Hello. Hopefully a simple question but maybe not. What causes end shake? I just put a end shake bearing in a friends model 18 and thought I know what it is but how does it come about? Thanks Bill

Recoil causes the body of the cylinder to move forceably forward (frame moves back-the cylinder remains static) and impact the end of the yoke barrel inside the cylinder cavity. This repeated impact during firing causes the gauge to open up between the end of the yoke barrel and the cylinder assembly.
This can also cause end shake on the yoke as well, at it's interface with the yoke screw.

Other lesser wear contributors include rotation of the cylinder during use, dirt and shooting debris inside the cylinder assembly, and opening and closing of the cylinder.

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Ret. LE, FA Instr, S&W Armorer