I have recently fallen foul of this issue myself. Anyone remember the recent crash of the lorry on the M25, and the subsequent 29 mile tailbacks everywhere? Yeah, I do, vividly
I was inching along on a very hot day for miles and on one of the uphill section it went pop and started steaming. So I pulled over and checked the engine bay. The valve cap was nowhere to be seen and the coolant had ****ed all over the hot pipes, hence the steam.
As I am not very good with cars, I called the RAC and the guy temporarily fixed it by screwing a self-tapper into the remainder of the plastic. This worked really well and lasted me until today when I got a metal valve cap fitted by Adrian at AlfaAid (Maidenhead).
For anyone worried about the difference in thread pitch, yes there is a difference, but as long as the metal thread in the hole hasn't been damaged, just running an M10 x 1 tap down it first will allow the metal cap to screw in tightly
Shouldn't need to buy a new pipe or anything.
Job all done in 30 mins, no other parts needed, all in all, relieved