I had the same issue...... When i switched to a D3.... Just took a plastic bag and poked the motor shaft through. Keeps the metal shavings out of motor and bearing. Made the flat long enough that any pinion fits now.
BTW....... I just figured out why newer setups don't work. The 6.1 setups make my TC6 pushy. Just went back to the KHebert setup above and WOW! Car drives MUCH BETTER.