Oil Pressure Switch thread size


What is the size (dia., thread count, type) for the oil pressure switch? I'm trying to determine the correct adapter to attach an oil pressure gauge.

The gauge hose has a 1/4 18 NPT male end. It came with an 1/4 18 NPT female to 1/8 27 NPT male adapter to fit the other adapters (1/8 27 NPTfemale to etc). Unfortunately none of the adapters are the same as the oil pressure switch (which seems to be a 1/2 20 UNF or possibly 1/4 20 NPT will work).

94' NAS 3.9 v8


Well-known member
I think I responded to you on the other site but for posterity I believe it's 1/2-20 UNF. I had the same issue and got part no. 5-1/8 F5OG-S from Hydradyne for my particular oil test gauge which had 1/8-27 NPT. Worked perfectly. Maybe you can find similar for 1/4 18 NPT. My only other suggestion would be to not think you can get away with using multiple adapters because it's a weirdly tight spot by the time the hose is connected.


I have a 1/8-27 npt male to 1/8-27 npt female 90° that should fit and work perfectly with the 5-1/8 F5OG-S.

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I ordered the 5-1/8 F5OG-S from


The gauge was installed using the adapters from my oil pressure gauge kit.

The engine oil is Castrol 03111C GTX High Mileage 10W-40 Synthetic Blend Motor Oil

The numbers:
33lbs @ idle
50lbs @ 2500 rpm
16lbs @ idle
28lbs @ 2000 rpm
30lbs @ 2500 rpm

These pressures sound reasonable. However, I hear a muffled rumble on acceleration. The frequency is that of the valve train. Moreover, at times there is a very brief knocking at start up. I'm assuming the oil is draining off.
I'm trying to determine what is causing this noise and if it can be resolved short of a (at the least) top end rebuild.

Note: I did not purge air from the oil pressure gauge hose.


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