I bought new Auto Meter guages and went to put the temp sensor bushing in and it won't screw into the head. The bushing is 3/8" for an electric guage. The engine came out of a 1955 Bel Air and from the research I did it came with a mechanical temp guage. I don't know if the mechanical bushing had different threads than the electric? I've went to the auto store and hardware stores looking at different bushings and nothing will fit. Hopefully someone can help me because I have no Idea what to do next ???

Thanks for any help,
Greg

I believe what you have is a head with a straight cut thread and your new bushing is a pipe thread (NPT) which is a tapered thread, they won't work together. One solution is to retap the head to the new bushing size.

 

S10 racer, can this be done with the head on the engine? Also is there a plug i could get with straight cut threads so I could block off the hole?

 

S10 racer, can this be done with the head on the engine? Also is there a plug i could get with straight cut threads so I could block off the hole?

A picture of the hole would help. I'm just guessing that what you have is straight threads. Normally straight threads are not used for sealing connections, they are used for mechanical connections. Nuts and bolts have straight threads, anything that needs sealed uses thread such as NPT or O-Ring.
If there is room to work in the engine bay you should be able to re-tap the existing hole for a NPT threaded bushing. It all depends on what size of hole that is in the head now as to what size you need to tap. A 3/8" NPT requires a 9/16" hole, a 1/2" NPT requires a 45/64 hole.
Most straight threaded plugs use an O-Ring for sealing and without a machined surface around the hole, it will not seal. The AN type fittings are straight but then again, they need a machined surface to seal against. Your best and fastest solution is to tap the existing hole in the head to NPT even if you have to drill it out to the next larger size. Someone else may have a better solution so stay tuned for more.