What is the thread size on the back of oil pressure gauge??
Picture? Model/ brand? geez It is most likely either US pipe NPT IE "1/8 pipe" which is close to 3/8 fine thread or else it will take a 1/8 compression nut and ferrule which used to be "standard." See if there's a champer on the inside edge
This is what you used to almost universally see........a 1/8 compression fitting and 1/8 pipe adapter, except "on the gauge" the right hand side of this adapter would be female to screw onto the gauge
Picture? Model/ brand? geez It is most likely either US pipe NPT IE "1/8 pipe" which is close to 3/8 fine thread or else it will take a 1/8 compression nut and ferrule which used to be "standard." See if there's a champer on the inside edge
This is what you used to almost universally see........a 1/8 compression fitting and 1/8 pipe adapter, except "on the gauge" the right hand side of this adapter would be female to screw onto the gauge
