Measure Cylinder Wear Without a Micrometer

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.)

My substitute micrometer uses a wood block 3/4 x 3/4 inch cut in length to the diameter of the cylinder.

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.)

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.

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.

The steel rod needs to be cut into two pieces so the opening between the two pieces is in the notch you made in the block.  Lightly grind a point on the outer ends of the pieces of rod.  Gently round the inside ends.  The gap between the two inside ends should not be more than 0.020 to 0.040 inch when the outer ends are pressed against the inside of the engine's cylinder wall, but that is not critical.

This device is actually easier to use if you drill a hole straight down from above one of the steel rods and insert a locking screw to keep that rod from moving. 

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.