Measuring Stroke with a rod



M-Block 351M/400 Technical Reference

Measuring the Stroke with a Rod

Copyright © 2003 Dave Resch (Used with permission from Dave)
All rights reserved.

One way to measure the stroke of an engine is to insert a rod through a spark plug hole and touch the top of the piston. Then rotate the crankshaft to cycle the piston through its stroke. By measuring the position of the rod with the piston at top dead center (TDC) and at bottom dead center (BDC), you can determine the stroke length.

Angle of rod is the same at TDC and BDC. Note that rod touches the piston top at different places at TDC and BDC.

As long as the rod is aligned with the bore axis (i.e., straight up and down in the cylinder) at TDC and at BDC, you get an accurate measurement.

Unfortunately, you can’t use this method with M-block engines because the angle and location of the spark plug hole on Cleveland-type cylinder heads won’t allow you to insert a rod (even as small as a coat hanger) straight in line with the cylinder bore axis.

The best measurement you could possibly get, by making sure the angle of the rod relative to the bore axis was exactly the same at both TDC and BDC, would be:

rod measurement = stroke / cos A

where A is the angle between the bore axis and the measuring rod.

This will always give a long measurement, which will be longer as the angle between the rod and the bore axis increases. (This also assumes a flat-top piston, with no valve relief or dished area to alter the measuring rod’s height when it touches a different part of the piston top at TDC and BDC.)

Angle of rod is different at TDC and BDC. Note that rod touches the piston top at the same place at TDC and BDC.

If you try to position the rod at a fixed point on the piston top, your measurement will be far less accurate because the rod angle will be different at TDC and BDC, and you are then measuring the difference between two different cosines. In that case, the measurement will always be short.

For example, the worst measurement would come from positioning the rod at the far edge of the piston top (i.e., against the cylinder wall opposite the spark plug hole), where the angle is the largest. In that case, the rod would only move about 2.25 inches as the piston moved through a 4.0-inch stroke.