How Plastigauge Works
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. 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:
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.
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 ).
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.
Fig.1 It is recommended that surfaces be clean prior to use of Plastigauge
Fig.2 Plastigauge placed across the bearing surface
Fig. 3 Compressed Plastigauge ready for measurement.
Fig. 4 De-formed Plastigauge compared with calibrated scale.
Plastigauge ® is highly effective for measuring the clearance in large bearings which include Marine Drive Shaft bearings, Turbine housing bearings, Pump and Pressure System bearings.
Shaft end-float lends itself to direct measurement by Plastigauge ®.
Flatness and clearance in pipe-flanges and cylinder heads are readily checked and measured with standard Plastigauge ® .
It is recommended that the surfaces be separated by a ground spacing washer or shim of (typically) 0.002". Plastigauge ® is positioned in those regions where distortion is suspected and any variations of flatness will be shown, after the assembly is dismantled, by variations in the width of the compressed Plastigauge ® strip.
For large bearings, multiple measurements are recommended by inserting two or more gauges around the bearing.
Plastigauge ® is particularly effective for the measurement of separation in moulding tools, and wherever it is required to determine the separation between hidden surfaces.
A WIDELY USED APPLICATION
Perhaps the most widely used application of Plastigauge ® is in the measurement of clearance in plain automotive bearings. This application is described in detail:-
Remove the engine sump cover to reveal the big-end and its retaining set screws. Remove surplus oil and release the big-end shells by unscrewing the set screws. Wipe the exposed surface of the journal and shell. Apply a smear of grease to the journal and squeeze a small quantity of silicone release agent on to the shell.
Trim a length of Plastigauge ® to fit across the journal using the grease to hold it in place. Replace the shell and tighten the retaining set screws to the manufacturer's recommended torque setting without rotating the journal.
Now remove the shell once again by unscrewing the set screws to reveal the Plastigauge ® which will have been spread across the bearing surface as a stripe or band. Match the width of the Plastigauge ® stripe against the calibrated gauge card supplied and read off the clearance.
Ideally you should remove the Plastigauge ® stripe with a clean oily cloth or industrial de-greasing solvent, but users may be assured that any Plastigauge ® left behind is oil soluble and cannot harm the engine in any way.