Bicycle Mechanics - What is this brake washer?

Hey, this is my first post after lurking these forums for ages:)
My main bike is a 1993 Trek 1100, and to day I took the Shimano Exage brakes off today to clean right in behind them (it was unexpectedly muddy today.) When I took the rear one off, this washer fell off in two pieces! This is what it looks like super-glued back together, but I'd like to replace it if possible. I haven't been into the LBS yet but wondered if anyone knew what it was called or where to get one?
It goes between the locknut and the brake mount on the frame, I guess to keep it in position.

Thanks!

It's a serrated washer. It goes between the pivot or mounting block of the brake and the frame. The serrations bite into both the brake and frame to prevent the brake from moving off center without the need for high bolt torque.

If you use this, be sure the brake is centered as you first tighten it, because it bites into both creating sort of an index effect which sometimes makes small corrections more difficult. I'm not a fan, and use coarse lapping compound (grit in grease) between the brake and frame, which gives me the bite, but doesn't cause the indexing effect.

Thanks! I'll see if the shop has them tomorrow, if not I may take you up on the offer:) I had wondered if I could replace them with something else, it seems like a great way to damage the soft aluminium on the brake mount.

You can also get non-serrated washers, and really...if you set up your breaks correctly...you don't need it to be serrated.

That's what I wonderd, if I can just put a couple of regular washers in there instead of having a gnarly one chewing up my bike. I'll see what they have at the shop as I don't even have any regular washers of that size laying around. Cheers:)

That's what I wonderd, if I can just put a couple of regular washers in there instead of having a gnarly one chewing up my bike. I'll see what they have at the shop as I don't even have any regular washers of that size laying around. Cheers:)

You don't need or want plain washers between the frame and brake. Washers (non-serrated) are to allow movement between two parts, such as when under a nut. In this application you don't want movement, so washers would be counter-productive and you should not use any.

In case you need a spacer to push the brake out away from the frame, try to use a single spacer rather than stacked washers.

Serrated steel or smooth aluminum brake washers are available in various thicknesses at any LBS.

Depending on the shape of your frame's brake bridge they can be cupped on one side for tubular bridges or flat on both sides if the bridge has a flat area where the brake mounts.

If you don't want them I'll take them. I love them.

The bike shop gave me one and it works, so awesome! Thanks for all the advice, it can be frustrating not knowing what something is called and sounding like an idiot asking for 'one of those thingies' at the shop (if you are unable to take the part in.)