For what it's worth, on my recent rebuild I had 2 pre-chambers wanting to keep falling out during re-assembly, even though I had to wail on them pretty good with a hammer and punch to get them out when I was taking it apart. I peened them to hold them in place. Just my opinion but I would think once you get the head back on there's little chance of them coming out as about half their diameter is mashed down against the head gasket. Seems to me they'd have to break apart to come out.
