From memory I think the stock Chevy manifold and later heads have a 3/8" NPT, I'll try to confirm that and also post a picture of mine. I do have some '70's sbc heads and Q-Jet manifolds I can look at for comparison. The thermostat housing with the threaded port is not used for the temperature gauge, that is a later housing and the port would be used for a temperature switch used in some spark/emissions equipment.

Last edited by 78buckshot; 05/14/2024 9:34 AM.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy