Thanks Duncan,
I need to clarify both things we said. Both are right.
The WP 9 and 20 can accept the same size Tungsten electrodes...what I meant to say.

The WP 9 and 20 physical size of the collets are different than our other series...what you said.

To sum it up, if you have a supply of wp 18 size collets, then you will need to get some more to add to your inventory for the WP 20/9 series. Both series interchange with standard weldcraft and other brand torches. They all can be bought locally at a very small price.