Reproduction Parts for 1916-1964 Chevrolet Passenger Cars & 1918-1987 Chevrolet & GMC Trucks
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Several questions involved in this blurb. I have a 1939 GMC Panel Delivery Van with an original in-line 6, 228 CID motor. I was experiencing some oil pressure problems, so i dropped the oil pan to get access to the pump. The pan had at least a solid 1/4" layer of old sludge on the entire bottom surface and the bottom of the baffle plates. The oil pump suction screen was at least 75% plugged with the same type of sludge that had solidified and it did not clean up until after it had soaked in gasoline for 2 days. I have removed the valve cover gasket and i intend to flush as much of the oil system as i can by spraying choke and carb cleaner throughout the valve spring area and any passages i see and let the cleaner just gravity feed through the crankcase and run into a pan on the garage floor.....then re-assemble.
Here are my questions....
1. I was told by someone that GMC may have installed some oil pumps where the discharge pressure was adjustable. Has anyone else heard of this? on the side of mine, there is a large acorn type of nut that may cover an internal pressure adjusting screw....i do not know but while i have the pan off i was going to investigate this question.
2. I was told that this year vehicle should always use a lubricating oil with a high ZINC content. As i investigate this at the auto parts store, they quickly throw out the term break-in oil of which there are several types and weights and even synthetic oils. What should i be using? There is no filter in this system and i will be using advice i received on this forum and i will simply be changing my oil every thousand miles.
3. The previous owner installed new instrumentation and he got rid of the oil pressure gauge that actually had tubing run to it and replaced it with an electronic sending unit that sends a milivolt signal that moves a needle to represent PSI.......is this reliable or should i use an old direct reading pressure gauge as a back-up?
As usual, any and all advice is appreciated......i don't want to make mistakes on the lubrication system, it is critical to the life and performance of the motor.
Greg
Hi Greg
1. I'm no GMC expert, but I can't find any evidence in my parts books or otherwise that the oil pressure was ever adjustable on GMC 228 or 248 engines.
2. You will find a lot of conflicting information on zinc in oil.
Zinc is an anti-wear additive aimed mainly at preventing wear on heavily loaded valve train parts like camshafts with flat tappets in some engines.
The valve train in a 228 is not heavily loaded.
And, there was no zinc in the oils sold when your vehicle was new, and very little to no zinc was in oils commonly used in gasoline engines for many many years after that.
The fact that your engine still has a running camshaft and tappets is proof that you don't NEED zinc in your oil.
All that said, all high quality oils now sold do have zinc in them, some more some less.
Zinc IS NOT the whole story, and must be balanced with other additives such as detergents as they are both polar molecules and actually compete for space on metal surfaces.
The stuff in the bottom of your oil pan and plugging your pump screen is showing you how important detergents are in modern oils.
And for that reason, do NOT use break in oil long term as it usually sacrifices detergency for anti-wear properties and is only meant to be used very short term.
Just find a quality API SP oil of a viscosity appropriate to temperatures you will be driving in and you will be fine.
I wouldn't however go any heavier that 10W-30 (and you will notice that most high zinc oils are heavier or much heavier than that and really not suitable on that basis alone).
Here is a very good video by an actual tribologist that will explain all this better than I can:
https://www.youtube.com/watch?v=erxjPicpYyw
3. I'm kind of old school so I am biased toward mechanical gauges fed with a real oil line.
Electric is probably just as good without the risk of a line springing a leak, I just like the simplicity and fewer places for inaccuracy to enter into things.
And, I like my factory original gauge. 🙂
Ole S Olson
Saskatoon, Sask, Canada
1946 DR 3/4 ton stake
1139 old site posts
Greg, the pump is not adjustable, however there is a relief valve in the pump and the spring may have lost it's tension over time. I use 10W30 in my 213 flat head engine and have not had a problem. My oil pressure gauge has a copper line from the block, the only problem is that over time the inside diameter of the line gets smaller from the dirty oil. Like your engine, there is no filter on mine.
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