Overview
The most common way to measure bearing clearances is using a product called Plastigauge®. Plastigauge is a thin waxy string used for measuring engine bearing clearances. The concept is so simple, yet critical, you'll never build another engine without it.

The thin wax string is placed between the journal and bearing, and then the bearing cap is torqued to specification. The Plastigauge flattens out to the exact distance between the two surfaces. The flattened wax is then measured using the supplied ruler.

Using Plastigauge is a cheap and easy way to check main and rod bearing journal clearances, and can save you the heartache inflicted by spinning or overheating a bearing because of insufficient clearance. Follow along as we show you the simple process.


Plastigauge comes in several "colors", each corresponding to a measuring range

Use the "green" Plastigauge for measurements between 0.001" and 0.003" inch - typical engine bearing clearances.

With the lower bearing and crank in place, cut a small piece of the Plastigauge wax string and place on at least one main journal. Be sure the journal surface is free of any oil or grease.

Install the main caps and bearings and torque to specification. Do not rotate the crank. Then break the torqued fasteners and remove the main caps carefully.

The Plastigauge strip will have compressed and can be matched to the calibrated marks on the package. The acceptable range is typically between 0.0005" and 0.0025", however check your shop manual or crank manufacturer's specs for exact requirements.


You can also use Plastigauge to measure rod bearing clearance. Follow the same process, torque the rod caps to spec and then remove the caps. Aim for 0.0005"- 0.0025"-inch clearance

Conclusion
If you find clearances are too tight or too loose, the crank can be cut to the next bearing size. In any case consult your machine shop if you feel the measurements are not desirable.

When done, Plastigauge can be removed by wiping with a clean rag. It is oil soluble and if left on the journal it will not damage the engine or clog the oiling system.

There are other ways to measure bearing clearances. Machine shops will directly measure the diameter of the crank journal and subtract it from the inside diamater of the main or rod bore, with bearings installed. However this method requires expensive micrometers and bore gauges as well as a good knowledge of using such tools.



Color Shim Gage
A product called "Color Shim Gage" actually works similar in concept to a feeler gauge. A soft aluminum shim of precise thickness is placed on the journal surface, and then the main or rod caps are torqued down. An attempt is made to rotate the crankshaft. Thicker shims are placed until the crank requires excessive force to turn. This method, in our opinion, is cumbersome in that it requires several torque and remove procedures to obtain the measurement.

Plastigauge can be obtained from all reputable machine shops. If your shop doesn't stock any, you ought to consider using another machine shop.