# How to Measure the OD of a Clutch Disc

by Teeter Allen Morrison

The clutch disc has splines in the center hole that mesh over the splines of the transmission input shaft. That rigidly connects the disc to the wheels via the transmission. The disc is pinched solidly between the flywheel and the spring-loaded pressure plate when engaged. When the clutch pedal is pressed, the clutch disc is released so the engine can rotate while the wheels are stopped. High-performance vehicles use a larger diameter clutch to help prevent slippage when a lot of horsepower is transferred to the wheels. There are several ways to measure a disc's outside diameter (OD).

## Step 1

Lay the clutch disc on a flat, level surface. Place two small carpenter squares, one on each side of the disc, 180 degrees apart. The squares should be placed against the disc like backwards book ends. Measure the distance between the squares.

## Step 2

Place the clutch disc on a level surface with the straight edge of a yardstick over the center line of the clutch disc and align both outside edges of the disc with the inch markings on the yardstick to gain an accurate measurement.

## Step 3

Raise the rear wheels, of the vehicle with the clutch to be measured off the ground and spot two jack stands, one under each axle tube near the wheels.

## Step 4

Remove the bellhousing inspection cover using a box-end wrench. Tape and attach one end of a flexible dress maker's tape to the pressure plate cover disc. Use a small strip of cellophane tape to hold it in place.

## Step 5

Weave the flexible tape inside the mounting brackets of the pressure plate. Be sure the measuring tape gets an unobstructed circumference measurement of the round cover disc on the pressure plate. The OD of the cover disc is the same as the OD of the clutch disc.

## Step 6

Calculate the clutch diameter using the circumference measurements acquired with the flexible tape. To extract the diameter from the circumference measurement, divide the circumference by pi (3.14). For example: say the circumference is 34.5 inches; 34.5 divided by 3.14 equals 10.987. This would equate to an 11-inch clutch disc.