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 How Plastigauge Works.
Fig.1 It is recommended that surfaces be clean prior to use of Plastigauge
Plastigauge is comprised of a rod or thread of a compliant plastic material of accurately determined cross-section - either circular or square. The surfaces between which measurement is to be made are first separated, PLASTIGAUGE is inserted (see fig. 2) and the surfaces are returned to their standard position.
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 Fig.2 Plastigauge placed across the bearing surface
The once circular (or square) section will have been flattened but the area of the cross-section will remain unchanged. We can, therefore, state that:
P D² / 4 = W x T \ T = P D² /4W
or in the case of a square section of side 'a' :-
a² = W x T \ T = a² / W
Hence T, the clearance to be measured is equal to a constant divided by the width of the strip after deformation. By opening the surfaces to reveal the deformed gauge, W can be measured directly and from this the clearance, T, can be determined.
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 Fig.3 De-formed Plastigauge ready for measurement
In practice the need for this calculation is obviated by the use of a calibrated scale on which an array of marks has been printed (see fig. 4).
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 Fig 4. De-formed Plastigauge compared with calibrated scale.
The width of the strip can be compared with the scale (see fig. 4). The actual clearance is shown alongside the mark which most nearly corresponds to the width. For greater accuracy the clearance may be interpolated between the two most nearly comparable scale marks. Plastgauge is soluble in oil and although it is recommended you clean all residue off no harm will come to your engine if any residue is left behind, it will simply be dissolved by your oil.
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