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4.7 mm front brake nut thread engagement enough?

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4.7 mm front brake nut thread engagement enough?

Old 05-23-15, 04:22 PM
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4.7 mm front brake nut thread engagement enough?

I've got dual-pivot caliper brakes on a vintage fork with non-recessed mounting. My brake nut is a nylock. The total depth of the nut from crown to base is 7.7 mm. A depth gauge tells me that when fully tightened up, I've got 4.7 mm of nut thread engaged with the mounting bolt. Of the total nut depth, a good 1.5 mm at the crown is taken up by the nylon bushing, so I estimate a total threaded depth to be 6.2 mm. Is engaging 4.7 mm of the threads enough for front-brake safety? I saw various articles online written about modern-style forks with recessed mounting nuts indicating a minimum of 10 mm of thread engagement. That is 3.8 more than I would have available to me if the nut threaded farther onto the brake stud than it does, and 5.3 mm more than what I can achieve (fork-crown-brake-stud-mounted front light at the front of the fork, fender tab and fender washer at back side). I would rather not drill out the fork to take a recessed nut. Does anybody know the common standard for a vintage non-recessed fork and vintage fastening nut? Average thread depth engagement? I sure would hate to lose my front brake at speed.
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Old 05-23-15, 04:47 PM
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The general guidline for threaded fasteners is that they achieve 100% of maximum strength with engagement equal to the diameter. If you look at most commercial nuts, you'll note that their thickness is usually equal to the thread diameter, or close to that, because there's no point to making them thicker.

The reality is that you reach 100% or very close to that before the full engagement. So 4.7mm of true engagement on a 6mm bolt is borderline, but most likely close enough to pass. But there's a catch. The threads on the bolt don't usually go to the end, and odds are there's a chamfer, so while you may think you're engaging 4.7mm, in all likelihood, you're only engaging 3.7mm and that's kind of pushing it.

OTOH- I suspect you may have the nut upside down. Usually Nylok nuts are installed, thread first so the nylon ring ia the last to engage. In your case, it may not engage at all, but better thread engagement is preferable to the nylok feature. If you're concerned about loosening, you can use a nylon paint or even trap a bit of dental floss in the threads to serve as the "nylok".
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Old 05-23-15, 05:53 PM
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nylock at exit not entrance

I hadn't even considered the chamfer on the stud. I think this is a risky setup. Note: nylock insert is at the top/exit of the nut, not the entry, luckily.
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Old 05-23-15, 06:03 PM
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Originally Posted by motorapido
I hadn't even considered the chamfer on the stud. I think this is a risky setup. Note: nylock insert is at the top/exit of the nut, not the entry, luckily.
Is there a place to find a millimeter or two, ie. remove a washer someplace? If not, and if this is on the front, you can drill the back of the fork to 8mm (5/16") and use a recessed nut. No need to counterbore, the flange of the nut can sit outside the fork. If you go this route, you'll need to shorten the bolt about 4-5mm or so.

If this is a rear brake, odds favor you if you go ahead. The brake won't tear loose in use since it's pushing not pulling on the bolt.
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Old 05-23-15, 07:49 PM
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It's the front fork. I suppose I will drill out the fork and get a recessed nut. However, on a non-recessed setup, there is a washer with a shape that hugs the curve of the back of the fork. A recessed nut, without such a washer, would contact only at the two left/right sides of the curve on the back of the fork, right? What keeps this from rocking, without having the contact of the specially curved washer used for non-recessed setups? Would you not need the recessed nut to make purchase 360 degrees around the drilled-out hole?
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Old 05-23-15, 08:21 PM
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BITD- when brakes used regular nuts in the back, brakes were held onto forks like yours with the adapter in front and a plain washer in back.

You have a few choices, one of which will make your life much easier.

By skipping the rear adapter spacer, you might gain enough thread to go back to your plain nut. Or you can use a file to make a bit of a spot face in back, and stay with the nut.

Or you can drill both the fork, and back adapter and go with a recessed nut. Or you can drill the fork, and file a spot face. Or use a step drill that will make the through hole and spot face or counterbore in a single pass.

So, you know what you have, and can make a decision among the various options depending on the tools and skills you can bring to bear.
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Old 05-23-15, 09:03 PM
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Originally Posted by FBinNY
BITD- when brakes used regular nuts in the back, brakes were held onto forks like yours with the adapter in front and a plain washer in back.
Ah ha. This is new to me. So the curved adapter/washer was only to be used in front and a plain nut and a flat washer in back? Mine has the adapter/curved-washer in front AND in back. So in back, do I use just the plain nut and a plain, round, flat washer? That will allow 100% OF the nut threads to engage with the brake stud, and then some.
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Old 05-23-15, 09:11 PM
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Originally Posted by motorapido
Ah ha. This is new to me. So the curved adapter/washer was only to be used in front and a plain nut and a flat washer in back? Mine has the adapter/curved-washer in front AND in back. So in back, do I use just the plain nut and a plain, round, flat washer? That will allow 100% OF the nut threads to engage with the brake stud, and then some.
Problem solved. As I said, you might want to file a bit of a flat spot, maybe down to where the washer is supported for most of it's periphery, but there's no real need.

BTW- the fork is strong enough that it's OK to skip the adapter in back. But if you have a plain round rear bridge which takes an adapter, you need it on both sides there to prevent crushing the tube.
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Old 05-24-15, 12:50 PM
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Originally Posted by FBinNY
Problem solved. As I said, you might want to file a bit of a flat spot, maybe down to where the washer is supported for most of it's periphery, but there's no real need.

BTW- the fork is strong enough that it's OK to skip the adapter in back. But if you have a plain round rear bridge which takes an adapter, you need it on both sides there to prevent crushing the tube.
Based on this photo of the rearward side of the fork crown, am I OK putting a plain, flat washer against the back of the fork, followed by the fender mount bracket, another plain flat washer, and finally the nylock nut? With that setup, the end of the brake stud protrudes past the top of the nylock nut, assuring me of contact with 100% of the nylock nut threads. Is this variety of fork crown robust enough not to have the specially shaped washer in the back? Photo here (can't get the photo uploader tool to work, daggonit) https://1drv.ms/1SxNKSC
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Old 05-24-15, 01:00 PM
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Yes, there's no structural issue. You have about 1/8" or more steel under the nut and there's no way you can damage the fork before snapping the bolt. The only purpose the rear spacer serves is to protect the washer from bending, and to make a nicer more finished look.

If you want a nice professional job, use some body filler or "plastic steel" type product under the 1st washer letting the nut squeeze out the excess, and trimming it flush. When finished it'll look as if the fork had a flat seat back there.
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Old 05-24-15, 01:09 PM
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As always, FBinNY, I appreciate your thorough, logical and helpful replies. It's great having experienced forum members to assist with various issues, and I thank you. As to applying a cosmetic finish around the washer, I am afraid that I will skip that. I am a Fred, this is a Frankenbike, and it serves me as my urban transportation, grocery-getter, long-haul touring rig, and rail-trail gravel grinder. She's been my sweet angel since I bought her new in 1976 with my newspaper route money, and at this point I am not concerned with aesthetics, although I do think she is a dazzling beauty, in the manner of elderly Italian actresses who do not resort to plastic surgery, and instead continue to look stunningly beautiful, showing the history of their wonderful lives in the lines on their faces. The bike is a Schwinn Super Le Tour 12.2 (Panasonic 4130 frame is all that remains original, save for the seat post bolt, which is stamped with the Schwinn S.)
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