2-71 Detroit Diesel Low Oil Pressure – Solved!

Many of you may be familiar with my repowering a Farmall 400 with a 2-71 Detroit Diesel engine modified for the installation. A concern I had with the engine since the initial installation was that the oil pressure appeared to be too low dropping to 20 psi or less at full speed and fully warm with 40 weight oil. In searching the internet I found various people at sites discussing a similar problem and a variety of comments and suggestions including that “Detroits” often have low oil pressure and that this was acceptable to a degree. You can find discussions of 2-71 low oil pressure at the Allis Chalmers forum, Smokstak, and Yesterday’s Tractors internet sites.

My operating manual indicated normal oil pressure to be:

  • 30-55 psi @1800 rpm
  • 20-45 psi @1200 rpm
  • 5-12 psi @Idle

Minimum safe operating pressures were called out at 25,18 and 5 psi @ 1800, 1200, and idle respectively. When the engine was originally built the standard oil was 30 weight, and with the 40 weight recommendation in later years one would think even higher levels of pressure would be achieved. With my engine running below this I was concerned.

A look at the Emerson Matkin’s specifications ( a major refurbisher of 2-71 engine generators). It shows a minimum oil pressure of 18 psi @1200 rpm, which is in agreement with the previous mentioned specifications.

My first efforts to resolve this was to look at the troubleshooting tree in the manual to look for possibilities. I checked my gauge and even moved my measurement to a point nearer the pump. I pulled the oil pan and carefully inspected for missing plug in passages and found none. My 40 weight oil was new and undiluted. I took apart the oil pump, found some wear on the cover and lapped it out. While apart, I lapped the housing face to set the gear end plate clearance to a minimum. I replaced the regulator spring, assured myself the regulator valve moved smoothly and was could not stick open and added extra shims to the spring to raise the operating pressure. The effect of all of these was minimal.

When the engine was rebuilt, the block was “tanked” at NAPA and I had new cam bearings installed. I had rebuilt the engine with a reground crank and installed the appropriate undersized bearings. A check of clearance with plastigauge showed clearance to be within spec. I rechecked the rods to make sure the orifice in both rods rifle drilling were in place. Nothing stood out as an issue.

Some internet advise indicated the loss of oil pressure was most likely from the crank and rod journal clearance. I took the “old timer approach” of shimming both the rod and main bearing tighter with “Dechwar Tapered Shims” (http://deckwartaperedshims.com) to reduce the main and rod bearing clearances to a degree. This resulted in only a small incremental change in warm oil pressure.

I next concluded that perhaps the crank was somehow reground wrong. I had another 2-71 power unit that had good fully warm oil pressure that needed a rebuild. I tore this engine down and found a perfect standard size crank with bearings with minimal wear. I removed my engine from the tractor, and replaced it’s crank with the other engine’s and use 0.002 undersized rod and main bearings resulting in bearing clearances near the 0.0015 inch minimum spec. I also used the oil pump from the donner engine that had good pressure. The end result was only incremental improvement – leaving me baffled.

Finally – The Solution by Accident

In doing some consulting work I needed an example of a clinched bushing, one where the ends are connected with a dovetail kind of joint. Recalling having seen that type of bushing in cam bearings I got out the original cam bearings that NAPA removed. In looking at the bushings I found they were stamped “.030” indicating undersized bearings. They had installed standard size bearings I had provided not realizing the cam and balance shaft were ground undersized. The result was obviously to have extremely high clearance on the 3 cam and 2 balance shaft bearings.

Checking a 1964 Detroit Diesel 2-71 parts book I found they listed 3 cam bearing sizes, standard. 0.015″ under and 0.030″ under, part numbers 5194609, 5194610 and 5194611 respectively. They were listed as 1.5″ wide bearings. Further investigations revealed that there were 3 other part numbers for 2-71 cam bearings 5192804, 5192763 and 5192765 for standard, 0.015″ under and 0.030″ under bearings respectively that were listed as being 1.125″ wide. These were used in earlier engines (like mine) with serial numbers prior to 2A-37623. There was also a listing that even earlier engines, prior to serial number 2A-5793 had another bearings, part number 5155135 (standard size). I have no information on that bearing, but speculate it may have been one needing sizing after installation.

I looked at another set of parts from another engine core I had acquired and found it had 0.015″ under bearings that were 1.125″ wide and had stamped part numbers corresponding to what I stated earlier. Below is a picture of two bearings showing the different widths and the undersize stamping.

Bearing Size Comparison , 1.125 " and 1.5

Once the bearings were replaced, I decided to increase the main and rod bearing clearance as the oil pressure seemed extremely high reaching approximately 95 psi at the pump. I replace one of the 0.002 undersized main and rod bearing shells with standard shells to add another 0.001 inch clearance. This resulted in bearing clearances of about 0.0025 inch. The resulting oil pressure as a function of engine speed is shown in the plot below.

The oil pressure was measured at 2 location. One at the taped hole at the upper left rear of the engine block by the governor. This location is off an annulus around the outside of the rear cam bearing and is furthest from the pump. I believe this is the standard location for pressure measurement in the Detroit Diesel specification. The other measurement is at a tap I added off the oil bypass housing to the right of the fuel pump. This pressure is near the pump outlet and before the oil cooler. The cool curves represent data taken immediately after a cold start at approximately 70 degree F temperature. The hot curves were measured with the engine fully warmed up. Problem solved!