Mercury Engine >> MAIN BEARING KNOCK

Mercury Engine >> MAIN BEARING KNOCK

Front Main Bearing Knock On
Some 1975-1977 Ford, Lincoln, Mercury 7.5L (460 CID) Engines

It has been reported that some 1975-1977 Ford, Lincoln and Mercury 7.5L (460 CID) engines may have a front main bearing knock. The thump or knock may be distinctly heard inside the vehicle at half engine frequency when the engine is hot and is most noticeable between 800 and 1500 rpm.

Ford engineers recommend checking the engine timing. Following this move the vehicle outdoors, close the windows and shut off all accessories. Slowly raise engine speed to 1500 rpm in drive while depressing the brake pedal. If the front main bearing is defective, a definite recurring thump will be heard at half
crankshaft frequency.

To solve the noise problem, Ford Motor Company has recommended selective-fit standard and undersize bearings:

Upper main bearing (Std.)
Upper main bearing (.001)
Upper main bearing (.002)
Lower main bearing (Std.)
Lower main bearing (.001)
Lower main bearing (.002)

Any combination of the above main bearing halves can be used. The use of two different size halves on one journal is approved, however, the larger undersize should be installed in the upper position. A properly selected combination of bering halves should allow for a free-turning crankshaft when all bearing cap
bolts are correctly torqued to specifications.

Installing these selective-fit bearings should enable you to provide .0004 to .0015 clearance at the front main bearing. Try the .001 undersize bearing first and check the clearance with Plastigage or equivalent.

The AERA Technical Committee

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