(I do not have an engine to show in this Instructable, and have decided to use a holesaw to represent an engine cylinder. The bottom of the holesaw represents the top of the piston.)
Step 1: Measure the Cyclinder's Diameter
Step 2: Round the Ends for Better Fit
Step 3: Drill-end-to-end
I made guide lines in pencil and drilled by hand. Because bits this size are short I had to drill from each end to make a hole that runs the full length of the block. Happily, my holes met in the center. Even if they do not meet, this device will still work, but the holes should at least be close to meeting.
Step 4: Make a Notch
Step 5: Cut the Rod in Two and Finish the Ends
Step 6: In Use
Raise and lower the piston to the area of the engine cylinder you want to check for wear. Let the block rest firmly on top of the piston. Position the wooden block so the steel rods are at the widest part of cylinder. Insert blades from a feeler gauge until you find the maximum thickness you can place between the inner ends of the steel rods. Record the sum of all blades by reading the numbers on each and adding them. Turn the block one quarter of a turn and take another measurement. Do this at the top, center, and bottom of the cylinder. Subtract the smallest reading in any position from the largest reading in that position to determine how many thousandths of an inch your cylinder is out of round. If it is within the specified tolerance, you need buy only slightly over-sized rings, not to have the cylinder ground.
This method of checking relative amounts of wear in an engine cylinder is simple, but effective.
If you need to check a different size of cylinder, you will need to make new steel rods and maybe a new wooden block, too.