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Bearings

July/Aug 2001

Bearings in our engines are known as "plain" as opposed to frictionless. Frictionless automotive bearings are ball and roller types which are not usually found inside our engines.

Plain bearings in our engines include main, rod and cam bearings, oil pump shaft and oil pump idler bushings, piston pin bushings and fuel pump push rod bushings.

Main Bearing

Main bearings from 1932 to mid-1936 were poured in place in the block and main caps. The areas into which the bearing alloy was poured had blind drilled holes and grooves to keep the bearing material (babbit) from rotating. These engines were full pressure lubricated, much like later flatheads.  The babbit lasted quite well as long as it was closely backed up by the cast iron of the block and cap.  As the bearing material wore and was carried away by the oil, shims between the main bearing caps and the block could be removed to maintain proper clearance and oil pressure.  The shims, when new, were typically .004 to .007" thick.  These days the shims may be "peelable", but all one piece for easier assembly. Sometimes, only in the bad old days I'm sure, the main caps would be ground or filed on the parting line to try to make a high mileage engine go a little farther.

Bearing life decreases rapidly as the thickness of the babbit becomes greater. This happens when the crankshaft is turned down to eliminate roughness and out of roundness due to wear.  The fear of undersize crankshaft journals is due mostly to this shortened service life of babbitted bearings, I believe. Some sources show a loss of 80% of bearing life with a doubling of bearing thickness. With precision inserts, as used in Fords from mid-1936 on, these fears are groundless.

More modern diesel babbit material is harder, and it can tolerate use in somewhat greater thicknesses. Bearing materials need to have good corrosion resistance as well as the ability to carry the loads generated by combustion.  Imbedability and conformability are additional parameters considered in selecting bearing material. Tin, cadmium, copper, silver, lead, and numerous alloys have been used in automotive engine bearings.

The precision inserts bearing used for main bearings in our flatheads starting in mid-1936 are a large improvement. One advantage these have is that they are easily changed.  The main advantage is that as the crankshaft journal size gets smaller from regrinding, the layer of babbit or other bearing material does not get thicker. It can stay the optimum thickness by increasing the thickness of the bearing backer the steel part.

Ford did make precision main bearing inserts for using the 1932 to mid-1936 crankshafts in the late 1936 through 1938 blocks. The steel back of these bearings is about .2" thick.

In 1939, probably to aid commonality with the new Mercury engine, the main bearings changed again. This was only a diameter increase; other features remained the same.

In 1949, and in 1948 trucks, the final change was introduced.  The front and center main bearings were the same as in earlier years, but the rear got a smaller locating tang and the length of the housing became smaller.  This allowed space for a thicker thrust flange on the bearing. The rear main bearings on 1939 through 1948 do not interchange with 1949 through 1953 in either direction.

Main bearing diametrical clearances are typically about .002" with .001" to .0035" being acceptable.  Thrust clearance is usually about .007" with .005" through .010" an acceptable range.   Main bearing cap nut on stud (or bolt) torque is 80 ft-lbs.  Where bolts are screwing into the block, the torque is 100 ft-lbs.

We will address other engine bearings in the next issue.

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