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Any idea what might have caused this damage?


Jonny

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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 ?

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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.

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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. 3BCC7032-942C-457B-BA42-C1968FEE2881.thumb.jpeg.05b05ce8eb9494d0f5717be687516eca.jpeg

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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 by Jonny
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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 by Gixer1460
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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.

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