amount (or maybe type?) of silicon added to the aluminum alloy.

I have run quite a few "dykes" and "head land" style forged pistons with the top ring located much higher than the KBs are with very tight gaps, some 4 in motors as tight as 12-13 thousandths with no sign of butting. I would not recommend anyone run this close a gap unless they are prepared to a lot of fairly expensive "testing", some that's fairly destructive.


9th-August-2006, 10:30 PM
Now THAT is a budget engine!!!! How long did you hold it together, Mike?

I never had a chance to find out. The guy that owned the Nova we ran the engine in sold the car with my engine in it! Some guys came up from Mexico with a suitcase full of cash and bought both his cars. He gave me the iron headed 406 that was in my car for many years before it was time to rebuild. That one ran 10.80's on motor--9.80's w/ juice. That's with a stock cast crank I still have.
The guys that bought the car didn't want the cast crank, KB piston, cheap rod stuff, so he added a forged crank, J.E. pistons and some high buck rods--the car would barely break into the 10's after that!! hehehe!