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Continued on next page... The CGA 510 connection has left-hand threads, internal on the cylinder outlet; the CGA 300 connection has right- hand threads, external on the cylinder outlet. Tighten both regulator connection nuts firmly with a wrench. (Avoid use of adjustable wrenches. Wrenches with two or more fixed openings, designed specifically for use with gas welding and cutting apparatus, are available from all apparatus suppliers.) PRECAUTION: Should it ever be necessary to retighten the regulator union nut after the outfit has been set up and the cylinder valves opened, be sure to close the cylinder valve before tightening the nut. Rotate the pressure-adjusting screw of each regulator to the left (counter-clockwise) until it turns freely.  It is essential that the valve in the regulator be closed before cylinder pressure is admitted to the regulator. To achieve this, the pressure-adjusting screw must be backed off until it no longer is pushing against the pressure- adjusting spring in the regulator. Fig. 6-2. Always tighten regulator connection nuts with a wrench. Fig. 6-1. Always crack cylinder valves, to blow out dust and dirt from the outlets.