I used 15mm x 5mm pitch screws with a 5:4 belt reduction. Originally 5:2 before I upgraded the drivers.

If the Gibbs aren't held firmly in position the slides are tight in one direction, loose in the other. You just can't get the adjustment right.

I made some studs that screwed in where the Warco adjustors used to screw in. That gave me a very fine pitch 8mm male thread and a flanged adjuster nut rather than a bolt. I threaded the stud internally 5mm to hold a cap that pinches on the end of the Gibb. Can't turn, can't slop.