They should have a Metal nodual on the inside of the cap which when you push and turn the Oil cap they lock together...
Failing that and be carefull with this one...Stuuf some rag in the Oil filler top {Make sure you can retrieve it}, and drill the Oil Cap and top of The Oil filler on block and self Tap screw...i used to have to do this on my old TD as it had pressure problems and it was that bad it used to blow the Cap off!!!! :shock:
didnt move when drilled ..