Re: housing bore for main bearings on 7th series 8

Posted by 32model901 On 2010/9/6 10:29:24

Thomas,

I noticed something mentioned at the start of this post that should be checked out. It was mentioned the mains were line honed. I hope this was not the case, and the mains were line bored.

Soft Babbitt type bearing should never be honed, only bored. The reason, abrasive particles from the hone will break off during the honing process and become embedded in the soft Babbitt.

Hard type bearings such as brass, bronze, steel, and cast iron can be honed because the material is not soft enough to allow the honing stone particles to embed into the bearing, and can be washed away with thorough cleaning.

Regarding the bearing clearances for your engine, data from a Motors manual listing '31 Packards is as follows:

Main bearing clearance - .001

Main bearing end play - .003
Thrust on main bearing #7

Rod bearing clearance - .0015
Rod bearing end play - .003

Plastigage is the best method for checking main & rod clearance because it measures the actual oil film clearance.

If one used a micrometer to measure the crankpin diameter and a bore gauge to measure the I.D. of the main bearing you could have a tight bearing even if you measured .001 clearance between the bearings. Why, if the crankshaft had a .001 bow in it, this bow would eliminate the .001 oil clearance that was measured.

Plastigauge can be purchased in .001 to .003 range. The bearing and crank should be clean before you make your measurement. Cut a length of plastigauge the full length of the bearing you want to measure.

You may want to check out:plastigaugeusa.com/how.html

Torque the bearing caps to the correct torque, Motors lists 65-70 ft. lbs for the mains and 40-45 ft. lbs. for the rods.

The Motors data on the '31 only shows one value for the clearances, not the service limits.

Just as reference info Motors shows the following service limits for a '35 model 120:

Main bearing clearance - .001 to .003
Crankshaft endplay - .003 to .008

Rod bearing clearance - .0008 to .0028
Rod bearing endplay - .004 to .010

Good luck,

Dave