Hi niko,
Thread size not really critical, as long as the size is the same for all six knobs.
I'm thinking the knobs would be minimum 50mm diameter, with a 50mm bolt.
Or...
The knobs could be threaded, i.e. 'female', to screw on to bolts fixed in the eyepiece box and the scope.
Hope I'm explaining my ideas OK?
I suspect Bunnings might have them, but $15 on fuel just to find out is a bit rich!
Hence I'm doing some research before I hit the road!
Thanks
Chris