10mm axle.. 9mm dropouts..
Bicycle frames come in many different sizes.. some for quick release hubs, others with bolt-on type axles..
On this bicycle ( Norco Origami - Folding bike ) the front forks are 9mm.
The Crystalyte 408 axle however, is 10mm wide on the flat portion.
The first time I installed a hub motor on a front fork, a number of years back, my first reaction was to grind the forks and make room for the axle.. However, a better/safer option is to grind the axle on the motor to fit into the dropouts. Considering that all that is needed to make this motor fit is 1 mm, or 0.5mm per side, it's not a huge amount of material to remove, but the fork should be left with as much metal as possible..
First step is to tape up the area to prevent fine metal dust from getting into the bearings. Then take out the dremel tool and a bunch of cutting disks. I found that by stacking a number of disks together you get a nice wide grinding surface to work with..
Then using a caliper, measure out the 9mm dropouts on the fork ( you could just eyeball it and keep taking a bit off at a time, then check the axle on the fork for a tight fit.. but i find that a caliper saves time.
Now, don't be in a hurry here, take a shaving and then check with the caliper.. and keep working little by little until it just barely fits..
Almost.. but not quite.
As you can see above, the axle was not ground quite enough and would not seat flush into the dropouts.. the flat washer on the inside of the fork has a gap between it and the axle.. As much as you really just want to bolt this thing on and ride.. resist the temptation to leave it be and do the job right.. back to the dremel and another few minutes of work..