I have used two gum seal washers from faucet parts for the pulleys:
![](http://images.meredith.com/diy/images/2009/02/p_SCP_062_02.jpg)
Mine are 14mm outer diameter, 4mm inner, 4mm thickness. They are easily found on every hardware store here in Spain. The shafts have about 6 mm dia., so the new pulleys will remain immobile in its place without glue. We have some clearance here, because the potentiometers and shafts adapt themselves to the ball diameter by means of springs.
But 14mm(plus a little of enlargement due to the fitting) is probably bigger than the original, and they touch on the upper flange of the pulley housings. It is necessary to cut a little of plastic on the ceiling cap of the mouse ball housing. Here in this picture you can see the flange that needs to be cut.
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With the ceiling cap done and mounted in place, I have tested some diameters of ball, but I only had 30mm and 35mm ones. The first is a silicone bouncing ball that has probed to be excessively thin to serve us, but also the second, made of solid plastic, doesn't enters completely on the hole. I think that the ideal bore will be 33,34mm, being an English manufacture surely they have used an 1-5/16'' diameter.
The next step is to buy some iron balls for measurement, from 32mm to 34mm diameter. Once found the exact measure, I will search for a definitive gum or silicone solid ball. There is no necessity of a weight inside the ball, only that it wouldn't slip the pulleys or the table surfaces.
Edit: Today I have measured correctly the balls with a caliper, one has 28,5 to 29,5 mm dia., and the other 33 to 33,5mm dia., they are very irregular. So the optimum measure has to be between 31 and 32mm.