[Shotimes] rod bearings

[bjshov8@comcast.net]

> Just wondering.  Replacing the rod bearings.  Yes it gives you piece of mind
> that they dont spin, but isnt the crank woren (sp) into them to specific 
> clearances?  So that new ones will not fit correctly and then spin anyways 
> at a later date?  Maybe im just thinking this the wrong way.

The rod bearings are much softer than the crank, so while the crank does wear a little, it is much less than the rod bearings.  What happens is that as the rod bearings wear, the clearances open up.  As the clearances open up, the oil pressure goes down a little, and the lubrication of the bearings reduces.  At some point it becomes critical and the bearings either wear really fast or spin.  They also start knocking which can damage pistons, bearings, or rods.

When you put in new bearings you just restore the clearances back to close to factory specs.  In general it is good when doing bearings in any engine to check the diameters of the crank journals and if out of spec you will have to have the crank turned and put in oversize bearings.  Also I think it is good to actually check clearances with plastigage when you put the bearings in but there has been discussion here in the past to the contrary.