The correct procedure for tightening the ATD is a) make sure the tapers are clean and undamaged. b) with the ATD removed completely, fit the wedges to hold the mechanism open, make sure the nut is pushed all the way into the ATD trapping the dished washer and horseshoe washer c) Offer the ATD to the mag spindle gently, left hand on the mechanism, right hand keeping the nut pushed against the horseshoe washer until you feel the nut touch the mag spindle threads d) gently turn the nut clockwise onto the threads whilst pulling the ATD away from the engine to maintain pressure between the nut and horseshoe washer e) run the nut up finger tight then back it off about 1/2 a turn and pull the mechanism off the taper to enable free rotation whilst timing the engine. If you push the ATD onto the taper with the nut very loose you run the risk of the left hand extractor threads biting (as mentioned before) before the right hand threads of the nut engage on the mag shaft. The only other explanation I can think of is the nut bottoming on the mag shaft before the taper is tight. The mag shaft thread is 3/8 BSF so if you get a 3/8 bolt or bottoming tap you can run it into the ATD and compare with the length of the mag shaft, or even try the ATD on a spare magneto I you have one. Having repaired lots of mags and ATD's it is very unusual to come across an ATD that doesn't stay on if it's tightened up properly.
