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MG MGA - Drop in Oil Pressure

yet more questions. While test driving a prospective MGA today I experienced some decline in oil pressure that I'm wondering if I should be concerned about. For the first 5 minutes or so oil pressure stayed right around 65 while under accleration, during the 5 to 12 minutes the pressure dropped to 60 for a while, then pressure dropped to 40 while under acceleration and stayed there until I finished the test drve.
Is that normal? Or is that a sign of of some greater issue?

Thanks, Tysen
Tysen

No. An engine in good condition should hold 50 psi at 2500 rpm or higher with hot oil (maybe higher if you shim the relief spring). When it drops to 40 psi at road speed, I figure it's time to change the crankshaft bearings.

Barney
Barney Gaylord

Thanks Barney
University Motors recommends rod bearings and an oil pump while in there.....is that the same as what you recommended?
Tysen

Of course, some drop in pressure while the engine warms IS normal. It's just a question of how low the pressure goes. My engine generally starts around 75PSI and warms to about 60. The oil viscosity also effects these numbers. If the owner has 10W30 engine oil in the car, in the summer time, the pressure will be lower. Most of us use 20W50 in summer and 10W30 in winter.

Regards,
M.D.
'57 Coupe
M. D.

What they have said above is correct.

However, you can take the sump off with the engine in the car and check/change the big end bearings and oil pump quite easily. I have done this several times.

However 3 main bearing engines tend to get more crank flex than 5 main bearing engines and this wear the main bearings more. This is an engine out job to change these.

Remember there are two surfaces that wear, the crank and the bearings. If you change the bearing often enough then you can reduce the ware on the crank, as the bearings ware through the soft white metal and into the harder backing material.

In my experience if you've got 40 PSI, then you most likely have some crank wear and the crank will need a regrind...

My Twin Cam came with a pushrod engine which I conservatively think had not run for at least 10 years.
It had about 40 psi when I finally got it to run. As I did not want to spend buckets on a motor I was not going to use for long, I changed the big ends myself and got it up to about 50PSI...

Hope this helps...
Cheers
<MARK>
Mark Hester

Oil pressure is directly controlled by the relief valve. The relief valves on many older engines are not as good as they could be. Simply by changing the valve pressure can be restored to much higher values. Baiscally 40 PSI at medium RPM is more than adequate, hopefully this will reduce to at least 20PSI at tickover. These figures would be OK and some engine builders may adjust the spring to achieve these values as too much oil pressure can just eat up HP.
Bob (robert)

I would not replace an oil pump on a guess without inspection to determine the internal condition. Such things cost money. If the oil is kept relatively clean an oil pump will last a very long time. I have occasionally transfered a 100,000 mile oil pump to the next rebuilt engine.

Main bearings can be replaced from the bottom without removing the engine. There are a few tricks to it. In the front you have to remove the two bottom screws from the timing cover, as these go into the main bearing cap. The front main cap is above the cross member, so you use a screw with flat washer and a pry bar to pull it down. The top half of the main bearing shell can be rotated around the journal for R&R, but do only one at a time while the other main bearings hold the crankshaft in place.

Barney
Barney Gaylord

Several years the oil pressure in my 58 MGA was really falling off in very hot weather and was hitting like 10 pounds at a low idle at worst case. The motor had around 70,XXX miles. I replaced the rod bearings that were definately worn, the original old style oil pump, and the relief valve and spring. About 10,000 plus miles later I have 75 pounds cold and 60 plus when very hot and the idle has never gone below 25 pounds. Cost was pretty minor and job pretty simple (except for a couple of difficult to get at oil pan bolts). Worth a shot before you tackle anything major.
Bill Haglan

Always make sure the gauge is accurate. The first step
when a customer has a low oil pressure complaint is to hook up a real calibrated gauge and check pressures. Then find out what oil is in the car and what has been normal. I know that the gauges on our cars are more accurate than on most modern ones. But verify first repair later. Most doityourselfers under diagnose and over repair whitch makes it more difficult for those who follow.
Randy
R J Brown

This thread was discussed between 12/07/2005 and 14/07/2005

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