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Allis Chalmers Discussion Forum

AC/B Oil Pump/Oil pressure

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05-22-2010 19:22:45

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I just rebuilt my AC/B. I have good oil pressure when first starts, but after about an hour of runtime..the oil pressure drops to the low end of normal. The pump is with in specs.and I replaced the springs..the vanes are a little worn on tips, but not bad...HELP..Thank You

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06-15-2010 13:49:32

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 Re: AC/B Oil Pump/Oil pressure in reply to xeagerx, 05-22-2010 19:22:45  
Thank all of you that replied..The sounds like eerything is OK..I was just used of newer engines ,where the system is pressurized...but it must do the job ....it worked for 60yrs..Thanks Again..xeagerx

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05-23-2010 20:00:52

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 Re: AC/B Oil Pump/Oil pressure in reply to xeagerx, 05-22-2010 19:22:45  
xegarx, the guys bring up a good point. If you dont know the system, the pump puts out 85% of the flow down the hollow cam shaft to the front end. Oil is sprayed out of holes toward the piston and connecting rod bearings.... the ohter 15% of the oil goes toward the filter. There is a hollow 1/4 inch tube inside the filter with a couple of small holes in the top of the tube. That 15% of the oil is filtered and dumped back to the sump as "clean" it is not used. this is called a bypass system... If the tube is gone inside the filter, or the two holes are too big, you will loose pressure as you might leak off 25% instead of 15% of the total. Since you have pressure when cold, we can assume the tube is inside the filter. Also a new filter will have less restriction than an older filter.. PSI will actually go up over a period of time.

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05-23-2010 15:43:39

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 Re: AC/B Oil Pump/Oil pressure in reply to xeagerx, 05-22-2010 19:22:45  
the drop in pressure is not due to the pump, it is due to clearances in the cam bearings and weight of the oil used as olds said. the thinner oil will leak past clearance easier and therfore loss of pressure. low range of the "normal" should be about 5 psi, and that is all some will put out. with a "new" motor, you should be able to hold 10 psi at 1000 rpm, when hot.

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05-23-2010 18:15:32

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 Re: AC/B Oil Pump/Oil pressure in reply to steve(ill), 05-23-2010 15:43:39  
Camshaft bearing clearences have nothing to do with oil pressure on a B. It is totally regulated by the oil filter & oil viscoscity. It is a bypass system. Cam is lubricated by oil running off of the rocker arms and oil that gets by the pressure relief valve.

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Brian Jasper co. Ia

05-23-2010 19:40:44

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 Re: AC/B Oil Pump/Oil pressure in reply to chalmersbob(pa), 05-23-2010 18:15:32  
Bob, worn cam bearings will cause low oil pressure. The oil pump fills the hollow cam first and then oil flows from the cam through the cam bearings to the mains. It's not just the filter that helps regulate pressure.

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05-24-2010 18:50:17

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 Re: AC/B Oil Pump/Oil pressure in reply to Brian Jasper co. Ia, 05-23-2010 19:40:44  
not on a B, C, CA. That system starts on the D14

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05-22-2010 20:14:58

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 Re: AC/B Oil Pump/Oil pressure in reply to xeagerx, 05-22-2010 19:22:45  
Pretty normal and hour run time oil gets hot and thin so pressure drops. If your running a muti-weight oil switch to a heavier one of go to a straight weight oil like a 30W or 40W

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