Here is quick tutorial on how to determine the size of your machine.
First of all, you must realize that machine model size is based upon the spread of the brush bristle when the machine is on the floor, so for example, a 20 inch Model floor machine will not accept a 20" brush! If you measure beneath the housing, from inside edge to inside edge, you will most likely find 18 1/4" for a 20" machine. ( Another example is a 17" model machine, where you will find the inside clearance is 15 1/4" ). This means that your 20"model machine will accept any brush with a diameter of 18".
Pad drivers come with a built up spacer, called a riser block, and we have a different goal with these. We'd like the outside edge of the pad driver to be even with the outside edge of your machines rubber bumper. This means a 19" diameter pad driver on a 20" machine, and you should run 20" diameter floor pads. This will give the pads a 1/2 " of reach beyond the machine, allowing you to reach the edges of the floors your working on.
What is the brand of my machine, and "Which clutchplate do I need ?"
Oftentimes the brand of the machine has worn off, or is a "private Label" machine, built by one company, and renamed and sold under another's name. It's not too difficult for most modern machines, simply measure the round diameter of the drive hub ( the part that spins when you hit the trigger on the machine).
Most times you fill find 3", which is an industry standard, # 92 clutchplate. There are more possibilities , and we can usually figure them out with your help of measuring your hub. I'll then match your measurements up with the plates that we have here in inventory.