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SRAM road brake calipers (mine are specifically Rival from 2011) have a quick release lever to facilitate wheel changes, as many calipers do. The SRAM QR lever has detents which make a little "click click click" noise and feeling as you rotate them. Today I found out how that detent works, and wish I hadn't.

You see, I was replacing the M4x6 button-head screws on the QR "axle", as they were quite rusty (see SRAM Rival brake caliper screws/bolts are rusty, how to replace?). I had unscrewed one of them a couple weeks ago to check the screw size, ordered the screws, and now had them in hand. When I unscrewed the second one, I heard a little "ping" as something fell down on the floor. I searched around for it, and behold, it was a tiny ball bearing!

So in the process of opening up the first brake a couple weeks ago, I'm sure the ball popped out of it, and the floor has been vacuumed since then so goodbye.

The question is: can someone tell me what size the ball bearing is? I can tell you it's roughly 1-2mm, but I don't have sufficiently precise equipment to measure it. Bonus points for a link to purchase replacements (in small quantities!).

As an aside, this design seems comically bad: they use a screw which for some reason is more prone to rust than any of the others in the groupset, and when you unscrew it there is this tiny metal ball sitting on a tiny metal spring waiting to eject it from the assembly. It's no wonder why they don't call out these parts in the service manual!

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    I have to ask - what possessed you to replace the screw anyway?!?! – Batman Jan 26 '15 at 14:50
  • @Batman: They were very rusty. There were only five or six rusty screws on my otherwise shiny bike, so why not replace them with ones that don't rust? – John Zwinck Jan 26 '15 at 15:32
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    I would think that you'd actually have to measure it, as bearing could be made in any size, unless someone can point to information on definitive information from the manufacturer, It would be best to measure it. Take the one you have into a bearing supply store and they should be able to measure it with a micrometer, and find you a replacement. – Kibbee Jan 26 '15 at 15:41
  • Likely any bearing of about the right size will work -- doesn't have to be exact. And any bike shop should have an assortment of balls, if you can't find something in your junk drawer. – Daniel R Hicks Jan 26 '15 at 16:33
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    Then buy a micrometer. Or go to a jeweler or pawn shop - they will have one. A micrometer is a good tool to have and they are not that expensive. – paparazzo Jan 28 '15 at 1:36
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The balls are 2mm in diameter.

How do I know? Well, I found a bearing supply shop, and asked them to see some of their smallest balls. I bought one 2mm ball and one 3/32" (2.38mm) ball for all of 20 cents. The 3/32" ball was slightly too large to fit in the hole, so I tried the 2mm one, and bingo--a perfect fit. Putting the QR mechanism back together is a pain because the tiny 2mm spring wants to eject the ball again, but with the brake facing straight down it's doable.

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I have just spent 2 hours searching in the grass for that blasted ball bearing!

I took the other out and have measured it. Since I have no micrometer, I set up a camera and place the bearing on a high quality tape measure. I photographed the ball on the scale. Cropped the image and blew it up to 400 odd times. Then measured the bearing on my screen with the tape measure, compared it to the length of 1mm.

The result is that the ball was 134 mm on my screen, and in the same image, 1mm was 65 mm.

With this in mind, I find the ball to be 2.0615 mm. Since this is an american product, I am assuming it is an imperial ball bearing of size 5/64th inches. This is within my error bars.

Given the task the ball bearing has, I would say either a 2mm ball bearing or a 5/64" ball bearing would do the job perfectly. As my housemate says, "that's as close as feck is to swearing".

Mr Winter (Physics teacher too impatient to wait for a micrometer)

  • You're right that SRAM is an American company, but most (all?) of their hardware is metric-sized, because they're compatible with Shimano and the rest of the world. I'm sure a 5/64" ball would fit too, if you can find one--my local supplier didn't stock 64ths. – John Zwinck Aug 23 '15 at 1:36

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