Thread size is 1/4"-20 for side panels.
If you are talking about shimming the pivot of the damper and control plates:
Damper 5/8" ID Shims
Control 1/2" ID Shims
Grab some .005 and .010 shims for both sizes so you can dial it in, as well as new retaining rings. It helps tighten things up if they aren't distorted.
More info here:
http://cubfaq.com/hydrolurch.html