For .60 size Sierras I used No.6 cap screws 3/4" to 1" depending on the rail thickness. The Sierra case is stouter (and heavier) than a lot of the other retracts, so I liked to screw them in good and a hex cap seem to work better than various screw type heads.
For other retracts (Robart air and various mechanical retracts) I use 3mm self tapping screws I get from Airborne Models. Generally the cases on these retracts are not as strong. If the rails are not exactly square, pulling them down really tight will put them in a bind. May hang up then. 3 x 14 for .46 planes and 3 x 18 for .60 planes. These screws come with Airborne's planes with mechanical retracts and seem to hold better than others I have tried. Don't know why.
I use the pilot hole and CA method mentioned earlier.
The Sierras for my GS TF P-51 came with a solid, rigid base plate the retracts screwed onto. I think the rigid plate/heavy retract case spread the landing shock out more evenly across the rails. I did not have any problems with retract rails after I installed these Sierras.
Brian