The problem has been not only the tap size but to match the sending unit with the proper size tap and the proper output(ohms). Last night, I was reading a post on the highperformance pontiac page. There was a discussion and a fellow had information on sending unit outputs. He did have a recomendation on a unit. I ordered one today from NAPA. I will get it tommorrow, test it and report back. It has 3/8" base and the ohms are faily close at low temp and very very close at the 200-220.