Bicycle Tutor

In cleaning my rather old bike yesterday, I noticed the plastic plate (does it have a name?) was brittle and cracking. To replace it, I believe I need to remove the cogs, pop off the old, and pop on the new. Has anyone done this? Are they one size fits all for your rim size?
Thanks
Bruce

The disc is there to stop the rear derailleur going into the spokes and damaging them.
You actually don't need to have one unless there is a danger of the derailleur overshifting.
There is usually a stop screw, on the derailleur, to counteract this.
If you look at current bikes they don't have the disc.
I would expect all discs to be standard and possibly you could make your own out of suitable plastic.

Thanks. I was at the bike shop today and got a similar answer. They say some people actually break them off and never use them.