S&W K Frame Model 66 Strain Screw Square Butt .357 Magnum – Part# 64173

Smith & Wesson K Frame Model 66 Strain Screw – Square Butt, Old Style

Fine-tune your trigger pull on your Model 66 with this factory-original strain screw. The strain screw threads into the grip frame and controls the tension on the mainspring — adjusting it affects trigger pull weight in both single and double action. This old-style square butt stainless steel screw is the correct replacement for the Model 66 Combat Magnum chambered in .357 Magnum.

• Part Number: 64173
• Fits: Smith & Wesson K Frame Model 66 (Square Butt)
• Caliber: .357 Magnum / .38 Special
• Part Type: Strain Screw
• Style: Old Style
• Material: Stainless Steel
• Frame: Square Butt
• Condition: Used (factory original)
• Installation: Gunsmith fitting recommended

Factory Replacement Quality

Genuine Smith & Wesson factory part manufactured to the exact specifications and tolerances of the original — same thread pitch, same dimensions, same reliable mainspring tension control. No aftermarket substitutes.

Strain Screw & Trigger Pull

The strain screw is one of the few user-adjustable components on a S&W revolver. Backing it out slightly reduces mainspring tension and lightens the trigger pull — however, this can also cause light primer strikes. Always verify reliable ignition after any adjustment. Gunsmith guidance is recommended.

Finding Your Model Number

Swing out the cylinder and check behind the yoke on the frame. Model numbers have been stamped there since 1957–58. If your revolver predates that, contact us with the serial number and we'll help identify the correct part.

Questions? Call or email us — we're happy to help you get the right part the first time.