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Its size is 2.2cm times 1.5cm. The width of axle is 0.92 cm (measured on the curved area, not probably the right way to measure) for axle in the hub Shimano Nexus SG 8R20 (12mm stop washer but cannot find its axle width from service manual). Its thread is unknown.

Please, see the picture. I want to learn how to measure the tread inside the thing and other conventions by which such things are named, such as the maximum torgue in Nm, so I can buy the thing in any unknown store. My local hardware store said that you cannot get it as bulk product because it has special thread. I am however interested whether there is any cheaper non-bicycle-store substitute to this thing. If not, I am still looking for ways to substitute this thing for inner hubs.

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[Update, more images]

You can see that the left cap nut is in a very bad condition, it was new 6 months ago. The washer seems to be a bit damaged when looked from the side. The metal spaghetti shows that the internals of the cap nuts are broken.

enter image description here

The serrated washer eats into the paint, making very bad to the frame.

enter image description here

I am looking something better options to this inner hub thing, the expensive easily-broken cap nuts and damaging paint are not the way to go again. enter image description here

Related

  • question with similar sizing problem covering nuts and axles here but front wheel
  • Harris explaining the axle hassle that affects the nuts here
  • investigating only the axle length, probably the reason why the cap nuts gets broken seasonally, here
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Are the washers attached to the nut or separate? Is there a lock or something at the outer end of the nut, or is that the shaft end showing through? Can you post a picture of the parts disassembled? –  Wayne Johnston Apr 17 '11 at 1:13
    
@Wayne Johnston: thank you for the notice. Images added, please, let me know if something is still uncertain. I will do my best to focus on the question. –  user652 Apr 17 '11 at 7:29
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1 Answer 1

up vote 3 down vote accepted

They called cap nuts, and often the washers will have an anti-rotation tab on them that is bent in so it slots into the dropout. They have an oval hole in them to lock onto the flats on the axle.

If you're lucky the thread is an M10 fine (M10x0.1mm) and an industrial fastener outlet will have them (I use www.bolt.com.au), but there are a few fairly common sizes around so it would pay to make sure. If you are working on older bikes and buying tools a small thread pitch gauge would be a useful thing to have, they're under $20 here. But trying to measure the thread inside a cap nut with one is very hard work and youŕe likely to get it wrong. The better alternative is to buy an axle thread set - basically a short section of each of the common threads on a backing plate. You try the axle or nut you have on each until you find the one that it fits (the tool is clearly labeled).

From your torque question and the pic above, I suspect you are trying to do up the nuts to stop the wheel sliding forward in the dropout. For that you need serrated washers that cut into the dropout. And damage it. The better option is a chain tug as previously discussed here. A torgue wrench won help much with old nuts and axles, as they tend to lose strength as they wear.

Edit after the new photos: The Shimano anti-rotation washers are nice and thick, also serrated, so need to go on the outside of the frame (where they are now). You've already done enough damage to the dropouts by digging things into them. So you need a thin chain tug to fit inside between frame and hub. Also one that you can file off any protruding aligning bits, since your dropout opens forward rather than backward. I would look for one that has two screws rather than one, so it will be more stable on that curve at the back of the dropout. I have one like that on my commuter.

Edited your photo to add the tug or at least an outline of where it should go - obviously behind the frame rather than in front of it as sketched.

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Oh, and do not ask why bicycle axles are available in 10mm x 26tpi, an abominable mix of units if ever there was one. Worse, you get both 26 and 28 tpi in 3/8" and 10mm. Nuts of one size will work on some of the other sizes, but not well. My theory is that it is because the French are bastards, but I'm sure it's more to do with being able to cut 10mm axles on an imperial unit lathe, but changing over the thread cutting gears to metric is expensive. So they used the lathes they had with the new metric sizing. A gift to future generations that just keeps on giving. –  Мסž Apr 17 '11 at 4:01
    
@moz: what is really the chain tug? The picture is from about 3 years old 8 hub gear bike. After every winter, I have the problem that the old nuts fail and the serrated washer eats into the painted frame. So the chain tug won't replace the cap nuts but they somehow tighten axle to the frame or? When using chain tug, can I use my old broken cap nuts for tightening or do the chain tug replace the cap nut and the serrated washer? The problem I have been having is the sliding axle in the horizontal dropout. Are the chain tugs one-fit-them-all? –  user652 Apr 17 '11 at 6:38
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one-fits-most. The chain tug replaces the washer, as long as the washer is not an anti-rotation one. If it is you need to put the washer on the inside of the dropout and the tug on the outside. Or vice versa. See the linked article for photos and links –  Мסž Apr 17 '11 at 6:57
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The DMR one will not work for you - the bolt part of it needs to sit in the slot of the dropout, but your slot runs forward not backward. The "genetic" one will work. Put it on the axle closest to the hub with the bolts on the outside (the printed part towards the hub). The bolts should rest on the curved part of your dropout, so they're accessible from the back of the bike. –  Мסž Apr 17 '11 at 23:16
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@hhh: that's there to annoy you. File it off :) Normally it would sit in the dropout slot and help align everything, but your dropout runs the wrong way for that so the stud is just an annoyance. –  Мסž Apr 27 '11 at 21:20
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