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C allis chalmers oil pressure bypass

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Category: Allis Chalmers
Forum Name: Farm Equipment
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=186359
Printed Date: 01 Apr 2026 at 5:06pm
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Topic: C allis chalmers oil pressure bypass

Posted By: Monte Mahan
Subject: C allis chalmers oil pressure bypass
Date Posted: 10 Feb 2022 at 7:04pm

reading in my manual there is an oil pressure bypass thet limits the pressure to 15 pounds. It isn't in the oil pump, where is it?

Replies:

Posted By: steve(ill)
Date Posted: 10 Feb 2022 at 7:16pm

the pump pushes oil down a hollow cam shaft to spray on the internal parts... there is a 1/4 inch TUBE that comes out of the flywheel housing and goes up to the FILTER BASE.. The filter allows about 15% of the total pump output to go thru, then DUMPS TO SUMP.. About 85% of teh pump oil goes to lube things and does NOT go thru the filter first.

The filter has to be Packed Cotton type and RESTRICTS the oil flow thru it.. There is also a 1/8 inch orifice tube inside the filter housing that restricts the flow... If you leave something out and the RESTRICTION is reduced, too much flow will go thru the filter and starve the rest of the engine.

there is also a spring loaded PLUG at the front end of the hollow cam shaft that allows some oil to DUMP into the front gear train area... The backpressure from the spring and plug help to control the 15 psi pump output.

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Posted By: Monte Mahan
Date Posted: 11 Feb 2022 at 6:49am

Steve. Thank's so much for you're detailed explanation about the oil system. I have lost oil pressure. I'm guessing I either spun a main bearing or the bypass stuck open.

Posted By: juniorp53
Date Posted: 11 Feb 2022 at 8:26am

had pressure and lost it ....or never had it ? tell us more!

Posted By: Lon(MN)
Date Posted: 11 Feb 2022 at 8:36am

The oil pump sometimes will lose prime. Take the oil filter off, squirt oil in the upright tube. A little air pressure will help. With the filter off turn the engine or start it for a short time. Oil will come out the tube.

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Posted By: steve(ill)
Date Posted: 11 Feb 2022 at 8:40am

Did you change the filter ? Is the 1/4 inch tube with the 1/8 inch orifice up the middle of the filter? Take the filter off and crank the motor and see if any oil comes up out of the small tube..

At 1500 rpm do you have pressure, just not at idle ? Remove the oil pressure gauge and crank and see what you get out.. This engine will run with 5 psi of pressure.. 15 is GREAT, but it will run with less... Did the GAUGE fail ?

It is possible for the pump to loose prime. The 1/4 inch pipe that comes up out of the bell housing, you can open and pump a cup of oil down the tube with a squirt gun.. Its possible the motor sat too long and the oil drained out of the pump ?

Look at the oil on the dip stick ? Old oil ? Maybe time to change it out ? .. You can also drop the oil pan and inspect the suction screen.

My LAST choice would be bearing wear.

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Posted By: Monte Mahan
Date Posted: 11 Feb 2022 at 8:55am

Thanks for all the suggestion's. Yes, I did have oil pressure after the overhaul. I did pull oil filter to put seal ring under filter (Dumb mistake) after that, no oil pressure. Reading thru a bunch of posts, I think I have lost prime. I will check that ball in the rotor and use some plastigauge to check pump end clearance. the veins and springs look good. Is losing prime a normal ting when changing the filter, if so I wonder how many motors have been ruined by this.

Posted By: steve(ill)
Date Posted: 11 Feb 2022 at 9:05am

It is not common, but it does happen.. Normally after setting for months. Sometimes you can reprime, and it never happens again.

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Posted By: juniorp53
Date Posted: 11 Feb 2022 at 9:16am

those pistol grip oil can come in mighty handy for filling those lines with oil. the check ball is just a 5/16 ball bearing and i am sure you found all the posts about the oil filters. then there is that cork ends in the rocker shaft ...#5 sealing cork found in hardware section at lowes. its my understanding that if it keep loosing prime the check ball is not sealing but steve would know more about that then myself

Posted By: steve(ill)
Date Posted: 11 Feb 2022 at 9:20am

junior is right about the ball.. Make sure the SEAT is clean... Sometims setting the ball on the seat and bumping with a brass rod and hammer will "reseat" an old seat.

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Posted By: Monte Mahan
Date Posted: 11 Feb 2022 at 10:12am

Thanks every one who replied. I will check all the suggestions. watching video, my end play seams excessive, will check with dial indicator and probably go back with only 1 .004 gasket, my old one checks out to be .008. I ordered new gaskets, expensive little burgers.

Posted By: steve(ill)
Date Posted: 11 Feb 2022 at 12:24pm

YES... with excessive clearance around the pump vanes, you can drain back to sump.

Suppose to be as tight as possible without touching... .002- 4 would be acceptable.

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Like them all, but love the "B"s.

Posted By: WF owner
Date Posted: 11 Feb 2022 at 2:45pm

Also, make sure the stand-pipe that goes from the filter base up into the hole in the middle of the filter is in place. It's not uncommon for the pipe to come out when removing the old oil filter.