You dont need to modify anything, in 17.5 I am able to run down to ratios of 3.2.

You will just need to fit a smaller spur such as a 68 or 65 tooth spur and bigger pinions from Robinson Racing which I got from Northern Beaches Hobby Centre ( www.nbhc.com.au )

Bolt the motor in an put the pinion on from the other side of the bulkhead once the motor is in. If you put the pinion on first you wont fit it through the bulkhead opening.

Hope this helps,

Cheers
James.
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