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[ed.lazda]

Had a good look at the circlip grooves. No sign of any damage.

[Chevy]

The round style circlips fitted to these pistons should fit into the groove just OVER half their depth, so any amount of pushing by the gudgeon pin can not dislodge the clip, only push it deeper. Check that the groove is deep enough overwise it will happen again. If this is all ok, it has probably not been fitted correctly on the last build.
Chevy

[ed.lazda]

Thanks Chevy. It's certainly possible that the groove is shallower due to a bit of carbon build up. I'll probably be putting in a new piston anyway, so hopefully that'll sort it.