Im changing the rear main on my 1980 351M. I was wondering if i should go ahead and change the rod bearings and oil pump while im under there. The rod bearings are only about 3 bucks a piece. if i do should i oversize? if i change the oil pump, should i put a standard or high volume? Is there any thing else i should change??

 

Welcome to FTE.

How many miles are on the engine? If it's less than 75K, the benefit of changing the bearings and oil pump is marginal. If it's 75K-150K, the potential benefit increases proportionately. At 150K miles or more, I would definitely change both rod and main bearings, as well as the oil pump.

While you have the oil pan off the engine, clean it out thoroughly to remove any accumulated debris or deposits. When you seal it back up, be very careful not to get excessive silicone gasket sealer on the inside of the engine. Chunks of silicone sealer and valve stem seals sucked into the pump are the most common causes of oil pump failure.

If you want to change bearings, measure all of the crank journals with a micrometer. If they are out of spec diameter, you'll need undersize bearings (that's undersize to fit a smaller journal diameter).

Check the diameter at several points around each journal, and at both both ends of the journal to look for taper. If you find any taper exceeding 0.0005", the crank needs to be machined to clean it up.

The service limits for standard size bearings are 2.994' to 3.002" on main journals, and 2.3103" to 2.3111" on rod journals.

If you have good bearing clearances (0.0008"-0.0015" on both mains and rods), you'll be fine with an OEM-type oil pump. If you have larger clearances, you should probably use a high-volume oil pump.

If you replace the oil pump, replace the pickup tube also. An old pickup tube could have hidden debris inside the tube, or hidden defects (cracks, poor screen attachment, etc.) that could kill the new pump. I wouldn't take the chance on such a false sense of economy.

 

On some engines the rod big end is prone to getting a little egg shaped or out of round with high miles or high RPM's. Is this not an issue with the 351M rods?

 

If you let them run long enough with excessive bearing clearance, it is an issue. By the time the rod's lower bore is deformed from excessive clearance, the engine is usually making bad noises in the bottom end.

In that case, a complete rebuild is necessary.

The service spec for the connecting rod lower bore is 2.4361" to 2.4369", with no out-of-round measurement.