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I like a thumb safety on a firearm whenever possible. One of the ways to lock a Sten from firing is to restrict the disconnector (tripping lever) from moving forward. I modified the full auto selector so it would do that.

For the purposes of these drawings, converting the fire selector to a safety assumes that the receiver tube has been cut such that the disconnector is retrained in a narrow channel and cannot move sideways as on the full auto.

Yellow arrow indicates spline. The hole needs a slot for the spline to ride in. I also ground off the R & A markings and stamped F & S.
Mill the selector on the bottom, as shown, flush with the full auto slot. This will allow the selector to pass over the top of the disconnector. Braze a small spline on the top (or bottom) of the selector to keep it from rotating while in the FIRE position.

3/4 view showing function.
I ground a notch in the top of the disconnector and inserted a small section of an old file that had been shaped to fit. I used a heat sink to keep the sear notch from losing its temper and deep welded the piece in place. With the selector in the FIRE position the vertical safety bar will slide into the full auto groove allowing the disconnector to slid forward and release the sear. With the selector in the SAFE position the vertical bar is prevented for moving forward.

Semi-auto Sten pistol.

The photos below were scanned from the September 1943 issue of Popular Science. Left click on an image to enlarge.

British paratrooper with Sten MK2.

Canadian Sten factory.

Firing the Sten.

Sten shipment.