I don't understand why you think this happened due to the lack of umbrella seals? Without stem seals, oil consumption would go up a little. Mostly you'd notice a puff when you started it after sitting and smoke under decel.. That piston looks to have suffered a lack of lubrication either from no oil or too tight of tolerances. It got hot enough to destroy the temper in the rings.

There was small pools of oil in the dishes on top of each piston. Each exhaust port/valve stem has large amounts of oil. The only change I had made since the last event was fitting a different head with smaller combustion chambers and no stem seals. The way I look at it is that excessive amounts of oil doesnt like to compress very easy, put together with a higher comp ratio to start with. It probably started with heat build up, then small piston failure, then it would have snowballed from there.

Any other theories /explaination of events for this failure from anyone else?.... Understanding how this happened is all part of the fun.

Benny

Hmmmmm ouch, sorry to see this happen to you Benny
I would also like to hear other explanations for this happening because I am another one that doesnt run any valve guide seals other than a 2mm section viton O ring on each stem to stop oil running down the valve stem. It's an old racers trick.
I can understand why you would think no valve stem seal could cause this to happen, especially since its a recent mod on your engine. Maybe your stem to guide clearances were a little too big but I would be examining the head very closely and checking everything before assuming anything.
This is the first time I have herd of this happening if it is the true cause of all you grief.
Very interesting, now you have me a little worried
Good luck with the refresh Benny.

The guide to stem clearence is so to say, not the tightest tolerance. Im thinking a set of K-liners would fix that problem........

believe it or not, I think this kind of thing is normally caused by coolant getting into the combustion chamber...

If there was a lot of oil geting into the cylinders, then it's possible that the oil was detonating [oil has a VERY low octane rating] & this may have contributed, but detonation usually punches holes in the tops of pistons & THAT much oil would surely produce a lot of smoke.

This looks like it has had parts of the piston crown grabing in the bore, causing the damage below it. You obviously need to find the cause before doing it all again, but i'm going with detonation at this stage.
Just how high is that compression ratio anyway?

If the guides are a wory, then fix them & fit umbrella seals to the inlets only, but have a real good look at all of the head in case there is a crack that has been missed.

Sorry that i don't have more to offer.

Yes L18, partial head gasket failure was my first thought, then compression ratio issues, even fuel quality (even though they guarantee RON), even possible sticky rocker due to heat etc.
Sorry Benny but looks like you have some homework to do
This is all part of engine developement I guess.

Benny - which cylinder????

Good luck and keep us posted mate.

The Pic shown is Piston #4, and in the worst condition. Pistons 1 --> 3 are also damaged, #1 being in the best condition. calculated Comp ratio was 11.3:1. Head was fitted on a saturday afternoon and raced on Sunday. [I hate not having at least 1 week lead time before an event].

Suprisingly the bores appear somewhat undamaged [cant rule out warpage though] as I dont have the right tools to measure. I quick hone and nearly all marks were removed.

R-fast has passed on some replacement pistons to me [thanks R-Fast]. I totally agree that detonation is the cause of the damage + residual oil remaining in the cylinder [overheating and pre-igniting] as well as the increased compression. I leaning towards that it wasnt just one problem that lead to the damage, but a series of small problems which led to the destruction of the pistions.

Benny

Forgot to mention, Yes Dodgman, there was lots of smoke, in fact shed loads.

Benny

Just be cautious about jumping to conclusions that this was caused by detonation - that might cost you another engine. There should be heaps of other evidence of detonation in the combustion chambers or on the piston crowns if this was caused by detonation. If this was a detonation problem, you would certainly have heard it happening!

Are they full moly rings or moly tipped cast rings? or chrome cast rings? unless they are actual solid moly, detonation extreme enought to cause this piston failure would have cracked the rings.

I'd be checking the surface of that new head to make sure its within tolerances. And having a close look at the head gasket and head and block surfaces for evidence of leaking or black marks beyond the gasket's firing ring. My money is on this being caused by coolant.

I did some research into diagnosing piston failure a couple of years ago (when we were blowing up turbo engines), and coolant being sucked in at high rpm/high load can cause the edge of pistons to become brittle and chip like this. I think it was said to de due to extreme temp differences between the hot piston and the liquid coolant That's the start of the damage. Then when its opened up enough for the combustion process to burn away the oil that should be lubricating the skirt, the rest of the damage you see here happens.

Well, Harry makes a well reasoned assessment of the situation, based on the limited detail & the single photo. I suggest that it is one of several possibilities, & maybee several things that were happening all at the same time as Benny has suggested.
I tend to still stay with the detonation theory, once again, based on limited info, for the following reasons.

1. The cylinders had a small amount of liquid oil in them on teardown.If the guides are as bad as is suggested, then they might be the cause,... maybee. As i have said, oil has a VERY low octane rating.

2. There was plenty of smoke. I am assuming that it was blue smoke. I also assume that smoke was evident from the start of engine runing & not just at the time of engine failure. If this is true, then it indicates the presence of excess oil during the run.

3. While ring failure is often connected with severe detonation, it is not always seen in a detonation affected engine. It's possible that an engine can be stoped, or power is dramaticly reduced, before ring failure actually occurs. I have seen this situation before.

4. Water, or water /alcohol injection is often use to control detonation, although it is normally provided as a mist or vapour & not as a raw liquid. My limited experience has been that raw liquid often results in severe misfire or in the worst case, hydraulic lock, although the actual effect would be entirely dependant on the volume & the temperature. A small leak often shows up as a completely decarbonised combustion chamber if left for long enough.
If coolant is used in the cooling system [instead of water] there is sometimes evidence of it in the chamber or the exhaust, although, once again, this is dependant on the time in service & the size of the coolant leak into the cylinder.
There has been no mention, so far, of any obvious coolant leak via the head gasket, nor has there been mention of evidence of coolant in the exhaust ports or pipes. [yet]

5.This problem occurred in all cylinders, while a coolant leak into the conbustion process is usually seen in either one cylinder only, or in two adjoining cylinders. The fact that the problem gets worse from the front to the rear may possibly be a reflection of the fact that the rear cylinders of an inline engine often run hotter than the front, although this is a generalisation.

I suspect that a thorough examination of the engine will reveal more clues, & you need as many clues as you can find, & hopefully this problem will be overcome.

I suggest that the head should be major overhauled after a thorough crack testing job, & hardness testing too, then the engine be dyno tested if possible as i have seen an engine destroyed for something as simple as the fact that the outer ring of a harmonic balancer moved from its intended position, causing the timing mark on it to be too far out of register, leading to wrong timing & a wasted engine. Dyno testing will reveal the correct timing points for this engine at all engine speeds.

Oh yeah, it would be a real good idea to ensure that the radiator is good as overheating can cause rubbish in the block to dislodge, & the radiator core can filter out all the larger bits to make sure that they don't return to the block

Good luck with your sleuthing.

Edit, Please tell me that it wasn't something as simple as someone forgetting, in all the rush, to fill the radiator, or leaving a hose clip loose causing loss of coolant externally, or even for the hose to come off at one end. Another possibility is that the rasdiator was filled, but the traped air wasn't purged from the head before racing, leaving insufficient coolant in the system. Say it isn't so.
Simple overheating from this cause can result in severe detonation which does this kind of nasty stuff, & as you say, it took all of three to four minutes to do it.

I think i'm wrong, but it looks to me like lubrication problem.
There was maybe no oil supply to the pistons anymore and they overheated starting to break up.
How many laps did you do before this happened.

Looks like a condition of preignition by the top edge looks melted and and the v shape on edge in center on edge

Benny did you figure this out. I have a vw 1.8t with 200 miles on the rebuild with similar looking failure. I believe busted pcv that hooks directly to the intake manifold was sucking large quantity of oil into the combustion chamber. 20psi boost with mid teens timing, A/F was in the 11s. Heres a pic




Plugs from motor look perfect?



The last one is black because the cyl lost compression