Ask the Mechanic

Steve Thompson, Contributing Columnist

READER: I know my Ford 8N tractor is old, but it has a special problem with losing oil pressure. It will start out at 35 pounds but will drop slowly as the engine warms up to near zero pounds. After the engine has time to completely cool down, it will repeat the same drop in oil pressure. The engine runs very clean at the exhaust, has no oil leaks or blowby but just will not hold oil pressure. The guy who sold me the tractor said it had been recently overhauled. Where is the oil pressure going?

STEVE: The oil pressure is dropping when the engine warms because the engine's oil viscosity is thinning from heat from the engine (see photo accompanying this article). The cool oil is much thicker than the heated oil and is filling up the leaks, but when the oil heats, the viscosity of the oil is much less, and it can't fill the excessive clearances in your engine. This problem can happen in any engine, not just your tractor's engine.

Since the oil pressure is holding when the engine is cold, we can eliminate any problem with the oil pressure regulator valve on the front of the engine. The engine in your tractor does not have cam bearings, so that will not be the problem.

The next two areas that usually cause your oil pressure to drop are the main bearings and the oil pump. Proper clearance between main bearings and crankshaft is between 0.001 and 0.003 thousandths. I bet if you listen to the engine when it starts cold, you will hear a slight rumble (not knock) coming from the engine before the engine picks up oil pressure. That noise is the crankshaft "rumbling" in the main bearings, which will require oil pan removal and clearance inspection using Plastigage (a measuring tool used to check bearing clearances).

The other area you need to check is the engine oil pump. A kit is available to rebuild it, but you will need to measure the width of the gears in the pump. One pump has larger 3/4-inch gears. Also, look at the pickup tube plate that is removed to access the gears. If it shows wear at the location of the gears, replace it or have it milled smoothly at a machine shop.

Oil pressure can leak by any wear on the cover plate of the oil pump. Do not run the tractor again until you fix the problem to eliminate further damage to the engine.

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-- Email Steve Thompson care of askthemechaniccolumn@gmail.com, and be sure to include your contact information and phone number.

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