Firstly, thanks for a great thread - a quick search has given me all the answers I was looking for.

Can I just confirm this is correct:
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As the adjustment is on the top of the box, you need 19mm socket to break the lock nut loose, 19mm wrench to counterhold the lock nut & a long 6mm allen socket to adjust. Turning the 6mm allen COUNTER-CLOCK WISE takes out the play!
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No reason to doubt the poster, except that it seems illogical to somone who doesn't know the internals of these steering boxes!

Cheers,

Nick Vyse
London