Ring end Gaps

[modok]

Should be mentioned that different combos the wear can be all over the place. Used to seeing antique and industrial engines, where the rings really DO wear to nothing, but in other cases I see them go the life of the engine and hardly wear at all!
I've seen a few cases of chrome or moly rings get absolutely tortured and yet not wear enough to open the gap more than .004 what it was to start with, which is very odd, but it can happen.
Just as a "overall" view--
Olden days, round skirt piston and cast rings, sure you can run tight to start, but remember that was totally different hardware than what most of us are running now.

Nowadays, slipper skirt piston with rings high up, and pre-lapped steel rings that hardly wear..... I think we find out that ring gaps are...... same as a lot of clearances in that the min isn't the ideal. If the spec is .020-.040, right in the middle is where I'd want to be, to start, because if we get it right it's not going to wear.
Some of them the second ring gap spec....you'll need to stack 2-3 feeler gauges it's so huge

[Brian W]

Dirt track 2bbl deal. Bore was extremely wore out... Unsure what ring gap was when new because I didn't build it but would say this is a little excessive! Lol
Thats a 3/16 punch...!

[Cougar5.0]

Your right Bill...run a tight top ring gap on a KB piston and see what happens...not pretty.

Are you sure?

DSCF0631.JPG

[user-23911]

That's from detonation.

100% preventable by using some electronics.

[In-Tech]

That's from detonation.

100% preventable by using some electronics.

Sorry Joe, that is probably not from detonation. Have you ever used KB hypers?

When they first came out I used quite a few sets. The heat they reflect and the raised top rings makes the rings grow a LOT. One example I can give is back in '94. Supercharged 383 chev 7.5:1 compression AND Aluminum heads. Customer wanted 500hp dead smooth idle and exhaust note quiet as stock when idling. It's a fairly long story/journey so I will try to be brief, 1994 GM ecu so knock sensor actually worked correct, .032 top ring end gap, ~11:1 AFR at WOT(no intercooler). It never knocked and blew the top ring off like above picture. Broke the cylinder while it was at it. Sleeved it and now 4.040 bore from 4.030. Now ~.045" ring gap, all seems well and about to give to customer, one final flog in the 6300lb Tahoe for about a half mile and blammo, did it again.

Another block, JE pistons, .024" top ring gap, same tune, NEVER hurt it again. Ran 5 years after that and Tahoe destroyed after giving to his nephew who rolled it a few days later.

There are quite a few things that COULD have been done so THOSE hypers could have lived but it is not what my customer wanted. He wanted a dead quiet stealth Tahoe and he got it. What he ended up having to deal with was a slight bit of piston rattle when cold.

Just like my sig line says, "Heat is energy, energy is horsepower, but you gotta control the heat."

[cgarb]

Yep, broke right at the valve reliefs. That's where most of them break out, the weakest point. I did see one where the whole top came off. That's was from idiots building a motor though. It was file fit rings that never got fitted. I'm sure even the most expensive forged piston would have suffered the same fate.

[Steve.k]

I think with boosted or heavy nitrous application deto can hit without detection at certain times of engine cycles. With a tight (quiet mild mannered engine)cam this would be even tougher to detect and control. So with that yes forgings would offer more longevity. At .045 ring gap i have doubts they butted and lifted the crown? But hey ive been wrong before.

[cgarb]

One thing I just thought about, that picture the ring looked stuck in the piston. Is that how it was disassembled? I wonder if the ring gap would end up right at the valve reliefs near the edge of the piston with no ring under it to support it that it could deform the piston in that area and cause the ring to stick. It would act like the ring butted but it is just stuck at the ends.

[Orr89rocz]

When they first came out I used quite a few sets. The heat they reflect and the raised top rings makes the rings grow a LOT. One example I can give is back in '94. Supercharged 383 chev 7.5:1 compression AND Aluminum heads. Customer wanted 500hp dead smooth idle and exhaust note quiet as stock when idling. It's a fairly long story/journey so I will try to be brief, 1994 GM ecu so knock sensor actually worked correct, .032 top ring end gap, ~11:1 AFR at WOT(no intercooler). It never knocked and blew the top ring off like above picture.

In my experience with boosted sbc, there should not have been an issue with that low compression, that wide a ring gap and that air fuel ratio....what was the timing like? How long was it at wot? Being not intercooled probably did it. Which to me would be detonation after a while of wot when chamber temps got up. Or maybe temps got up just before it would detonate and the piston is just junk

[Steve.k]

Ive worked on numerous 2-stroke snowmobile engines. They suffer from this type of failure quite often. More often than not ring is still in place and top part of piston broke out.On the pic above there looks to be minimal damage to ring which would suggest clearance likely wasnt the issue. The valve releif is the thinnest area on piston ie weakest point. On snowmobiles breaks them out exactly the same way without having the valve releifs.

[Belgian1979]

The profile on the break surface doesn't imply detonation. It looks like lack of strength and just plain broke off in one effort.

[Cougar5.0]

I'm fairly certain it wasn't detonation, I didn't note any of the tell-tale signs and others didn't see signs of it when I posted the pictures back when it happened. I pushed the thing over the threshold by adding a KB blower (non-intercooled, ~6-8 psi) to a short block not built for boost. Ring gap was insufficient (not sure what gap was, I didn't fit the rings/pistons), but it was built as a ~9:1 N/A application with the KB pistons pictured. I may have rotated the rings to the broken area to facilitate taking photos of the ends.

The ends of the rings had polished areas which seemed to imply that they were butting (tough to see, but differences in reflectivity imply butting):

Butted_ring.jpg

Other pistons had cracks in the same area (which is definitely weak where the valve relief gets close to ring groove)...

KB_crack.jpg

[In-Tech]

When they first came out I used quite a few sets. The heat they reflect and the raised top rings makes the rings grow a LOT. One example I can give is back in '94. Supercharged 383 chev 7.5:1 compression AND Aluminum heads. Customer wanted 500hp dead smooth idle and exhaust note quiet as stock when idling. It's a fairly long story/journey so I will try to be brief, 1994 GM ecu so knock sensor actually worked correct, .032 top ring end gap, ~11:1 AFR at WOT(no intercooler). It never knocked and blew the top ring off like above picture.

In my experience with boosted sbc, there should not have been an issue with that low compression, that wide a ring gap and that air fuel ratio....what was the timing like? How long was it at wot? Being not intercooled probably did it. Which to me would be detonation after a while of wot when chamber temps got up. Or maybe temps got up just before it would detonate and the piston is just junk

A little more detail. The customer was a good pay and he wanted what he wanted and didn't mind paying again 'till he got it. The rub is the exhaust restriction to keep it quiet. 12psi boost with exhaust, 7 psi boost without exhaust. EGT's stabilized at 1650 with exhaust, 1400 without exhaust. When it is corked up the heat HAS to go somewhere and in this instance the forged piston absorbed more heat than the KB hyper. Still not an endurance engine by any means but fine for his occasional full throttle blasts in a 6300lb school bus

Early TBI Whipple kit, intercooled wasn't much of an option back in '94.

[Belgian1979]

I'm fairly certain it wasn't detonation, I didn't note any of the tell-tale signs and others didn't see signs of it when I posted the pictures back when it happened. I pushed the thing over the threshold by adding a KB blower (non-intercooled, ~6-8 psi) to a short block not built for boost. Ring gap was insufficient (not sure what gap was, I didn't fit the rings/pistons), but it was built as a ~9:1 N/A application with the KB pistons pictured. I may have rotated the rings to the broken area to facilitate taking photos of the ends.

The ends of the rings had polished areas which seemed to imply that they were butting (tough to see, but differences in reflectivity imply butting):

Butted_ring.jpg

Other pistons had cracks in the same area (which is definitely weak where the valve relief gets close to ring groove)...

KB_crack.jpg

Looks like it is heat induced/expansion cracking. Any signs the pistons were scuffing ?

[MadBill]

Looks like a good candidate for TBC.
(and maybe abradable skirt coated forged pistons, to get strength without noise)