1931 Pierce Eight Oil Pressure

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    Maybe a general discussion about the oil pressure to expect would help newer folks like me: I admit to pure panic when I started my new engine and it did not immediately jump to 30 PSI. A search of the Service Bulletins somewhat calmed me: 30 PSI warm at 40 MPH, maybe as little as 2 PSI at warm idle 30 weight oil.


    I apologize for not responding sooner, low oil pressure on a fresh engine always a concern. I would expect the engine to go up to over 20 psi fairly quickly when started cold. After it gets hot very low oil pressures are not necessarily a problem. My Packard after rebuild 30,000+ miles ago has always barely registered at hot idle, goes to 35 or so on the road. My ’74 Alfa has 170,00+ miles and it’s hot idle reading has always been so low it basically doesn’t register on the gauge and decades ago I confirmed with a separate mechanical gauge that it really was above the 2 psi that would light the low oil pressure warning light. Before it’s rebuild at 99,000 miles (with hardly any wear and no issues when torn down) I discovered a crankshaft oil gallery plug had blown out. After rebuild it hardly read any higher. That may be because often the new inserts (not relevant to P-A) can have more clearance than the originals. I theorize that bearing suppliers may be worried about liability exposure if a customer rebuilds an engine too tight and it seizes, which is much more likely than any failure from higher clearance.

    2 days ago I fired my ’35 Pierce up with an original gauge installed on the dash and was surprised it jumped to over 50 psi where the newer mechanical gauge I had been using mounted directly on the oil port never read above 40. I don’t think either of these gauges are paragons of accuracy.

    What is your gauge reading cold at low idle and higher rpm when it reaches a steady point?

    If it is reading zero that of course is a big immediate concern. If just reading low then there are a number of possibilities. The main thing is that oil is circulating, the oil pressure you read on the gauge is just an indicator that oil is circulating, and the ability of the bearings and pistons to ride the oil film without grinding directly on the sliding surfaces has basically nothing to do with the oil pressure reading as long as the oil reaches the bearing or cylinder wall. Oil pressure is an indicator that the oil is pumping and and reaching the places it needs to go. Note that many inexpensive low performance engines don’t even have an oil pump and rely only on oil splashing around.

    Low oil pressure can be from several sources besides the gauge. The oil pressure regulator could be set up with too low a spring pressure to bypass oil sooner at high rpm. This shouldn’t be an issue at idle as the spring should be preventing any oil from bypassing. Another is the oil pump is significantly worn. The oil pump intake screen could be plugged with debris.

    The most common cause of low oil pressure is excessive clearance in the main, con rod, and cam bearings, usually from wear. The whole oil system is like a sprinkler hose getting fed water with lots of little holes for the water to stream out of. If the holes -represented by the bearing clearances – are consistent and even then the all the holes will stream the same amount of water. If one of the holes gets a lot bigger than the others for some reason, then more water will flow to that hole and less will reach the other holes. If the wear in the bearings is reasonably consistent throughout then the lower pressure needed to get oil everywhere needed is not a big issue. If it is because of some major failure at some point then that is something different of course.


    In the 70s at the dealership we were told if the cars had 2PSI at warm idle they were fine. Ford trucks put back in gauges because everybody wanted an oil pressure gauge, it was nothing but a mechanical idiot light, it never moved hot, cold idle, highway.
    Jim L


    I really think it’s fine: new engine, etc. When I tried to adjust the oil pressure at idle and got little response from the gauge I wasn’t prepared for that. On cold start it does go to at least 20 psi, warm itvregisters 4-5 psi, and it’s not really roadable yet so I haven’t checked the “30 psi at 40 mph” standard as yet.


    Well, and anecdote is in order.  Long story maybe short, maybe longer.  Had an engine fire in my 1931 model 43.  Decided to re-restore it, since last restoration was done in 1960.

    Everything apart, decided to do engine, and a good friend of mine did a great job on it.

    Cut to the chase, it’s about 2 months before the 2001 Buffalo meet, and I’m desperate to get the car back together for that event.  Problem is, I’m working full time, and weeks spent out of state.

    I call a good friend who has a modest restoration shop.  He comes and looks at frame over there, fenders in the corner, body there, engine on a stand.  All painted and all the parts ready to go, just the time to put together.  So, he does, and a fine job, and I’m off to Buffalo.

    While there, oil pressure driving down the road no problem, but when engine warms up and I sat at idle, gauge is as close to zero as I can tell.  So, I put a slight rev on the engine, it jumps slightly to a few pounds (realizing the gauge is not so calibrated, but it at least jumps off the peg).

    Car does fine, I worry about low oil pressure, get home.  Talk to my friend who rebuilt the engine, he comes over, we drop the pan to check things out.  Whoever put the oil pump in cross threaded one or both of  the output fitting studs, so it didn’t tighten gasket  and seat, and so not all the pressure was going to the engine.

    We checked all the journals, no damage done, corrected problem.  Now, oil pressure at idle shows 5 or so PSI, and at speed varies with oil temperature.

    Whew, I feel like I dodged one on that…



    Thanks, Dave. I went so far as to remove the pan for a look; only a few tiny particles of white metal so good. I saw nothingvamiss. I removed the oil pump pipe ( fully gasketed, not split, etc) and oil pump. All seemed to be fine so I put it back together and replaced the oil with new.
    I even blocked off my rigged-up bypass filter ( with 1/16 inch orifice) which resulted in no noticable change in oil pressure. So far, I think it’s fine and I am inexperienced with Pierce engines.

    I think the next complex job might be duplicating the toe board!

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