You can actually get away with pretty small screws on large engines. A 4-40 screw is quite strong. I've used 4-40 screws to hold a YS160DZ to a mount, holds just fine. 1000+ flights, etc. These were however run into blindnuts that were sunk into the bottom of the nylon rails via application of heat. If just tapping the rail, I wouldn't use a 4-40 on a larger engine and would scale up bolt size as Hoss posted. BTW, the blind nut trick on the bottom of the rails is a good solution if you don't have easy access to the bottom of the rails to use a locknut, or if you just don't want to mess with the lock nut method.