Originally Ruckstell used a fiber thrust washer inside the bell,P-159.. When new, they specified an .085 fiber washer. I'm assembling a near mint original Ruckstell, and it's going to need a .095" - .100" washer, and that is allowing for .005 clearance. A neat way to check the clearance is with a pointed feeler gauge, which I stick through one of the three slots in the bell. Works good. The standard Ford steel thrust washers are only about .077-.081 thick. What I did was assemble one of these Ford thrust washers in there, and then determine the added amount I will need with the pointed feeler gauge.
I believe original Ford steel thrust washers should measure out at .086 - .088. New repros are ground to .087 - .088.