I've just now stripped my newly built chassis to see if there's a problem with the bearing not sitting flush in the moulding, turns out the teeth on the idler are catching on the edge of this bearing, as shown:
And a pic of the protruding bearing, by 1mm:
I've shown in the pic below from the top of the step to flush is, as you say 3mm, the full depth of the hole is just over 4mm. Comparing this with the original tc02c g/box moulding (not pictured), which also has a step but measures 4mm from the top of the step to flush, in other words there's less of a step thus allowing the bearing to sit flush.
I guess if I hadn't inspected it again, in my instance the edge of the idler teeth would've been worn away by the bearing. I thought I could hear a slight 'notchy' sound when I rotated the tranny by hand, that's why.
I may end up dremalling the step to 4mm deep but I guess I shouldn't really have to...
The only part I don't get is if my layshaft still rotates freely with the 1mm protruding bearing, will it end up with 1mm of side play if the bearing is seated flush ? The same layshaft when fitted to the tc02c didn't have any play..