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A crankshaft is not measurably weakened by grind...

Shop Solutions: March 2008

Debunking the 10 x 10 Crankshaft Myth for Customers; Extending the Life of Crankshaft Polishing Belts; AMC (Jeep) Rear Main Bearing Leaks

Debunking the 10 x 10 Crankshaft Myth for Customers
We’ve heard a lot of customers say they need a .010˝ x .010˝ undersize grind crank  because “the vehicle will be towing a lot of weight” or “this guy drives his car/truck real hard” or “this boat runs at high rpms and I don’t want to take the chance of the crank breaking and destroying the engine.” With all due respect to our customers, these people are mistaken.    

A crankshaft is not measurably weakened by grinding it down to .020˝ or .030˝ instead of .010˝ undersize.  Crankshafts are over-engineered from a strength standpoint. OE manufacturers design the crank with a wide margin of error to ensure against the chance of breakage. Grinding a crank with a 2.100˝ rod journal such as a small block Chevy .030˝ instead of .010˝ under reduces the journal size less than 1 percent. On the mains the reduction is about 0.8 percent.  

More proof that grinding a crank .030˝ under or more lies with bearing manufacturers. Leading manufacturers such as MAHLE Clevite, Federal-Mogul and ACL, who supply bearings for most engines up to .030˝ undersize, and for popular engines such as big block Chevys and small block Fords up to .040˝ or even .060˝ undersize.  The engineers at these companies would not allow these undersizes to be made if there was a reliability issue.  Subsequently, the steel backing bearing layer is made thicker as the undersize increases. This allows the somewhat softer bearing material to stay uniform regardless of the undersize so there’s no soft mushy liner to beat out.  

Some people believe that by grinding a crankshaft beyond .010˝ the surface hardening layer will be ground away.  In the first place, a vast majority of engines have no surface hardening, This includes most Ford, GM or Chrysler engines, and most imports. There are BMW, Mercedes, Porsches and others, including Hondas, that are surface hardened from the factory.  These cranks should be heat treated to restore its surface hardening after grinding regardless of undersize, because the surface hardening is usually only .0001˝ to .0002˝ thick. 

There are several things to be concerned about when putting a reground crankshaft into an engine, but a .020˝ or .030˝ under crank should not be one of them.

Engine Pro Technical Committee
 

Extending the Life of Crankshaft Polishing Belts
Periodic cleaning of crankshaft polishing belts will greatly extend the life of the belt. To clean a belt, crumple into a small ball, soak it in mineral spirits and repeat the process until it is clean. This procedure can be repeated several times.

Darrin Anderson
Braymer, MO

AMC (Jeep) Rear Main Bearing Leaks  
Improper flywheel or flexplate torquing procedures on 1987-’93 AMC (Jeep) 4.0L engines may result in distortion of the rear main bearing seal, resulting in leakage at the rear main seal. When installing the flywheel or flexplate, be certain to tighten the bolts to 105 ft.lbs. in an alternating sequence.

David Mussell
Midstate Machine Shop,
Santa Maria, CA

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