^ " b l " is right on ^ You only need 3 threads to get the maximum holding capacity of the 1/4"x28tpi bolts. Put your finger inside the action on the hole and run the screw in until you can just feel it. This should be about 3 1/2 threads, but if you don't quite make 3 threads, it's OK. Just make sure the bolt is not sticking up above the inside action surface. Mark the bolt on the outside of the action with a sharpie. This is the amount that sticks up past your stock into the action. The length needed for the screw to come through the stock, plus the amount in the action is the total length you cut the screw to, above the head.

During the 90's, I dealt with McMaster-Carr almost every week, and found them to be one first class out fit. They were a real pleasure to work with. May it still be so. :-)