Plastigage Versus Micrometer - Accuracy Test

Another trick is to use the same mic that measures the crank to set up the dial-bore gauge. This will reduce tolerance stack-up and inaccuracies. If you set the dial-bore gauge to the crank-journal diameter, the difference between the inside diameter of the bearing and the crank journal will be the oil clearance.

Plastigage
The reason many backyard engine builders like to use Plastigage is that they don't need expensive specialty tools like a mic or a dial-bore gauge. As long as the bearing surfaces on the rod and crank are clean, merely lay down a short strip of that green string and torque the bolts down to the factory spec. The wider the string, the less clearance there is between the bearing and the crank. Plastigage comes in three different configurations: Green (PN SPG-1) for 0.001-0.003-inch, red (PN SPR-1) for 0.002-0.006, and blue (PN SPB-1) for 0.004-0.009-inch clearances.