That's what it looks like when a small piece of something that doesn't want to be compressed ends up in a combustion chamber. Judging by the damage it was small enough that it only made piston-to-head contact when it was on the crown of the piston and at the edge of the combustion chamber, so not as bad as it might have been.

Best answer is to have the head cleaned up when it gets decked, do a valve job at the same time, and replace that piston at least.

You might be able to skate with running that piston as-is, but the edges of those craters will become hotspots in the combustion chamber, and be much more prone to detonation as a result.

 

That's what it looks like when a small piece of something that doesn't want to be compressed ends up in a combustion chamber. Judging by the damage it was small enough that it only made piston-to-head contact when it was on the crown of the piston and at the edge of the combustion chamber, so not as bad as it might have been.

Best answer is to have the head cleaned up when it gets decked, do a valve job at the same time, and replace that piston at least.

You might be able to skate with running that piston as-is, but the edges of those craters will become hotspots in the combustion chamber, and be much more prone to detonation as a result.

Thanks Wreckless you’re a beauty. If it leave it as is, would I be able to sand down the high points a touch? Won’t be able to swap out for a bit.

T

 

+1. Something small and roundish and metal getting pounded between the piston and the head, repeatedly. I had a quarter-moon shaped stamp pattern like that at the top of one of my pistons and bottom of head back 8 years ago (before the engine rebuild that it caused).

 

So before I throw in my 2 cents...

Is your car fully stock or did you by chance do the egr delete mod?

 

No EGR delete. 3 inch downpipe, apexi intake and 57mm ct turbo are the only after-market things installed. The previous owner had seized the stock turbo, starting to think something must've flown in there from having it seized. Honestly looks like a ball bearing went rattling inside the cylinder.

 

Were you using toyota pink coolant?
Looks like a coolant leak past the fire ring as well into the combustion chamber.
Pistons were steam cleaned on the sides.

 

Unfortunately no, the previous owner also decided to start using all purpose (green) preston coolant 😕

 

Doubtful...

Every single "something" in the piston that is round happens to be in the exhaust side.

What you are looking at there is definetly damage by detonation and some serious detonation.

The tell tale signs.. it is worse towards the edge then interior of the piston.

In the head picture, you can see some damage in the below cylinder also. Tiny but definitely detonation.

 

I haven't personally seen detonation cause that kind of piston damage without matching flash/scorch marks on the piston and the valves. The BHG could have complicated that I suppose.

But looking closely at it again, you can see the scuffing toward the water jacket on the exhaust side, where the composite HG got pushed out exactly alongside that pockmarked edge of the piston crown, which is a classic sign of detonation-caused BHG.

Very good call @figgie! Thanks for chiming in with your perspective on that.

OP, I'm sorry dude but you really need to make sure the ring lands on that piston are intact before you make any further decisions. A full rebuild is definitely the best path forward. Contact @Captain for a 7M deck plate if you need one for a piston overbore.

 

one final thing and this is a question for anyone with tuning experience.

What octane would anyone recommend on this timing map... (@3p141592654 yes, i still have some of this stuff )

 

Due note to consider against the owners manual - that's assuming a 100% stock car at stock ~6.8psi of boost pre-intercooler (usually around 4.5-5psi on the gauge post intercooler)

OP's car was modified, and I absolutely promise that Toyota would have required 91+ octane ( [RON+MON]/2 rating method aka US Octane rating) if the factory boost pressure had been 10+psi.

one final thing and this is a question for anyone with tuning experience.

What octane would anyone recommend on this timing map... (@3p141592654 yes, i still have some of this stuff )
View attachment 308790

No wonder my old '87 Turbo with a 62-1 CT26 absolutely LOVED running on Trick 101 octane.

Crazier still, remember the old SOGI 'trick' of setting base timing to 14* advance instead of 10* BTDC? Same timing values, but +4* everywhere? Freakin' yikes!

 

Kaboom

 

It was alien blood.

 

Welll... there we go, a memorable lesson to cherish. I'll clean my head in the coming days and post pictures.

 

For what it's worth - I nuked three stock CT26's (see my avatar) before realizing that they didn't like making 15psi of boost at 6000ft+ density altitude. 😬

Definitely pull apart your block for an inspection, and if you're going that deep you might as well get the head and block decked together so you can go to a metal headgasket. If costs are an issue, I'm sure we can find a set of good used stock pistons and you can make a good set of 6 out of the combined total, and get a stock rebuild going at least!

 

Check the main and rod bearing wear locations.
Would verify detonation, depending on what they look like.

But it's quite apparent already from what you have shown.

 

I do not see how it would be possible to cause heat-compression input based detonation on an engine that tolerates 100's of thousands of Watts of waste heat with just a few thousand at cruise plus or minus

But then again its a 7M so all bets are off

 

Of note...

The group A head had the squish area removed as that is just a detonation prone area.

 

figgie, LoL, looks like one of my heads...But you are right. 1) Keep in mind, that the 7M ECU, when it detects pre-ignition will, on it's own, roll back the timing about 2° at a time, until the 'ping' is reduced. It will roll it back to the point that exhaust temp will go up to the point of damaging the exhaust valves. That is why we can get away with 91 octane, and not the 96 that they have in Japan. 2) Pre-ignition damage on pistons is a lot courser than what OP's pistons are showing. It looks more like fractured rock. That is impact damage, the bottom of the depressions is too smooth. Something was ingested by the engine, and it may have been a single piece, even though OP reports damage on other cylinders. I've seen a bit of spark plug transfer from one cylinder, to the next due to reversion, and only one plug was missing the piece. 3) With the carbon build up you (OP) have, no wonder you were detecting some ping. Love the new way Wreckless put it, "Pink painted purple feathered..." 4) invest in a knock meter...