Piston damaged

[sean]

any damage to cyl head ? did you check the bottom of the valve guides any broken pieces ?

[David Tinsley]

Surprisingly the cylinder head has no damage and the guides appear OK, but worn, so getting the head overhauled.

[olev]

Something fell down the plug hole?

[Jules]

very strange damage to that piston, its definitely had something bouncing around in there which has caused all the smaller indents, I dont understand why there is no indications on the head or valves though, that makes no sense - unless as Swarfy suggested the piston was reused from elsewhere.....it also looks on closeup as if there is "something" still stuck in the RH hole (maybe?), maybe the small indents were from another earlier usage and these 2 major failures have occurred in this application??
Certainly not reusable IMO.......!

[limeyrob]

I'm wondering if there was detonation and a weak mixture that softened the piston crown then a soft spot came out at TDC on due to the piston acceleration. Its a very long shot but it could explain why the edges of the damage are above the surface. The other explanation is like an amour piercing shell that leaves a raised edge which would imply debris but where from?

[berger]

i know , the mystery ball bearing from worty's engine has been hanging about with that piston

[mugwump]

Does the hole nearer the centre of the piston correspond with the spark plug electrode. Have you checked the plug for damage.

[David Tinsley]

I did wonder if it was related to the plug position, but it isn't. I fitted new plugs and chucked the originals away. The new plugs are still fine.
When I get a moment I am going to saw that piston in half across the damage and see if it gives us any more clues as to what happened.