Plastigauge measures the clearance, not the undersize. Suppose the clearance range is from .001" to .0025". The amount the Plastigauge flattens shows pretty much what the clearance is. The green Plastigauge has a clearance range from .0005"to .002", I believe. Red is for slightly wider clearances, and blue is for BIG gaps!
On my race engines, I don't trust Plastigauge. I torque up the rod and main bearings with the crankshaft removed, and measure the actual inside diameter, down into the .0001" range, then measure the crank journal and subtract the difference to get the clearance. Then I polish each crank journal individually to get an exact clearance on all the bearings.
Jerry
