Hi Paul,
Some of the dimensions are down to the motor - NEMA14's have a face that is 35.2mm square, hence that as a weird dimension. Others are based on measuring my mount to se what fit. But from what I have heard from others is that there is so much variation on the powder coating of the mount (and even the machining of it) that the dimensions will work for some but may need fine tuning on others.
If you can't get it to fit flat on the RA, I presume it is because the powder coating curves? If so, the easiest way would be to try and round the bottom of the bracket with a file. There are two 'rails' on the RA mounting point which the bracket should sit neatly in between to help keep it aligned straight with the worm gear so the pulleys aren't at funny angles from each other.
They brackets are designed so that you can adjust the positioning once you get the belts and pulleys to ensure enough tension on the belt and alignment of the pulleys.
Regards,
Tom.
