I'm finally in the process of reassembling my TT Ruckstell rear. I got to the fiber washer and, of course, it needed to be thinner. I rubbed it on a wide piece of emery cloth on a flat surface for a while and got nowhere fast. I also sanded my finger tips off trying to hold it. Finally my brain warmed up and I came up with the solution that I wanted to pass along for anyone who gets into the same predicament. I'm sure there are better ways and better equipment but I am limited.
The washer from Chaffin's was about 0.128" thick. The Ruckstell rebuild manual by Glen Chaffin stated that the washer should be 0.08" - 0.100" depending on the wear of the unit.
I placed the washer on a heavy steel plate with some small weld beads that fit just inside of the washer I.D. The height of the beads was less than the thickness of the washer so that they wouldn't interfere with the sanding. I then used a sanding block with 80 grit and started sanding. I stayed in the middle of the washer and turned it constantly with the non-sanding hand to ensure an even removal of material. When I got down to about 0.110" I began to test fit the washer. To fit the washer, I put it in place over the thrust shoulder and on the inner thrust face. I then installed the outer thrust piece (P1004). When the washer it too thick, you can squeeze on P1004 and the bronze ring gear and the ring gear will lift up slightly to P1004 because the thick washer is creating a gap between the two parts. As the washer is sanded thinner, this gap lessens to a point that there is no more gap between the bronze ring gear and P1004. Once this was achieved (about 0.104" on mine), I switched to 320 grit to polish up the washer a little better and allow for the washer to be somewhat thinner to allow it to float. The washer ended up being 0.100" when completed.
This is the process that I used and if it is wrong or anyone has any more good suggestions, please add to it.
I also like to use fiber washers in Ruckstells. Where did you obtain the material, and was it the correct size? I have several originals (from cars) and they are .105".
It was from Chaffin's and the size was perfect. They leave them thick so that you can make it to the proper thickness based on the wear of your Ruckstell.