Also, main bearings have nothing to do with oil pressure, or at least nothing concerning lubrication. They're more apt to waer from contaminated dirty oil. The valve train and top end suffers with low oil pressure.

 

Hmm I was always under the impression that loose or out of spec bearings on the mains would lower oil pressure. The tighter the fit the more pressure present to sent to the top end.

Glenn

 

A low oil level in the oil pan, oil leaks, dirty oil, diluted oil (with gasoline or antifreeze), too low a viscosity oil, a plugged oil pickup screen or oil filter, a worn oil pump or worn main bearings can all contribute to low oil pressure. Complete loss of oil pressure usually results from a broken oil pump drive shaft (if the pump is driven off the camshaft). A sending unit mounted on the engine block monitors oil pressure. Oil pushes against a spring-loaded diaphragm, which in turn is connected to a resistor or set of contacts that trigger a warning light if pressure drops below about 4 or 5 psi can also be faulty.


1981 F100 Custom SB Inline

 

My husband is 64 yrs old and worked with his dad on all kinds of vehicles. I drew his attention to this thread and relaying his answer.

My 82 van's 300 is acting up similarily. Went through this forum to find any help when I found this thread. The previous owner put in a new oil pump, filters and new 10-40 oil. It does not use oil and the stick comes out clean. But the dash oil gauge registers in the middle cold and then drops to near Low at warm.

My husband figures it might be one of two things causing this. The sending unit is bad OR it is the bearings.

The sending unit is original and will be replaced tomorrow....along with a change to straight 40wt oil. He prefers straight 40wt in older cars than the new viscosity "10-40". If the pressure does not change....then it could very well be worn bearings.

(He drives an 83 Designer Series Ford Van with a 302 V8 and it keeps great oil pressure. 40lbs at idle hot. Runs about 65-70lbs cold. 55lbs at highway speed.)

He had a 1964 Curb VAn Chevy with a 250 Straight Six in the mid 70s. Drove it for 10 years. Near the end, the oil pressure would drop to 25lbs hot highway driving. He put in new cam bearings and that did not make a difference. So he added a high volume oil pump and it was running 75lbs cold at idle. It was running 60lbs highway speed hot. After six months driving he washed all the coating off the bearings. High pressure oil pump will give you great pressure for awhile but watch out!

 

Sheltrow,
I hope you don't mind my asking, but how much did the Ford Dealer charge you for replacement of the main and rod bearings?

thanks srod

 

Found out my problem. It was the old oil sending unit. Replaced it, drained the 10-40 and put in straight 40wt. Now I get decent oil pressure.

 

Worn bearings main, rod or cam cause low oil pressure. So does excessive clearance
at the lifter bores. Had a block we couldn't get pressure up on when we put a leakage
test on it the problem was the lifter bores. I also have seen one of these blocks that
the lifter bores were sleeved. The big problem with the oiling system is the oil comes from the pump and enters the cam bearings thru the groove in the bearings it goes to
the mains and of course thur the top half goove in the mains to the rods. Any wear
on the cam bearings results in an awful lot of pressure loss. One of competitors of
Ford machines the groove in the cam bearing bores, and drills a small hole in the cam
bearing. This meters the oil to the cam bearing no matter how loose they eliminate the
pressure drop at the cam bearings. Needless to say this is a more costly but better way.

 

Good discussion and I have learned somethings.

And I know we have at least one long time professional mechanic on this thread making comments. (I'm NOT)

My thought though is so what if your oil pressure is LOW (but not ZERO of course). As Viking babe correctly pointed out worn cranks and other parts will cause a drop, but is that bad? No. Oil pressure is created by the oil pump pumping oil to differrent parts of the motor, it creates pressure by doing this. The crank is bathed in oil, it sits it and then sloshes oil all over itself by its rotation. If oil pressure goes to zero because the pump goes out, then I'm betting the head will run try and all the lifters, rocker arms, etc will start to overheat and mess up.

Now my question that I ask you to comment on, OK worn main bearings will cause low oil pressure but I'm betting NOT to a point that could be considered destructive to the motor UNLESS the main bearings are just about totally SHOT, AND then that would be YOUR REAL problem.

THANKS for your comments in advance. I know we are belaboring this topic, but I beg your patience at least one more time.

 

You have to remember that when the oil hole drilled in the crank to feed the rod bearing indexes with the groove in the top half of the main the rod gets oiled. If the mains are worn to where they loose too much pressure the rods may get next to none. Not a good thing, that is when you knock out rod bearings. As simple as I can put it, if all is well the oiling sysem is balanced to provide what is needed, we get a leak(i.e. loose main or cam bearing) and things suffer. By the way racing engines usually have main bearings where the ungrooved lower insert is replaced with a grooved insert this provides 360 degrees of oiling to the connecting rods.

 

Wow! Didn't think this post would get this much response.

SROD to answer your question: $1300.00. Here is what he did:

1. Pulled engine
2. Removed oil pan, pump, and pickup
3. removed clutch slave, throwout, and clutch assembly
4. Installed new FORD bearing halves (rod and main, standard size)
5. Plastiguaged upon assembly
6. Installed new FORD oil pump, pickup, screen, rear main seal, oil pan, oil pan gasket, clutch, slave, throwout, and Throttle body gasket.

Keep in mind that all the parts were FORD new-not reman. I'm a little skeptical that the crank and rods weren't machined but I do trust him. He's been a mechanic there for over 35 years-not to mention he owns the dealership with his brother. That should count for something. Their father started the garage back in 1941 or so-back when cars were near impossible to get due to the war!

So far over 1100 miles on the reconditioned lower end and no problems. Still holding 50 psi hot cruise, 35 psi hot idle, 70 psi cold idle or cruise.

 

Thanks Sheltrow,
Looks to me like you got a good deal of work done for a fair price, I've priced a few of those parts through Ford over the years and well over half your bill must be parts.