ADJUSTING THE CROWN WHEEL & PINION

With thanks to the Bristol Austin Seven Club magazine 1983 and Solent A7C

Adjusting Back Axle Crown Wheel/Pinion Back Lash

The Austin Seven Companion, full of technical information, gives lots of advice on back axles, including setting the crown wheel/pinion back lash to 4 thou. equivalent to 1/16 inch movement at a radius of 7 inch from the centre of the pinion flange.

The problem is that you cannot tell if the back lash you are measuring is between the crown wheel and pinion or elsewhere in the differential. To be sure you must prevent the crown wheel from turning and this is one way of doing that.

Take an old back axle drain plug, preferably a brass one on which the hexagon has been damaged by use of stilsons etc. Drill the plug to 7/16 inch and tap a 1/2-inch BSF thread; insert a 1/2-inch BSF setscrew on which the end has been ground to a blunt taper of a suitable size to fit between the teeth of the crown wheel. When you screw the drain plug into the drain hole, by good fortune, the set screw lines up the crown wheel teeth.

Now you are ready to set up the clearance to 4 thou. You have bolted a 7-inch pointer onto the pinion flange on the front end of the torque tube (or a 14-inch pointer if you prefer to see 1/8 inch movement). Screw the set screw into the crown wheel teeth using your fingers. As you feed in the screw rotate the pinion and you will feel the screw enter between the teeth. You will feel the screw lock the crown wheel and any play at the pinion flange is the back lash which you are trying to measure. Do not use more that light finger tightness on the locking set screw and ease back the screw when you make any adjustment to the position of the crown wheel and differential assembly.

Dave Fulton Solent A7C