M140 Track Stop for M140 Track. Two CNC machined 1/4? ? 1? aluminum flat bars with contours that match the curviture of the M140 Curtain Carrier, so as not to ?ding? the carrier wheel. The rear bar is threaded for a 5/16″ socket head screw to fasten the track stop to the track without needing a nut. The M140 Track Punch will quickly and efficiently add a hole to mount the M140 Track Stop.