Possible Detonation - Local Ring Groove Wear is questionable

[VFR750]

Did this piston experience some local detonation?

Looking at my KS-Alusil piston, I noticed the local wear on the edge near the exhaust valve, just above the #1 compression ring.

Plus the side gap looked big. It measured at .25mm (0.01" feeler) over a small arc. Everywhere else was within spec.

Could this local wear really be the early signs of detonation?




Other pistons have similar, but less groove wear. Three others are at/above the wear limit, and all are worn in the exact same location, directly aligned with the exhaust valve.

The engine was burning oil due to leaky valve guides, and had poor leak-down numbers. (#2 was bad due to failed head seal/loose head stud nut.)

[HawgRyder]

Detonation is usually found on the top of the piston in a "melted" look.
This is not to say that yours is not det damage....but I would take a careful look at the ring land for possible expansion due to ring "flutter".
When a ring moves up and down in the land...it sort of "puckers" the edge making the land less sharp (90deg?) at the edges.
Looking at your post...is it possible that the top rings were lined up with the exhaust vavles?
Perhaps the mixture is a little lean...or even too rich...allowing post combustion burn (more heat in localized area).
I have seen engines that had mismatched head/piston combinations...resulting in improper combustion.
When this happens...the flame front is directed towards the exhaust valve so that when the valve opens...it directs the flame out the ex port giving that area of the piston more heat than elsewhere.
Poor sealing of the exhaust valves can also contribute to the problem...they "leak" and allow flame to exit during the power stroke causing valve head burn/piston damage/you name it.
I would be interested in seeing the end result of all the detective work.
Bob

[VFR750]

I don't know where the ring ends lined up.

This is the first rebuild since new in 1982. So it was factory cylinder/piston/head.

The heads have rebuilt and the exhaust valves looked pretty beat. They have been replaced.

The cylinders were good, but one head/cylinder was replaced due to damage from a leaking head gasket. This piston was not from that cylinder.

[Eagledriver]

That doesn't look like detonation to me. Here's the deal, you can spend 3k plus on new cylinders or you can reuse what you have. .004 inches is supposedly the limit but if you use the old rings it will not be a problem. You will probably find that any new rings you buy have bigger end gaps than what you have now. I would recommend you clean them up,measure them an put them back in.

-Andy

[BURN-BROS]

Detonation usually results in broken rings on the 3.0 liters.

I typically find more wear at the exhaust valve side of the piston. This area of the piston is operating at a higher temperature than the intake side.

Your issue may be the result of a lean condition for that cylinder or possibly a plugged piston squirter.

[911pcars]

At any rate, the ring side clearance is beyond spec (~0.004") - ring end gap is a different spec. Replace the piston.

What were the engine build specs and operating conditions (compression ratio, base and ignition timing curve, fuel octane, piston-to-cylinder clearance, air temps, altitude, A/R ratio, driving conditions, stock or aftermarket ECU?). They all have a bearing on potential detonation damage.

Sherwood

[Lapkritis]

That looks like exhaust gas leaking past the ring and taking piston material with it over time. Side clearance is one of those things... I know some builders who don't even look at it. Others will tell you it's super dooper critical. At the end of the day, I side with some of the old schoolers who don't hang their hat on the number unless it's way, way far out of spec. It truly means nothing as long as the piston holds together. At TDC and BDC, the ring will be changing direction with the help of the surrounding piston. If the piston cracks then you'll know it and then you'll have to replace. If the engine is out of tune then it won't matter if you're in spec or out - you'll be replacing pistons anyway.