I have had good results by snugging the three guard screws up first, then tightening the front screw as tight as I can with a good fitting screw driver, the rear next and nearly as tight, the middle just in snug. If it is a Standard and has a forend screw it gets run in very snug, backed out, and then turned in until it stops. This has always worked for me and I have had a lot of pre 64 model 70's.
