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Since I've had the truck in the shop to put on the PTO and winch I also cleaned up the engine bay to try and figure out where my persistent oil leak is coming from.

One spot is here at the oil pump on the outside bolt on cover. It doesn't appear to be leaking at the engine to pump gasket. Is this cover something I can remove and replace with a new gasket or is this a full assembly type of thing? I've made sure all the bolts on the cover are tight and I've still got some seeping.

Yes, you can remove just the cover, and replace the O-ring gasket.


Just be careful not to let the outside rotor fall out of the pump when the cover is removed, and replace the cover with the same side towards the pump internals.

As I recall (been awhile since I had an oil pump apart), the O-ring might be a square section ring. I don't remember the dash number off hand (and am away from my workshop today) but it's available from McMaster-Carr. Midwest Military also lists them as a separate item in their M37 engine section online.

Be sure to clean the inside face of the cover, the mating face of the pump body, and the O-ring groove in the pump body before reassembly.

Thanks, Elwood.

I should have checked my manual and parts list first. But I’m glad you’re around. You’ve saved my skin twice this month!

good advice, and something I try to do to make metal meeting surfaces as "flat" at possible is to remove high spots with a diamond stone. Sometimes an imperceptible "nick" will hinder a full contact surface. Easy to find this possibility with a diamond stone. Hal

Good point, NamVet. In suggesting the cleaning, what I should have really posted was to identify the cause of the leak, and fix it before reassembly. Just a bad O-ring, or something else, such as a nick in the oil pump body or cover plate that prevents the O-ring from being compressed as designed?

Elwood wrote: ↑Mon May 08, 2023 9:38 am Good point, NamVet. In suggesting the cleaning, what I should have really posted was to identify the cause of the leak, and fix it before reassembly. Just a bad O-ring, or something else, such as a nick in the oil pump body or cover plate that prevents the O-ring from being compressed as designed?

The cause of the leak is quite interesting...there was no seal in the pump when I took the cover off.

Did the old seal disintegrate? Probably not, there wasn't even a trace of old seal.
Did someone take the pump apart and put it back together without a seal? More likely. I may never know for sure, but it is quite odd.

The two surfaces must've been pretty darned flat for it to seal!

The fun never stops around here.

After installing a seal in the pump, now I've got next to 0 oil pressure.

To summarize the salient information:
1. Prior to adding the seal, the oil pressure gauge read 60psi all the time, unless the oil was low and then it would read around 30.
2. After the oil seal was installed, the gauge now reads slightly right of 0.
3. I am aware running the truck with the oil gauge pegged either way is not good practice.
4. There are no symptoms of low oil pressure: e.g. engine noise, burning smell, poor performance. I'll dig out a manual gauge and verify my readings soon.
5. Could there be a mismatch of oil sender and gauge? I have a 60 psi gauge in a 1963 M37B1. VPW advertises a "late style" 120psi gauge and sending unit in their catalog, but my understanding was this is for M35 and other multifuel/diesel rigs.
6. The outer pump rotor (CC-1314602 in Elwood's post) did fall out onto a paper towel, but I put it back in with the faces back the way they were.
7. Because there was no seal in the oil pump, I have to assume that other things in this truck are suspect.
8. I had power washed the engine prior to installing the oil pump seal, but the gauge still read 60 after that.

Anyway, that's what I am dealing with now.

Yes, I'd first verify the engine oil pressure using a known good mechanical gauge.

If the oil pressure gauge worked fine before, it's unlikely that the sender and gauge are mismatched.

Is the wiring to the oil pressure sender and gauge still good? Check to see that the sender and gauge are getting volts.

If you actually have low or no oil pressure, then it's possible that the installed square O-ring seal is too large in cross section, and is keeping the cover plate from seating against the pump body, allowing oil to escape around the rotors. Previously, with no O-ring, the cover plate would likely have been pressed flat against the pump body. The clearances (too much or too little) inside the pump are important.

When you reassembled the oil pump, did you use any silicone or RTV or other gasket maker on the mating faces in addition to the O-ring?

It's also possible that some bit of something was dislodged in the oil pump repair process, and has now jammed up the bypass valve or the sending unit.

No, no sealant was used when I reassembled. I torqued everything down as much as I dared and didn't notice any additional leaking afterwards.

Would the pump need reprimed after something like this? Oil did come out during the procedure.

I'll take a look and get some readings with the manual gauge tomorrow evening. I appreciate the help, Elwood.

Good on the no sealant.

Those are small fasteners, so not much torque is required. The O-ring does the work of sealing, not the clamping force.

With a dry oiling system, such as after an engine reassembly, I prefer to pre-lube before first start up (also recommended in the TM 9-1840A engine manual) by spinning the oil pump or using a pre-luber until pressure is visible on a mechanical gauge. But the first option requires removal of the distributor and a mandrel to drive the pump from a drill motor, so not the simplest process.

But it does sound like you might need to prime the gerotor oil pump. I'd start by removing the spark plugs, and spinning the engine with the starter motor until you (hopefully) see pressure on a mechanical oil pressure gauge.

I'd avoid starting the engine again until you sort out the oil pressure question.

A couple times over 50 years, I've seen the military 24V oil pressure gauge crap out.

I'm not advocating the change to a mechanical one, but that type of instrument is good for a test against the electric one.

PoW

I tried to prime the oil pump with the plugs out and the starter, but didn't see any changes.

I wasn't able to get my manual pump on there tonight. That's kind of a hard place to get wrenches in without either easing the generator out of the way or removing the distributor cap. I will have to meditate on that a little.

I did tighten down the cover of the oil pump a little more. I removed the shields from the bottom and got at it from the underside to give it a little more torque, they were still quite loose from where I got to it from the top side.

Friday and Saturday should provide me with more time to devote to this problem.

Success, in a way.

I was able to get the oil pump re-primed using petroleum jelly so I didn't have to take it off the truck and muck around with a pan of oil (not good when you have two barn cats that are very interested in shop work). Manual oil pressure gauge reads 35-40 at idle (hard to tell since the gauge itself goess to 600 psi).

So, I'm halfway there. Now I just need to understand why the electric gauge doesn't work and I'll be back in business.

PS: I found the old seal of the oil pump. It was a mess of shreds forced down into the groove of the pump. No wonder it wasn't sealing.