Any idea what might have caused this damage?

[Jonny]

If not clear I’m talking about the dimpling at the top above the valves. 

[wraith]

Hot spot?

[bluedog59]

Could be a bit of detonation, anything on the piston crown ?

[johnr]

is it on every cylinder?

[Jonny]

36 minutes ago, johnr said:

is it on every cylinder?

No just this one 

[coombehouse]

It looks like damage from something that has got into the cylinder at some time in the past. If the piston doesn't have matching damage, it's probably been fixed in the past.

[Jonny]

14 hours ago, bluedog59 said:

Could be a bit of detonation, anything on the piston crown ?

Yes there are some corresponding marks on the piston  

[bluedog59]

It looks like it has started to have a nibble at edge of the crown and I would guess that it's just started on it's way down to the rings.  Do the other cylinders show any damage in the same area or do they have a slight (normal) carbon build up ?

[Gixer1460]

Difficult to see but next piston seems blacker so either it's richer or this is leaner / timing too advanced? Seems focused on exhaust side - detonation usually seen all over unless particular hot spot!

[FJD]

A stone might have banged the dents in piston and head

[dupersunc]

I'm going to say detonation, if it was foreign object damage it wouldn't be localised to one part of the piston.

The exhaust side is usually where det starts. 

[Joseph]

14 minutes ago, FJD said:

A stone might have banged the dents in piston and head

That would mean running pretty big main jets in the carbs 

[george 1100]

It looks like detonation

[bluedog59]

7 minutes ago, FJD said:

A stone might have banged the dents in piston and head

I think that's very unlikely.

When the squish band is working correctly there is usually very little colour on the piston crown (as there's minimal mixture there to burn). 

Ignition timing ?  Is there any sign of damage on the corresponding piston in the crank throw ? The one which is 180deg to the damaged one.

Hot spot to promote a bit of detonation ? It is right next to the exhaust valves.

I would have a look at your squish clearance across the cylinders.

Is it a stock engine or has it had some work done ?

[coombehouse]

It looks like there might be a few similar marks on the other side of the piston too. There are corresponding marks on the head. Hard to see clearly on a phone.  I have seen similar when a spark plug fell apart but the damage was much more jagged.  

Edited September 4, 2022 by coombehouse
More info

[dupersunc]

29 minutes ago, coombehouse said:

It looks like there might be a few similar marks on the other side of the piston too. There are corresponding marks on the head. Hard to see clearly on a phone.  I have seen similar when a spark plug fell apart but the damage was much more jagged.  

Good spot. The marks on the inlet side do look more like FOD.

[Jonny]

1 hour ago, bluedog59 said:

I think that's very unlikely.

When the squish band is working correctly there is usually very little colour on the piston crown (as there's minimal mixture there to burn). 

Ignition timing ?  Is there any sign of damage on the corresponding piston in the crank throw ? The one which is 180deg to the damaged one.

Hot spot to promote a bit of detonation ? It is right next to the exhaust valves.

I would have a look at your squish clearance across the cylinders.

Is it a stock engine or has it had some work done ?

All the other pistons and parts of the head seem fine. The engine is a 1230cc so not stock.  There is far more carbon build up on the other pistons. Might be worth saying that the reason the engine is in bits is I had a leaky head gasket and that is the cylinder that oil was leaking into. 

[Jonny]

45 minutes ago, coombehouse said:

It looks like there might be a few similar marks on the other side of the piston too. There are corresponding marks on the head. Hard to see clearly on a phone.  I have seen similar when a spark plug fell apart but the damage was much more jagged.  

Yes there are marks on both sides although worse on the exhaust side. The piston shows matching damage but much less

Edited September 4, 2022 by Jonny

[bluedog59]

Ah, big bore kit upping the compression ratio then a bit of oil being drawn in raising it a bit more (oil being non compressible) and bingo, detonation !

[bitzz]

What does the valve seat look like?

[Gixer1460]

8 hours ago, bluedog59 said:

When the squish band is working correctly there is usually very little colour on the piston crown (as there's minimal mixture there to burn). 

Eh? Squish is used to force mixture into the centre of the chamber so that is where the best burn occurs.

Edited September 4, 2022 by Gixer1460
Spelling

[bluedog59]

22 minutes ago, Gixer1460 said:

Eh? Swish is used to force mixture into the centre of the chamber so that is where the best burn occurs.

Sorry, I thought it was obvious that I was referring to the squish band area of the piston crown. A case of knowing what I meant but not wording it clearly. 

[Jonny]

5 hours ago, bluedog59 said:

Ah, big bore kit upping the compression ratio then a bit of oil being drawn in raising it a bit more (oil being non compressible) and bingo, detonation !

Thanks! That’s good to know! 

[imago]

That doesn't look like detonation damage to me, the dimples are too deep, small, and uniform in shape. It looks like some bits have been drawn in or left in. Something small and metallic and hard I'd say. Too round for ring bits, but it could be anything really.

The other thing that makes me say material in there is that nearly all the damage is on the low side of the piston. When it's all closed up and in the frame anything in the combustion chamber drops to that side. It's where you see the rust if an engine's been sitting with water in the bore for example.

[TonyGee]

It could be a tiny stone or something !!!!!  how many times have you seen bits of crap round a spark plug ? and when you take it out the crap falls in to the combustion chamber