Any idea what might have caused this damage?

[Jaydee]

5 minutes ago, TonyGee said:

how many times have you seen bits of crap round a spark plug ?

Over here in Ireland, the council use a very fine grit sprinkled over loose road chippings and pothole repair. It gets pulled up by front tyre and gets everywhere. I make sure the rubber grommets are in good order on the rocker cover. 

[imago]

2 hours ago, TonyGee said:

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  

Or one of the tiny ball bearings that blank drillings in carbs, or bits of metal dropped off of a rag etc etc.

The universally accepted engineering term for the ingress of foreign bodies being "some shit in [insert name of place shit shouldn't be]". 

[kitkatman]

These dimples  or marks in the head, how will this effect  the running of a stock motor, is it ok to leave them alone, or how do you fix it. ???

[Gixer1460]

Problem is, as you can't determine what's caused them, you can only guess if there is more damage elsewhere! The piston damage isn't significant enough to prevent use, but if it IS detonation, and it continues, things will only get worse. I'd check compressions / do a leak down test to check rings and valves, check carbs and plugs (correct type) - basically a good service and see what happens. To save ripping the head off again in the future un-necessarily, one of those cheap bore-a-scopes may be a good buy?

[imago]

3 hours ago, kitkatman said:

These dimples  or marks in the head, how will this effect  the running of a stock motor, is it ok to leave them alone, or how do you fix it. ???

If it were mine, they'll do no harm so box it up and good to go. It looks like old damage and the engine was built up again afterwards.

[kitkatman]

I like the suggestion of not removing the rubbish that gathers around the spark plug, and it falling inside when the plug is taken out. So in future its out with the airline  before whipping out the plugs.

[Jonny]

On 9/9/2022 at 5:01 PM, Gixer1460 said:

Problem is, as you can't determine what's caused them, you can only guess if there is more damage elsewhere! The piston damage isn't significant enough to prevent use, but if it IS detonation, and it continues, things will only get worse. I'd check compressions / do a leak down test to check rings and valves, check carbs and plugs (correct type) - basically a good service and see what happens. To save ripping the head off again in the future un-necessarily, one of those cheap bore-a-scopes may be a good buy?

Well the engine is back together now with a new head gasket (and base gasket) and I’ve cleaned everything up so we’ll see. When I was cleaning up the other pistons I noticed some very small but similar marks on two of the other pistons. Wasn’t visible until I cleaned the carbon off, so I’m going to discount a foreign object in the chamber as a cause now.