Counting Undersize bearing--> Which direction?

[Geri]

Hello together!

I need your help again with my 1955 chevy inline 6 Motor. I have a logical Problem to understand the counting-system

I measure the Main Bearings and the Rod Bearings with a Plastigage stripe. The clearance is on top of the highest tolerance. I want to replace them with the next selectable undersize.
For example: On the main bearings its a stamp with 002". What is the next undersize to minimize the clearance? 001" or 003"? Sorry for the supid question.With counting the oversize i have no problems. But how is it counted by undersize????
Is more also more or is it getting less??? im confused

Thanks for help

Geri

[Speedbumpauto]

It's a silly thing that gets confused all the time. Typically, the crankpin/crankshaft is ground undersize so, in inches, it might be reground from 2.000" to 1.990" or .010" undersize. The bearings, then, are made thicker or "oversize" by .010" to maintain the same clearance. So....the crankshaft is machined or wears undersize and you buy oversize bearings to fit the undersize crankshaft. Make sense? In your situation, if the plastigage reads .003 and you want to go to .002, you would go to a .003 oversize bearing, although to confuse things completely, most of us who build engines refer to the bearing as undersize because they fit an undersize crank journal. What makes it easier is that virtually no one makes a truly undersize bearing because no one makes an oversize crankshaft. However, in my experience, very few manufacturers make select undersize(oversize) bearings in .001" increments except for serious performance engines and even then .001 in either direction is usually the limit. For most american engines, when the clearance gets too large, the fix is to have the crankshaft ground .010 and get bearings to fit. Also, plasitgage may not be the best method to set oil clearances if you've found .001+ select bearings. Sorry for the length, it's the coffee.

[1project2many]

What is the next undersize to minimize the clearance? 001" or 003"?

The plastigage measurement is the difference between the crank outside diameter and the bearing inside diameter. To reduce this value you will need a bearing with a smaller inside diameter. The stamp on the back of the bearing refers to the amount the crank pin is undersize. A .002" bearing is intended to fit a crank pin that is .002" smaller than original. If you want to reduce the clearance by .001" you will need a bearing that is .003" undersized.