I did a cut and paste on this from some site that sells the PC 7424. I have the 7336 and it works great.

In general, the 5-inch counterweight (p/n 874011) should be used with a 5-inch or smaller backing plate, whereas the 6-inch counterweight (p/n 699933) must be used with a backing plate larger than 5 inches. Weight is the true determining factor for the proper counterweight, not the backing plate or pad size. If your machine vibrates excessively, you may need to switch counter weights. You will need a Torx driver (size T15, not supplied with the machine) to swap counterweights.