Plastigage
A calibrated plastic thread used to measure clearances between internal engine bearings and crankshaft journals.
Plastigage is a specialized diagnostic material used by engine builders and mechanics to measure the oil clearance between plain journal bearings and shafts. It is most commonly used when checking the clearance of crankshaft main bearings and connecting rod bearings during an engine rebuild.
The product consists of thin, calibrated plastic threads packaged in paper sleeves with a printed measurement scale. To use it, a small strip of Plastigage is placed on the dry bearing journal. The bearing cap is installed and torqued to exact factory specifications, which flattens the plastic thread. The cap is then removed, and the width of the flattened plastic is measured against the scale on the package to determine the clearance.
If the bearing clearance is too tight, the oil film cannot flow, causing the bearing to overheat and seize. If the clearance is too loose, oil pressure will drop, leading to engine knock (rod knock) and bearing damage. Plastigage is a simple, highly accurate way to verify clearances before final engine assembly, protecting the engine from lubrication failures.