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This comes from a set of Weinmann Type 370 calliper brakes. The two nuts you see are normally firmly located inside the plastic hood, not as they are shown.

I think they are intended to function as some sort of self-locking/clutch system, so that the nuts don't work their way off, while still not preventing the callipers from moving.

In this particular case, I am not satisfied that the nuts are securing the callipers on the bolt as tightly as they should be. As I rotate the plastic hood, the nuts move down the bolt, but then further rotation doesn't tighten mechanism; the hood just rotates over the nuts instead of rotating them.

How is this actually supposed to work?

securing nut arrangement of calliper brakes

http://www.classiclightweights.co.uk/components/weinmann-components.html says:

In the late '70s or early 1980s the domed centrebolt on the standard side-pull calipers was replaced by a plastic cover over two plain steel locknuts. These brakes were centred using an inverted allen key tool.

but that doesn't shed any light on the how the two-locknuts-in-a-plastic-hood arrangement is supposed to work.

  • Generally thin nuts like that are called Locknuts and are intended to wedge up against another nut to stop them rotating in place. So you're right about "allowing the calipers to move" if the nut was tight it would compress the brake caliper parts into each other preventing movement. A photo of the whole brake unit might help the question. – Criggie May 5 at 20:35
  • Yes, I assume it's some sort of locknut arrangement - but since they are both inside the hood, it's not easy to see how the locking works. Perhaps the first nut is intended to stop rotating first so the second locks against it, but it's really not clear. You wouldn't see anything useful in a photo of the whole unit: just the hood at the end of the pivot bolt. – Daniele Procida May 5 at 20:53
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    My take is that the plastic thingie is just a cover, to protect the nuts (and maybe prevent someone from cutting themselves on the bolt end). It's not "structural" and could be left off. – Daniel R Hicks May 5 at 22:29
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IIRC, the cover slides off by pulling on it, the two locknuts are locked against each other to adjust the caliper play (usually with the goal of creating as little slop as possible with no binding of the arms), and then the cover is pushed over the two locknuts. I believe the idea is its got lots of little notches inside so it can grip both nuts at once, making it impossible for them to vibrate loose.

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    If you lock the nuts against each other the plastic cover would provide no additional resistance to twisting. – Daniel R Hicks May 6 at 1:15
  • The cover doesn't slide off - I had to fabricate a puller to remove the nuts from the hood, and used a vice to press them back in. There's a stiff plastic lip holding them inside. There's some friction between the hood and the nuts, enough to screw on the assembly to the bolt, but it's much less than finger-tightness can overcome. – Daniele Procida May 6 at 9:14
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Nathan is correct the black plastic part is just a cap.

This diagram is for the 605 model but it's the same principle.
enter image description here

Part #6 Cap for pivot bolt

It's a cover that clips to the pivot bolt lock nuts.
It doesn't really do anything, except maybe provide a guide for the 'Weinmann brake centering tool' or cover up the ugly steel nuts.

From Weinmann 605 Brake Restoration: Addendum
Weinmann brake centering tool
Weinmann brake centering tool

Ideally, you would pull the cap off before removing the nuts from the pivot bolt.

With the cap off they adjust like any other side pull brake.
You would (it's possible to do these steps with the brake on or off the bike)

  • Move the inner lock nut toward the brake arm until the brake arms move freely and have no slop.
  • Use the outer lock nut to tighten against the inner lock nut jamming them together so that (theoretically) everything is locked in place.
  • Replace the cap for pivot bolt The cap is hard to remove. It wasn't supposed to need to come off once things were set and locked down. Wear has a way of making things need adjustment.

Brake Centering
Also from Weinmann 605 Brake Restoration: Addendum
enter image description here

If you don't have the wrench that came with the brakes....

So, what to do? The brakes are all assembled, they are working smoothly but cannot be centered. Dig out your nutdriver socket set and select the 4 mm socket. Attach it to the screwdriver handle, slip it inside the black plastic sleeve and fit it to the front of the hexagonal shaft. Center the brake caliper over the wheel rim and tighten down the nylock anchor bolt on the back of the caliper. Check the brake action and done.

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