1978 250 Inline 6 cylinder oil pump issue Advise - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Kimble005 · 01-30-2021, 12:26 AM

So, I rebuilt my dads 1978 inline 250. Question is....why would the oil pump take so much force to spin? When I took it out of the box before assembly, it spun fine. Now its installed and when I initially primed the engine, the drill struggled with spinning it. And when I stopped spinning it, it spun backwards for a moment or 2. Now since break in is over, I figured it would loosen up, but it hasn't. It will actually almost stall the engine when 50 lbs is reached. Any thoughts or advise would be appreciated. Thanks in advance.

**hatzie** · 01-30-2021, 08:07 AM

Something is binding when the pump is bolted to the block.
Poorly machined and or cast part? Third world QC procedures.
You machine a part like this using a baseline reference plane. That plane becomes your zero surface and all other operations are setup in relation to that plane. If that surface is not properly machined the pump gears and shaft will be machined such that the plane of the gears and stub shaft are at the wrong angle when bolted to the block.
This could be from overlooking core shift in the castings or just plain sloppy machine work. IMHO it's sloppy workmanship to overlook core shift that essentially scraps the part before you take the first cut. Skeevy third world manufacturers often machine parts from castings that don't pass muster such that they will pass a cursory visual check. Then they ship em rather than reject poorly cast blanks. Hence my comments in other posts about riding these types of suppliers like a $2 horse.
The protruding oil pump shaft is contacting something in the block that it ain't supposed to because it was not properly aligned with the mounting holes?
The end play between the steel gears and the cover plate is too tight?
The hole for the steel shaft is not machined to the proper dimension so it's binding in the pump?

Kimble005 · 01-30-2021, 05:05 PM

Thats good info. Thank you. Im not sure whats happening exactly. Would you recommend just replacing the pump or is there a procedure to test those various examples you listed? Its not an easy process to drop the an on this engine due to the timing cover bolting/sealing against the oil pan. Any suggestions would help. Thanks again.

**Palf70Step** · 01-31-2021, 08:58 AM

I think you should replace the pump. As mentioned something is not right, and it needs to be looked at/pulled apart.

01-30-2021, 12:26 AM	#1
Kimble005 Registered User Join Date: May 2017 Location: River Falls, WI Posts: 4	1978 250 Inline 6 cylinder oil pump issue Advise So, I rebuilt my dads 1978 inline 250. Question is....why would the oil pump take so much force to spin? When I took it out of the box before assembly, it spun fine. Now its installed and when I initially primed the engine, the drill struggled with spinning it. And when I stopped spinning it, it spun backwards for a moment or 2. Now since break in is over, I figured it would loosen up, but it hasn't. It will actually almost stall the engine when 50 lbs is reached. Any thoughts or advise would be appreciated. Thanks in advance.

01-30-2021, 08:07 AM	#2
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: 1978 250 Inline 6 cylinder oil pump issue Advise Something is binding when the pump is bolted to the block. Poorly machined and or cast part? Third world QC procedures. You machine a part like this using a baseline reference plane. That plane becomes your zero surface and all other operations are setup in relation to that plane. If that surface is not properly machined the pump gears and shaft will be machined such that the plane of the gears and stub shaft are at the wrong angle when bolted to the block. This could be from overlooking core shift in the castings or just plain sloppy machine work. IMHO it's sloppy workmanship to overlook core shift that essentially scraps the part before you take the first cut. Skeevy third world manufacturers often machine parts from castings that don't pass muster such that they will pass a cursory visual check. Then they ship em rather than reject poorly cast blanks. Hence my comments in other posts about riding these types of suppliers like a $2 horse. The protruding oil pump shaft is contacting something in the block that it ain't supposed to because it was not properly aligned with the mounting holes? The end play between the steel gears and the cover plate is too tight? The hole for the steel shaft is not machined to the proper dimension so it's binding in the pump? __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

01-30-2021, 05:05 PM	#3
Kimble005 Registered User Join Date: May 2017 Location: River Falls, WI Posts: 4	Re: 1978 250 Inline 6 cylinder oil pump issue Advise Thats good info. Thank you. Im not sure whats happening exactly. Would you recommend just replacing the pump or is there a procedure to test those various examples you listed? Its not an easy process to drop the an on this engine due to the timing cover bolting/sealing against the oil pan. Any suggestions would help. Thanks again.

01-31-2021, 08:58 AM	#4
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 47,249	Re: 1978 250 Inline 6 cylinder oil pump issue Advise I think you should replace the pump. As mentioned something is not right, and it needs to be looked at/pulled apart. __________________ Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God!



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