Farmall B - Very low oil pressure when hot

Farmall B & BN Tractors, 1939-1947
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Farmall B - Very low oil pressure when hot

Postby Chadly-MD-B » Mon Jun 20, 2022 12:29 pm

Hi All,
I just picked up a Farmall B with Woods mower I plan to use to mow a few acres. Motor sounds great, fires right up, oil pressure is awesome... Oil pressure is awesome that is until it warms up. Once warm the needle drops way down.
On the IH gage going from 0 - 75 PSI the gage reads about 35. When engine is hot, 30 minutes of mowing (and its 95 degrees outside today) it reads as low as 5-8psi I think.
Would anyone suggest trying a heavier viscosity or is this nothing to worry about?
Sorry if this is already covered. New and learning the search...

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Re: Farmall B - Very low oil pressure when hot

Postby Urbish » Mon Jun 20, 2022 1:25 pm

I would drain the oil and refill with straight 30W if you are only planning to use in warm months. 10W30 if you plan to use it in the winter too. Don't forget to drain the oil filter housing and make sure the system primes at startup. It would be a fairly inexpensive way to see if somebody was running overly thin oil in it.
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Re: Farmall B - Very low oil pressure when hot

Postby Jim Becker » Mon Jun 20, 2022 1:46 pm

I would consider dropping the oil pan to get the oil pump out then check the oil pump for excessive clearance and/or a warped cover. Look here:
viewtopic.php?f=177&t=110045

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Re: Farmall B - Very low oil pressure when hot

Postby Chadly-MD-B » Mon Jun 20, 2022 7:32 pm

Thank you all. Good advice!

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Re: Farmall B - Very low oil pressure when hot

Postby Chadly-MD-B » Tue Jun 28, 2022 11:01 pm

Hi All, Just thought I'd post an update. I changed oil with straight 30w. Made a huge difference. Pressure still drops when warm but no where near where it was dropping. This tractor is only used in the summer so I'll probably just keep going with straight 30w. Thank you again.

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Re: Farmall B - Very low oil pressure when hot

Postby SamsFarm » Wed Jun 29, 2022 10:49 am

if they only had a hour meter......

Might be scary though! Lol
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Re: Farmall B - Very low oil pressure when hot

Postby BullDAWG » Wed Jun 29, 2022 11:24 am

Jim had great advice but along with the pump, the pressure relief spring may be weak. For about $10 I'd swap it out as I don't think any metal springs after 70+ years is going to be as good as it once was. Plus if I remember correctly it's in the block near oil filter so it should be an easy swap.
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Re: Farmall B - Very low oil pressure when hot

Postby Jim Becker » Wed Jun 29, 2022 1:03 pm

The pressure relief spring is inside the oil pump. You access it through the same cover I mentioned earlier as possibly warped. Since it is holding 35# under some conditions, I don't think the spring is a problem. If you want to test it, it should support 26# when compressed to 1-5/8 inches.


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