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Here is the piston just taken out of the Min engine, originally caused by servo failure with more damage at Goodwood Sprint mid 80s from a blown head gasket between 3 & 4, actually fully failed in 1999 on way back from Ireland. Pistons were Powermax forged ones

quick buff up with some Autosol and that will go again

Forgive my ignorance, but how does a servo failing lead to this? The gasket failure however...

A torn servo diaphragm could lead to a huge air leak straight into the manifold and an extreme lean condition

ahh of course, didn't think of that! Thanks

brake fluid burns at twice the heat of petrol, so it must have sucking brake fluid into the manifold for a day or so, then I disconnected the servo to compete at Goodwood, the headgasket failed between 3 & 4 cylinders (at the chicane) leading to massive pinking which i could hear through the helmet, i kept my foot hard down & won the class beating Paul Chudeki in his Golf GTI in the process, the only time it came back on a trailer.
When I took the head off you could see the no.4 forged pistonwas damaged. it took a further 15 years to really crack it.
The rings came out without damaging the head or bore , it used 5 gallons of oil to drive the 200 miles back from Galway to the Midlands.