I was gonna do a compression test on the vq35de, but the tester kit didn't have a 14mm thread adapter, so I used the one from my Harbor Freight leak down tester kit. It would not thread and that seems to be because the unthreaded part is quite wide. Does anybody know the make and model of an adapter that fits this engine?

Probably won't matter as you generally cannot get the adapter by itself, the gauge has to be bought to get them or make them yourself, which I've done. The inside threads to match to gauges are in no way all the same and what they do.

I've seen long 14mm thread adapters for sale as part of a set. Much cheaper than the whole gauge kit. What I'm trying to find out is of anyone here had any experience with them. Doesn't sound like you do. Making one is an idea, but I'm pressed for time.

Different gauges need different sets, they commonly do not use the same fixture on the gauge unless it is a more expensive Milton type quik coupler. I've got like 3 testers and none of the fittings for each fit the other gauges. The Milton type because of the break across it tends to leak and then your numbers are spit only.

What you are trying to say is that you need adaptors to fit YOUR specific gauge. There really is no industry standard there. If the end of gauge ends in a tire valve stem size you may need that one, I forget the exact size but it's on the net.