I whipped my head off ready for the soon to arrive modified replacement. As usual there is a but...

This is what I found in cylinder 4.

Any thoughts? Should I start an eBay search...

Looks like something like a piece of metal has been bashing it's way around. Drop the piston and check the bore as it could be still in there if it's not been blown out

Next question would be how it got in there or what it is. Could have fallen it when changing a plug or be part of a piston ring...

I’m dont think it’s a foreign object as it is localised to one small area near the inlet.

No damage to the bore.

I guess it it must be detonation. The is some minor pitting to cylinder 2 also.

Each mark is the same length and matches the print on both sides. (Looking at the R shape in the middle and the X on one side, hence why I'm leaning towards foreign object).

Well spotted... I think you are right there, I suppose that’s the better of the two.

There is reversed 7 and LR pattern on piston with mirror image in head that could only have been created by a short thin straight metal strip being squashed between the two.

Could something have come off from the inlet or exhaust side, metal exhaust gasket?

Defiantly not det
Definitely ingested something
Damage is mostly cosmetic but could hold some localised heat spots, 1 would flat off the raised bits on the piston and perhaps grind flat the combustion chamber then bolt it back up.

Thanks all.

A Facebook group suggested there is also evidence of the pistons touching the head? But this cannot be the case surely as the engine was bog standard out of a focus.

def foreign object airbox screw/fixing, valve guide, anything really , id be surprised if it was piston ring though piston still looks too good. as said clean it up it will be fine

I was thinking piston ring - it's very thin and sharp, can't think of much else that would be hard and thin enough to do that. Even a compressor blade would be pounded flat in a couple of shots. You only need to lose the tip of an oil control ring and you've got that happening with not much else going wrong.

[Edited on 19/2/18 by coyoteboy]

I guess I’ll never know. As said a bit of filing and I’ll run it again.

It’s been 2 years and approx 1200miles since I last had the head off. So it could have happened at anytime.

If I’m feeling bored on evening I might pop the valves out and look for evidence it’s escape...

Filing will remove metal.

What I do in these circumstances is to use a drift to hammer the edges flat, and then smooth with wet normal dry.