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Well, you could just take a motor into the hardware or local hobby store and see what size brass tubing fits. But to save you a futile trip; the standard motor shaft diameter is 2mm, aprox. The standard telescoping brass tubing from KS will not fit exactly. Their 1/8" O.D. is 3/32" I.D. which is too big. The next size smaller is 3/32"O.D. but the I.D. is too small for the motor shaft. This means that you would have to drill out the tubing to fit. I think this would be a pain in the rumble seat.
You're better off just waiting until a pen runs out of ink and store those up for future use. It's pretty easy to clean the pen tubes out with lacquer thinner and a pipe cleaner. Far easier than drilling out the brass and hoping it fits.