There must be some kind of jokey link between girlfriend and the item in question.
Mark’s “keeper” explanation satisfies my cryptic crossword clue rule, which is that correct answers go “ding ding ding”. Mark gets two dings because a girlfriend and a collet can both be “keepers”. His third ding is that a valve-collet recognising girlfriend would be the perfect mate for a petrol-head. Seems too good an answer to be a coincidence, and if there’s another amusing link I can’t see it.
Keeper and collet in technical English both date from the early 16th century. They have much the same meanings except ‘collet’ comes from the jewellery trade and is French, whilst ‘keeper’ is anglo-saxon and associated with big engineering. I guess machinists prefer collet because this particular bit of jargon was inherited from precision work – jewellers and clockmakers – rather than wheel-wrights. As engines span both worlds I’m not surprised that a “valve-collet” is also called a “keeper”.
Dave