Another issue on the 55-59 gas tank I got. It too seems to maybe have metric threads on the location on top that you are suppose to install the sending unit. I think the 49-54 tanks had an issue with metric on the bottom plug mentione earlier in this thread.

I tried to install the sending unit (on top for 57) using the correct 10-32 1/2 pan head screws and the screws only went about 1 turn or so and then I had to use a slight bit of force to get the screws to continue threading. After removing the screw and trying again, as the original sending unit hit the tank lip and would not go down flush to the tank, I noticed the screw would jump down several threads acting like it was too small or stripped. All 5 screws are doing that as I screwed them all in together a bit at a time to get it seated.

I think this tanks sending unit holes may be M5 metric not 10-32 STD. I called the vender (classic Parts) and they are checking on it for me. But these 10-32 will not work as they appear to be too small a diameter for a secure fit.

Also I bought some M5 metric machine screws to test but they do not seem to fit either as the threads are probably a bit stripped in the tank now from trying to use the 10-32 screws!

I can tap it out I guess but I want to know what size they really are. I am starting to think they were made as M5. I compared M5 to 10-32 at the hardware store. The number of threads/inch is the same but the diameter is different I think. M5 seems slightly larger. As a result I think the thread pitch is flatter on M5. So slightly forcing the 10-32 probably messed up the screw and the threads in the tank.

I can take the new 10-32 screws and the old ones and both go into my old tank fine but both have issues trying to work in the new tank.