It can be quite hard to see what thread that shaft is as the treads barely exit the lock nut but I was able to tell by looking closely. A magnifying glass might help these days though. A little wire brushing may clean up what is visible and make it easier to tell.
I only found out there could be either thread after getting it wrong and stripping the thread out of the aluminium cap. That meant buying a used replacement box off ebay.
The allen key required should be 3/16" (4.76mm), nut should be 5/8" (16mm) across.
There are two adjustments on the box that can cause slop in the steering. One is the sector shaft gears which are adjusted with that rod on the cover. The other is the input shaft thrust bearings.
The input shaft preload is set by undoing the lock ring, then screwing in the cap till it stops. Mark the cap and case, then mark the case 1/2" (12.7mm) back from the mark you just made. Unscrew the cap till the mark on the cap aligns with the second mark on the case and tighten the lock ring.
The proper method for sector shaft adjustment requires the use of an in.lb beam or dial type torque wrench. It is ideally done with the box in a bench vice, with the box being bled of fluid. This can be done by winding the box from stop to stop with a suitably sized 12-point socket wedged on to the input shaft, and can get quite messy. The steering box should be centered by counting turns stop to stop, and setting halfway.
With the box centered, the torque wrench should be used to check the torque required to turn the box between 45 degrees either side of center. The highest torque seen is the over center torque. The torque should then be checked at 90-180 degrees either side of center. The average of the torque seen at right and left is the preload torque. The adjustment shaft, should be screwed in (or out), in small increments until the over center torque is 3-7 in.lb greater than the preload torque.
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