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I have a rather old frame that doesn't have threads on the rear rack braze-ons, and thus requires a nut. This nut makes contact with the chain if the chain is on the outer sprocket, so I'd like to tap out threads in my braze-on so the nut won't be necessary.

If anyone has experience with tapping new threads in a braze-on, could you give some instructions on how to do it properly?

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It's not hard. You need the tap, of course, and, while you can make do with pliers, your basic T-handle tap wrench helps a lot to do the job right. You need some sort of oil (pretty much any oil will work). The hole needs to be close to the right size. Get a "long nosed" tap, not a "blind" tap, so it's easier to get it started - remove the chain or wheel if it will be in the way. Put a few drops of oil in the hole. Keep the tap as square to the work as possible. Work it in and out, about a half-turn at a time. Withdraw the tap all the way, wipe it clean, and re-oil about every 2 turns. –  Daniel R Hicks Mar 24 at 12:09
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(And ideally pick your screws first, so you get a tap with a matching thread. Bike threads should generally be metric, and there are a few standard sizes.) –  Daniel R Hicks Mar 24 at 12:11
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Until you get the hole tapped, you can do what I did. I had the nut in the chain problem on my bike. I ended up filing the head of the bolt down and using the nut on the outside of the frame. The eyelets on my frame are a larger size than stock so the bolts that came with my stuff fit in them, but wouldn't thread. Instead of drilling out holes, I just hit the nuts n' bolts bin at the hardware store. –  BPugh Mar 24 at 12:26
    
@BPugh's comment is an answer to the underlying problem, even if not to the question. With the right length bolts and dome nuts (called something else in en-US - acorn nuts?) you can avoid hard/sharp edges that may otherwise snag. you might have to take the wheel off the fit them, depending how long they are. Low-head bolts/screws are available mail-order at least, to avoid cutting down the heads. –  Chris H Mar 24 at 14:03
    
So I've actually already done quite a bit of shortening the bolt and nut combo-I can get close to letting the chain completely clear, but it's still a bit too close for comfort. The best solution that I've found is to get a bolt that is a bit longer than the right length, and than effectively shorten the bolt with washers on the outside to get the length exactly right. Then you just have to find the shortest nut possible and put on some lock tight, like Chris H suggested in the answer below. –  T.C. Proctor Mar 25 at 14:48

2 Answers 2

up vote 6 down vote accepted

It shouldn't be too different to tapping out anything else, the added difficulty being that you would normally clamp the workpiece in a vice and you don't have that option. I'd take the wheel off and lie the bike down on a bench with scrapwood in place of the axle (try to clamp/screw it in place) to give you something to push against other than the frame.

Is the frame alloy or steel? Especially if it's steel you really need to make sure the pilot hole is exactly the right size first - which probably means buying a drill bit specially. Getting a tap started in steel isn't all that easy especially as you can't get the frame in a vice, in alloy it's not too bad - so a brand new decent quality tap is a good idea for steel,as is just smoothing the edge of the pilot hole with a bigger drill or countersink tool before you start. If you're buying the tap, get a taper type - they start much easier than a second, and you've got plenty of room to come out the other side with the wheel off. A decent tap wrench is worth the extra money over the junk that sometimes comes in cheap tap/die sets: you want the tap to be held securely.

Even if the pilot hole is already the right size for your tap, running a drill down it to take the paint off won't hurt.

Chain oil should do as a cutting fluid if you don't have anything better, then once you've got a thread started remember to back the tap off every now and then (e.g. forwards half to one turn, back a quarter).

Make sure to clean up all the swarf afterwards so it doesn't get anywhere important.

I assume the chain is catching on the nut rather than the bolt itself - you can buy half nuts / lock nuts which are thinner. You'd need to threadlock it on.

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Thanks for the advice. How exactly should I go about choosing the tap diameter for my hole? –  T.C. Proctor Mar 25 at 14:43
    
@T.C.Proctor, you need the existing hole size to be less than or equal to the tapping/pilot drill size for your thread of choice, and you should pick a thread that's commonly used for such parts, like M4 or M5, and available in a sensible length. You should also check the size of the clearance hole in the rack you're attaching - try to avoid drilling that out as well. –  Chris H Mar 25 at 14:52
    
With regards to the nuts, "jam nuts" seems to be the name half nuts commonly go by, at least in the US. They are hard to find though - I could only find them on the internet. –  T.C. Proctor Mar 27 at 14:41
    
@T.C.Proctor Thanks for the translation - hardware terms appear to be one of the worst areas of confusion between different flavours of English. I usually resort to the many specialist shops on ebay for that sort of thing. –  Chris H Mar 27 at 15:25

I wasn't going to answer the question with this, but due to the latest comment from the seeker, I figured that some photos will be useful.

So until you get the braze-ons tapped, here is a backup solution (or a permanent if you want) that worked for me. I found that the hardware that came with my rack was too small to thread into my bike, but did fit through the holes. Bolts that thread into my frame didn't fit through the holes in the rack. So what I ended up doing was taking the original bolts with me to the hardware store, rummage through the bins for 2 hex bolts, lock nuts, and lock washers that are the same size, but longer.

I put them on the bike with the nuts on the outside of the frame, but I still had a problem of the bolt head into the chain. My solution was to file off the top of the bolt head until it cleared the chain. It ended up being pretty close to the chain, so if a replacement is wider for some reason, I'll have to file some more. However, this setup has held up well for years, a few thousand miles, and a crash.

close-up of bolts on inside of frame (full size image)

This photo (sorry about the dirt) shows the bolt head side of the setup. There is 2 bolts here, one on front for the rack, and the one on the back for the fenders. The shiney here is the fender bolt, but the bolt on the front is even closer to the chain. May not be able to get a piece of paper in there. Yes the chain runs right by it and not over it.

Bolts on outside of frame Here is the outside of the frame showing the nuts. Standard lock nuts with the plastic ring in them. The washers are the wavy lock washers and not the split ring ones.

I may update this with better photos later.

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I found that if you open the images in a new tab, you maybe able to zoom it in from there. Worked for me in Chrome –  BPugh Mar 26 at 0:00
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I cropped the first image to make it more obvious where to look. There's a link to your original. Don't be afraid to crop images rather than trying to use them straight out of the camera. –  Mσᶎ Mar 26 at 22:55
    
Note that, for the bolt-from-the-inside trick you can almost always find a "pan-head" bolt of the appropriate size. This will have a fairly flat, low profile head. Worst case you can buy a "carriage bolt" and file away the little tangs under the head. –  Daniel R Hicks Mar 26 at 23:52
    
Thanks @Mσᶎ I haven't found the time to edit it. –  BPugh Mar 27 at 12:37
    
@DanielRHicks is right, you can usually find appropriate sized bolts or nuts that will save the work of filing down bolt heads. "button-head" socket/Allen headed bolts seem to have heads that are even shorter than a "pan-head" bolt. –  T.C. Proctor Mar 27 at 14:38

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