What is this on the top of my pistons?

[cat007]

All 6 have it:



It looks like a crack that goes all the way around the piston in an even circle. But I can't feel it with my nail. I thought it was some sort of scratch or mark, maybe from sitting on a shelf on the face of the piston, but I can't rub it out.

Never seen it before....

[Soiled]

Casting mark?

[cat007]

Casting mark?

I guess so. I'll try and take a better pic. Just never seen it before

[atmosports]

Definitely a casting mark, possibly from where there is a split in the pattern to allow them to use the pattern for other piston domes etc.