The extension of the prechamber tip beyond the surface of the head is spec'd to be 7.6 - 8.1 mm. Spacers are available in 0.3, 0.6 and 1.0 mm... uhh... denominations. My guess is that the gap between the prechamber and the piston influences spray and propagation. If it's just for clearance, 8 mm is a canyon.

I don't recall being able to move the pistons but I didn't try.

I don't know what seam you're talking about where you're able to suck oil. Maybe it's not a seam but an oil passage filled in with gasket remnants?

Sixto
95 S420
87 300SDL