Originally Posted by
sosidge
It's a 3mm ID shim. TOP make a shim set (PA-100305), Tamiya make some, Yokomo make some Kyosho make some... it's not an uncommon part, just not always something that shops keep on the shelves!
Shimming the piston is an essential otherwise you get a heap of undamped movement in the shock. The car will feel a lot smoother on the track once you do. I actually went another route and fitted Tamiya pistons and shafts to the shock.
The tamiya pistons dont have the correct size holes and they are also slightly smaller diameter and thus will have more oil that can get around the edge of the piston within the shock body. This will give you a different feel on the track, and its a feel I never liked.