THE WORLD LEADER IN HIGH-PERFORMANCE PARTS FOR CROSMAN & BENJAMIN AIRGUNS
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DRILLING SIDE VALVE-SCREW HOLES
( with common hand-tools )
MARK HOLE LOCATIONS
1) At both edges of the bottom valve-screw hole, wrap two pieces of masking tape perfectly straight around the diameter of the tube.
2) Measure and mark the exact center between the tape strips on both sides of the tube. Use a scribe or similar sharp object to make precise marks.
3) Remove one tape strip. Before removing the other, use a razor blade to cut it perfectly straight across where the two ends overlap. A straight-edged ruler can be used as a cutting guide, line it up perfectly parallel with the tube to get the cut square.
4) Remove the trimmed piece of tape that is now the exact length of the tube diameter. Place it on a clean, smooth surface sticky-side down. Measure and mark the exact middle of the tape length near one edge. An easy way is to peel and fold one end back until it's even with the other, then mark the middle at one edge of the fold.
5) On the same edge as your middle mark, mark the tape at the exact center between the middle mark and each end. These are your side-screw location marks.
6) Apply that tape perfectly straight around the tube, the the middle mark centered in the valve-port hole, the end corners centered in the bottom hole.
Use a scratch awl or center punch to dimple the tube at the screw-hole center marks. This helps prevent the bit from wandering. Use sharp bits, cobalt works better, at slow rpm to avoid chatter. Cutting or motor oil should be used to help the bits cut clean. Start with a small bit like 1/16 or 5/64 to stay on center, then step up to larger sizes in small increments. The final size for 22XX tubes should be 9/32 and fit the heads of my custom 22XX valve screws (PCP pistol valve screws) relatively snug. The edges of the holes inside the tube need to be deburred after, the valve may not fit if there's any burrs or high edges protruding into the tube.
Do not use a lot of force on the tube when dimpling or drilling. This can indent the tube and cause the valve to not fit after. The Magnum Valves are slightly larger diameter than standard valves so this is more critical for those.