It's a 6 mm diameter brush but I can only find 5 mm (more like 4.7 mm) brushes and they are about 1 cm long or 2 cm if you include the spring.
Oh well, let's drill out the burnt old brush with a 6.5 mm drill bit, pull out the old spring at the base, clean everything up and see.
Oh dear our 5 mm brush sinks right to the bottom of the hole and there's miles of room each side.
So we need to raise our smaller brush up a bit in its hole. On looking around for something, it looks like one end of a domestic fuse could fit. So let's break the old fuse, tap and scrape all the old fuse wire and ceramic stuff out of one end and clean it up. Nice.
A parallel pin punch allows it to be tapped into the hole, open end first. Tap it in pretty hard and check that the new brush pushes in all the way. It didn't, so I used some old bolts etc as punches to cave the end of the old fuse in a bit more to let the brush go into the hole fully.
So we now have a hole for the brush which is the right depth but there is miles of room all around it so it needs a sleeve. Enter 1/4" brake pipe which with an outer diameter of 6.4 mm is bigger than standard car brake pipe, but it's cheap and abundant and has an inner diameter of, guess what, 5 mm. Quality innit.
So we can cut off about 8 mm of this pipe, use a needle file to open it out a bit inside so that the brush slides about easily and press it firmly but carefully into the hole.
The 1/4" copper sleeve now holds the spring for the brush in and let's the brush slide freely. It even works on the car!! Awesome quality innit ;-0 ;-0