1931 Pierce Eight Oil Pressure

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.