[patrickt]

I have a piece of plastic that has almost every size hole, and thread, and is designed for just that. You just see what size hole your bolt, or oilpan plug, fits in to and then you line the threads up against sample threads on the plastic and you know. It does both US standard, Metric, pipe, etc. It does not do British Whitworth threads. It cost about ten bucks. It does it the other way for nuts as well. It's easier to use that the little key set that came with all my taps and dies. I bet you can get it off Amazon.