Originally Posted by 2.5Turbo

The surface in the head that the tapered part of the prechamber seals against. Now that I think about it the prechambers have never been out (as far as I know) so it's probably unlikely they're bad. Don't have the splined socket. Anybody have experience with this one? It's reasonably priced and domestically available on Amazon but I'd rather pay more for a different one if it's garbage.

And yes, it is the car in the sig. The photo is at least 60k old. It needs a detail (or three) but it still looks decent and works (mostly).