As far as screw length goes, generally speaking you do not want to use a longer than stock screw on plastic tub chassis. If you bottom out the screw into the hole and try to keeping tightening the screw, you risk cracking the plastic. I have made this mistake when installing the 2 long screws that hold the plastic bumpers to the chassis (keep the lower a-arm secured). In my case I was swapping out the rear toe piece between a TT-02S and a TT-02 SR. These 2 cars use different length screws because the S kit instructions use the plastic rear bumper and the SR kit does not. Because I wasn't paying attention I actually weakened the plastic so much on the chassis that it broke during a mild racing accident. I had to replace the chassis.

If you aren't sure how long of a screw you need, you should be able to use a 2mm hex screwdriver or an allen tool as a depth gauge for screw length. Make sure you account for the thickness of any part that is installed over the hole you are measuring. Again I forgot to do this when the part I was using didn't require a rear bumper and used too long of a screw.

Always pay attention if you are using longer than stock screws when installing steering knuckles to the a-arms or gearbox covers. You want to make sure those screws aren't too long and come in contact with moving parts such as axles.or input shafts.