M925A2 Mechanical Engine Oil Pressure Gauge, Swapped In

Having had enough of the low resolution and inaccurate electric engine oil pressure gauge, I purchased and swapped in a mechanical engine oil pressure gauge. This is a military part, spec'd for Willy's Jeeps and such, but this one has the windows in the case for lighting (M35A2 style?). Bought it on eBay for a too expensive price of $70 or such, but needed to mis-spell the search as "military oil pressure guage" to find it (WildChild467 found it for me through his expertise, or mis-spelling ability). It was NOS in the original unopened package. You can find Chinese versions with no lighting or bulb-in-case lighting for $20 or so.

Had a Teflon lined, stainless steel braided, high pressure hydraulic hose made up for it, rated to 3000psi.

Works well, not sure about the night lighting yet, especially since all the other gauges have a full clear case vs this one has just windows, but even if dim, I'll accept the trade off for the vast improvement in resolution and accuracy. Now I just need to paint the bezel to match.

For anybody considering, the engine oil pressure port on the Cummins 8.3L block is 1/8" NPT female, with an adapter to 1/4" female threaded into it. The pressure shown below is at idle and about 60 seconds after a cold start (65F ambient), with Shell Rotella Diesel 15W-40 oil. Pedal to floor (2300rpm) at same condition drives the gauge to about 80psi, but not much over. Of course hot engine @ idle will be significantly less, but this is actually the area I was interested in "seeing", of which the electric gauge did completely nothing for, it simply showed a needle-width above 0psi. I asked for a mechanical gauge to be on the engine for each of the two M925A2's I looked at before buying one, because such a low pressure seemed concerning, but I noticed it was common on both. Even after replacing the sender, no improvement in low range resolution. All good now:

That is a good modification. I use mechanical gauges for both temperature and oil pressure in all my vehicles. (WYSIWYG) What you see is what you got.

dmetalmiki said:

That is a good modification. I use mechanical gauges for both temperature and oil pressure in all my vehicles. (WYSIWYG) What you see is what you got.

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Well put on WYSIWYG. Are you using a mil spec mechanical temp gauge? If so, what was it's original application, and how is the night lighting setup on it? Pic would be fabulous, thanks. I'd like to swap those next.

Update to report that the "window" type lighting seems to be about the same intensity as the full clear case type, I was surprised. It illuminates same as the rest of the gauges, I'm excited.

The resolution is so good that the needle dips 5psi at each and every trans shift point when accelerating. Once engine is warm, the oil pressure gauge pretty much follows the tachometer on acceleration, using nearly its full sweep. Much different than what the truck came with. I could not be happier, should have done this the day I bought the truck. Now just need to find mechanical temp gauges with the same lighting scheme. Engine oil pressure gauge is second in from the left on the bottom row:

I have been wanting to do the same thing, it is great to see someone else do the RnD... thank you... $70.00 sounds high for a gauge but if you think about the price of a new electric pressure sending unit that won't work any better then what we have now ( I have swapped 2 sending units still has all the same symptoms listed above, one was NOS dated 1968 and the other was new production) it is not to bad, Thank you again..

I put in a mechanical gauge as well

I installed a mechanical oil pressure gauge as well. I "t"ed it of the sender and installed it on the side of the of the shifter tower. Both show pretty much the same oil pressure which seems high for a 30+ year old engine, (90-100) at old idle. I tied light circuit into gauge lights. I like having 2 types of oil pressure gauges.

so is it common to see the 923A2 trucks which have the factory oil pressure gages reading very low?


ive been shopping for a truck and see that most listen on gov planet have very low oil pressure at idle, most seem to be 15-25 or so psi at idle, this is a little spooky to me when considering buying a truck

Absolutely yes. My gauge showed barely up off the pin when at an idle. This was both as-received, and after replacing the sender and gauge with new, didn't matter. Threw it all away and went with this mechanical one that has the original type lighting (through-case). Is hard to find and expensive, but worth it if this value is important to you.

Same as you, I was nervous about what I saw when I first looked at these trucks regarding reported oil pressure. I looked at two 925A2's before I bought the second one, and I requested each seller to have a mechanical gauge on the engine when I arrived, which showed perfectly well. It is amazing the difference the mechanical gauge makes, you can actually have a real sense of what is going on inside. When I accelerate now, the oil pressure gauge sweeps up just like a boost gauge or tachometer would.

74M35A2 said:

Update to report that the "window" type lighting seems to be about the same intensity as the full clear case type, I was surprised. It illuminates same as the rest of the gauges, I'm excited.

The resolution is so good that the needle dips 5psi at each and every trans shift point when accelerating. Once engine is warm, the oil pressure gauge pretty much follows the tachometer on acceleration, using nearly its full sweep. Much different than what the truck came with. I could not be happier, should have done this the day I bought the truck. Now just need to find mechanical temp gauges with the same lighting scheme. Engine oil pressure gauge is second in from the left on the bottom row:

View attachment 577928

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Cool mod. kinda like a tach. for your oil pressure, will be looking into this myself-- the willy's guage look right at home in your dash.

74M35A2 said:

Absolutely yes. My gauge showed barely up off the pin when at an idle. This was both as-received, and after replacing the sender and gauge with new, didn't matter. Threw it all away and went with this mechanical one that has the original type lighting (through-case). Is hard to find and expensive, but worth it if this value is important to you.

Same as you, I was nervous about what I saw when I first looked at these trucks regarding reported oil pressure. I looked at two 925A2's before I bought the second one, and I requested each seller to have a mechanical gauge on the engine when I arrived, which showed perfectly well. It is amazing the difference the mechanical gauge makes, you can actually have a real sense of what is going on inside. When I accelerate now, the oil pressure gauge sweeps up just like a boost gauge or tachometer would.

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ALL of this and, at 400-600 rpm idle... 10-20psi is still acceptable. General rule of thumb is 10psi per 1000rpm is good.

Tornadogt said:

ALL of this and, at 400-600 rpm idle... 10-20psi is still acceptable. General rule of thumb is 10psi per 1000rpm is good.

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Yep yep. Just like a Chevy 350....

To anybody who pursues this particular gauge, it is a direct fit and use, but the bezel is the darker green as you can see in my pic. Try to paint the bezel the correct OD green before you install it.