'46 John Deere D oil pressure mystery ....

Crazy Horse

Well-known Member
I am posting this for a friend who is stumped after doing an engine rebuild on a '46 John Deere D. He is
an experienced mechanic who in his life has rebuilt pretty much every engine type on the planet (including
many, many John Deere two cylinders), along with associated transmissions, differentials, steering
systems, and electrical components so he's not a rookie by any means.

To make a long question short, no matter what he does (and redoes over and over again through testing) he
cannot get more than 3 pounds of oil pressure after the rebuild. Everything has been rebuilt and tested,
including all the oil lines and connections, oil pump, filter canister, etc. The engine was a full
rebuild as mentioned, several oil pressure gauges were used which he knows work properly. He hates to
admit it but he is totally stumped and thought perhaps someone on this forum might be able to make a
suggestion that he might have overlooked. If you reply with an idea, I will pass it along to him and
reply back for him if that works for you. I await replies and thanks in advance ...
 
Pressure is caused by resistance to flow. Either the pump flow is low, or bearing clearances are greater than they should be which lowers the resistance, or there's a leak in the lube system.
 
Has he checked the oil filter can and it's top casting which many have a bypass relief in them. Should also be an adjustment screw somewhere in the system too.
 
I had a similar situation with mine and it turned out to be the line was clogged. Disconnect the small line that goes to the gauge. Blow it out but my problem was from the top of the block back to the oil canister head. Blow back thru that line with lots of air. Could be the culprit .
 
I'll second the call for shop air into the 1/8" pipe fitting at the governor case for oil gauge. It seems to show where any massive leak would be in the fastest manner. I'll also suggest he take out any gaskets in the oil pump body and use lapping to achieve .002 side clearance on the oil pump gears themselves as a last resort. Not being a rookie is a very good thing, but being a paid professional in a shop with other wrench twisters is way better because thru conversations with others in the same trade one learns of using Plastigauge for .002 clearance measurements for example where the lone wolf while quite good and doing quality work didn't ever get to know about plastigauge's existence in the first place. Parts stores carry it and it's quite reasonable in price. Or it could be he doesn't know about lapping with wet/dry sandpaper on thick plate glass, or...?
 

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