I gathered some junk lawnmower parts to build a go-kart for my kids. The engine needed new rings, but was the cylinder worn in an egg-shaped pattern, or did it simply need slightly over-sized rings? I did not have a micrometer for measuring inside the cylinder walls. I improvised.
(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
My substitute micrometer uses a wood block 3/4 x 3/4 inch cut in length to the diameter of the cylinder.
Step 2: Round the ends for better fit
I rounded the ends of the block so it fits inside the engine cylinder with only a little looseness. It is loose enough to insert and remove easily, but tight enough that it almost centers itself. (Ignore the black crayon mark from the lumber yard.)
Step 3: Drill-end-to-end
A steel rod in two sections will run the length of the block. It is necessary to drill a hole-end-to-end. Choose any scrap small diameter steel rod. A piece of wire from a coat hanger will do. Drill a hole just a little larger than the diameter of the wire so it can move in the hole easily, but without being sloppy.
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
Make a notch in the center of the block that extends below where the hole passes through the block. I have inserted my piece of steel rod for illustration purposes.