Modifying a Unit Wheel Bearing Hub for Greasing

Thinking about taking a used pair of unit bearing hubs that I took off and modifying them. Pressed one apart, basically a cylinder with 2 plastic ball bearing race ways, that hold two inner tubes, one splined for the axle. Thinking of drilling into the side of the hub, threading it an installing a grease fitting. One thing I would like to know is does anyone know what size hole and what tap I would use for the grease zerk? I saw on you tube and a few other write ups where guys are removing the abs wheel speed sensor, pumping in grease then reinstalling the speed sensor. Any and all thoughts and comments on this idea would be appreciated.
Thanks,
Bill

I did this exact modification to a set of boat trailer hubs but that they were std removable hubs and it was for convenience only. With a press-together assembly like this, you need to apply the proper preload to the bearings or it'll destroy them in no time. You'll need to do the modification to a new set of hubs and hope you don't get metal shavings in there. Or switch to early model knuckles and hubs, which are rebuildable. OEM at least; not sure about aftermarket replacements. The smallest std grease fitting is 1/16" NPT (which is actually ~1/4" dia).

I refreshed a pair of old hubs and put them on a set of spare shafts for quick swapping. I didn't press the hubs out, I just popped the seal off the back, washed out the old grease and junk, then repacked the housings with red and tacky.

The hubs with the rubber seals (OEM, SKF) are easier to open and reseal than the ones with metal seals (Timken)