@PigletJohn : Bear with me here.
It's all one piece but looks like a hexagonal "nut" with a flange.
It's completely smoothe ie. there is no thread or grooves inside the flange bit.
At widest point of nut- 20mm
Nut part - 3mm deep.
Flange height - 9mm
Overall height of whole thing - 12mm
Width of the hole through centre - 9mm.
The whole thing fits in horizontally, through the centre of our shower door roller. Pic of door and rollers attatched.
A bolt-type bit of kit, slots through the flange and sits in the centre of the roller.
It's purpose seems to be, to protect the hole in the glass door from being worn/ground away by the metal bolt as the door is slid to and fro.
The bolt is narrower (6mm) and can turn freely inside the flange.
The purpose of the nut bit appears to be to protect the glass door while enabling the roller to be tightened up.
I discovered the problem when shower door became a bit wobbly and I went to tighten the rollers with an Allen key.
Bits of perished plastic or perspex or whatever it is, just fell off as I removed the covers so I'm unable to adjust the rollers and, sods law, door is more wobbly than ever.
I'd be happy to buy new rollers if necessary but I can't find these anywhere either.
They all seem to be white, nylon types now, where these are slightly concave so they can roll along the top of the chunky rail.
All suggestions gratefully received.
DH has suggested a rubber washer and a bit of clear PVC tubing would do the job - I'm not convinced.