ballnut - balls back in via the side cover and using less balls than original

[Jonathan]

The main question is, am I ok with 5 or so balls missing, or is it going to cause issues? another idea to get around this is put everything apart from the 3, then use some grease on the back of the cover where the return grooves are and stick the 3 there, before putting the cover on.

Having balls missing will effectively reduce the load rating of the nut, which in turn will shorten its life. So if this ballscrew is for your average diy CNC router that's probably not an issue, as if you calculate the lifetime with the relatively small forces on a router, they'll last for many years. Many years minus a bit is still a long time.

However, you should be able to get them all in. The grease idea sounds good. You could put as many as you can in, unscrew the nut on to a tube, stick a large magnet to the side of it (so the balls don't fall out), then poke the rest of them in. That's the way you'd do it if there was no access through the return channel, as it's generally more difficult than the former. The following pictures show this process: