Disclaimer - I generally print holes small and drill out for optimum fit. Print the body upside down. The cross holes in the wheels and gear are meant to be tapped through for M4-0.7x3 set (grub) screws - one or two per instance (the latter for redundancy). The employment of the zip-tie style launched and gear is entirely optional - the car will travel far with just a moderate hand push. Add fender washers to the wheels and 8 -24 .500  diameter steel balls for inertia. Other parts you will require, beyond the afore mentioned grub screws and steel balls, include the following:

• a pair of 30x12 x 2.8 fender washers
• a pair of 23,8 x 13,2 x 2 fender washers
• Qty. 4 of 4-20 x .25 PH pan head screws
• a pair of M3 x 92 (or 100) long dowels - alum or preferably SS
• Qty. 4 - 40X 3 O-rings - preferably silicone
• a pair of 30 x 2.4 O-rings - preferably silicone

In our field testing we discovered that two rows of balls (16 balls total) gave the best result for both distance traveled and straightness of the run.