Curiosity question to frame builders: How exactly are DT bosses done?

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Bubba

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Jun 17, 2025, 12:41:11 PMJun 17
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Hello frame builders.  I don't check in often, but I'm here with a very specific question about the standard operating procedure around down tube shifter bosses.  I think I know the answer but I'm not 100% certain, and I'm not certain if there may be several ways to do it.  

The context is this:  on another forum a home-mechanic finds they can't get enough friction on their down tube friction shifters.  I observe that anecdotally sometimes the D-ring can bottom out on the threads, and a tiny bit shorter D-ring bolt -OR- a washer under the head can shorten the bolt and make it easier to get the friction one wants out of the stack-up.  

Another poster chimes in that "STEP 0" is to run a tap through to make sure the threads are all clean.  

That is the source of my question about how things are done.  I've held shifter bosses in my hand, and they "always" are drilled through, but my observation as a mechanic is that when installed, the act of brazing that on has made the hole blind.  In other words, the frame builder does NOT drill the down tube and then align the shifter boss to the hole.  The frame builder (I think) marks a spot on the DT and brazes the boss there.  

Are ALL down tube shift bosses blind holes?  If so, one would need a bottoming tap to chase threads near the bottom.  Are SOME down tube shift bosses drilled through?  If so, one could use a "normal" tapered M5x0.8mm tap and go all the way through.  I think all the bosses I've ever checked as a mechanic would have all been mass produced bikes, and in my experience I think they are all blind holes, but I'm not totally certain.  I'm going to spot check a few bikes in my stable, but a broken leg has lowered the priority on that exploration.  :-(

thanks for any info
Bill Lindsay
El Cerrito, CA

Jon Norstog

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Jun 17, 2025, 3:10:45 PMJun 17
to Bubba, Framebuilders
I've never drilled a hole under a DT shifter boss.  That said, most of the bikes I've built use a bar-mounted shifter and a plain cable stop on the DT, and if there is a boss, it just holds an adjustable stop for the bar-cons.  My guess is that the problem you cite may be caused by an abundance of bronze/silver in the bottom of the hole.  Like all things framebuiilding, mounting braze-ons calls for caution and skill.

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Alex Meade

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Jun 17, 2025, 6:57:50 PMJun 17
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Bill,

Like Jon and you, I've never seen DT shifter bosses drilled through.  As you note, the bosses come pre-threaded, and you are correct regarding how they're installed.  No hole.  If you have a bunch of extra brazing material in the bottom of the hole, a bottoming tap might not help; it'll only help if the material is just in the threads.  I suppose you could carefully drill out with the correct size tap drill, then use the bottoming tap. And I don't think much bad would happen if you drilled all the way through into the down tube: the boss will act as a beefy reinforcement ring around the hole.

I always chase threads on my frames, and I can't recall ever noting a significant amount of material in the threads.  The tap usually spins in until I feel it hitting the down tube, and I use a taper tap, not a bottoming tap.

Just my $0.02.

Alex

Mark Bulgier

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Jun 17, 2025, 7:32:16 PMJun 17
to Alex Meade, Framebuilders
Alex knows what's what.

I stood by and watched once as a bike mechanic extracted the braze-on from a frame by tapping.  Once the central point of a taper tap contacts the DT, it's extremely effective at peeling away the tube from the back of the braze-on.  It makes a deep dent in the DT at the same time, so you can't just re-braze it, there's no longer enough cylindrical tube shape under the footprint of the BO.  

That central point on a taper tap  is not useful, except for while making the tap originally.  Or when re-sharpening it, but no one bothers re-sharpening M5 taps, it would cost more than a new tap. So I recommend grinding that point back, approximately to the first partial threads, being careful not to get it red hot while grinding.  Or just use a plug tap or bottoming tap.  Whatever tap you use, pay extreme attention to when the tap is going to hit the DT, and don't proceed one iota beyond that point.  Better to have to shorten the screw than risk extracting the BO.

I agree with Alex that if the hole is filled with braze, tapping won't fix it. But that would be extreme malpractice by whoever brazed it.  No one could stay in business if he was that bad at brazing BOs.   Maybe if it's an amateur, home-built frame, but I've never seen that happen.  Anyway, in that hopefully-rare scenario, you'd need to drill first then tap.

good friend

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Jun 18, 2025, 1:38:39 AMJun 18
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I find it hard to imagine the threads in there getting damaged unless someone uses a wrong bolt, tho lots of sand or other similar gunk getting in might create a problem, so  ...
   I'd first try some compressed air to blow the "smutchz" out (maybe with a drop of penetrating oil?)
and, as mentioned already, look to the shifter to see if it is damaged or not a size made to fit that braze-on size...

noMadic   Thomas



m-gineering

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Jun 18, 2025, 1:45:27 AMJun 18
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On 6/18/25 07:38, good friend wrote:
> I find it hard to imagine the threads in there getting damaged


Ever wondered why Shimano stamps the threadpitch on their shifter bolts?
It's a remainder of 50 years ago when the French (who else) messed
around with m5 x 0.9mm
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Marten Gerritsen
Kiel Windeweer
Netherlands

Duane Draper

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Jun 18, 2025, 3:34:07 AMJun 18
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Here is a picture of my Erickson. Sorry about the crud in there but you can see it goes through. 

From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of m-gineering <in...@m-gineering.nl>
Sent: Tuesday, June 17, 2025 10:38:45 PM
To: frameb...@googlegroups.com <frameb...@googlegroups.com>
Subject: Re: [Frame] Re: Curiosity question to frame builders: How exactly are DT bosses done?
 
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m-gineering

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Jun 18, 2025, 3:49:12 AMJun 18
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Wouldn't surprise me if the hole wasn't drilled first, as an easy was to
position the braze-ons

On 6/18/25 09:33, Duane Draper wrote:
> Here is a picture of my Erickson. Sorry about the crud in there but you
> can see it goes through.
>
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tnort...@gmail.com

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Jun 18, 2025, 7:27:48 AMJun 18
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Mark, you mentioned that you saw a boss get peeled off? Would the tap bottoming out apply enough  force to break a silver/brass joint?
Tom

Mark Bulgier

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Jun 18, 2025, 8:11:42 AMJun 18
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Tom wrote:
Mark, you mentioned that you saw a boss get peeled off? Would the tap bottoming out apply enough  force to break a silver/brass joint?

Yes that's exactly what I meant. The point force from the center of the tap is so high that the DT is pushed away from the BO even though it has to break a braze joint to do it.

I wasn't there when that BO was brazed, so I can't tell you about prep, brazing skill/speed, temperature, amount of filler added etc.  Maybe a really good braze could not be "extracted" by a tap in this way, but I wouldn't bet my life on it.  That pointed tap is a perfect storm for BO extraction.

tnort...@gmail.com

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Jun 18, 2025, 8:13:40 AMJun 18
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Wow! Now I  want to do a text pie e and try it!!

Doug Fattic

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Jun 18, 2025, 8:38:02 AMJun 18
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I had a bike shop tap off a down tube shifter boss on a frame I made.  In the 80’s Shimano lever bosses were mostly round shaped and had a 4 X .7 mm thread for the tightening bolt.  This was a very difficult tap to find in the US.  So this shop decided they would tap the boss to a more standard M5.  Now taps come with various amounts of taper and for our purposes, we want to use a bottoming tap that has little or no tapper or we couldn’t clean up the threads near the tube.  Anyway the idiot mechanic at the shop just kept turning the tap until the boss popped off.  If one uses enough force on the tap,it will eventually give way.  

What really irked me was that the shop called me and asked if I guaranteed my work.  He said one of my bosses came off and I needed to repair it. Of course I said I stand behind my work.  He didn’t mention that they had done anything to the boss themselves.  When the frame arrived, i could see a deep crater in the down tube where the point of the tap had buried itself into the tube until finally the boss gave way.  It was easy to see that silver covered the entire area so my brazing wasn’t at fault.  Of course fixing it involves painting as well as rebrazing on a new boss.  Fortunately for me the owner of the frame realized that I wasn’t at fault and paid me for fixing the boss and repainting the frame. 

Doug Fattic
Niles, Michigan

m-gineering

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Jun 18, 2025, 8:57:47 AMJun 18
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On 6/18/25 14:11, Mark Bulgier wrote:
> Tom wrote:
> Mark, you mentioned that you saw a boss get peeled off? Would the tap
> bottoming out apply enough  force to break a silver/brass joint?
>
> Yes that's exactly what I meant. The point force from the center of the
> tap is so high that the DT is pushed away from the BO even though it has
> to break a braze joint to do it.

The peelstrenght of silver isn't great. So when the tap starts to deform
the tube the edge of the silver gives way. It's the very localized
stress instead of the pull on the complete joint
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Marten Gerritsen
Kiel Windeweer
Netherlands

Tel: +31 598 491865

www.m-gineering.nl

Duane Draper

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Jun 18, 2025, 10:52:48 AMJun 18
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That’s my thinking but the risk of a positioning tool getting brazed inside would seem high. Maybe there are tricks I don’t know. 
Sent: Wednesday, June 18, 2025 12:42:30 AM

To: frameb...@googlegroups.com <frameb...@googlegroups.com>
Subject: Re: [Frame] Re: Curiosity question to frame builders: How exactly are DT bosses done?
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m-gineering

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Jun 18, 2025, 10:56:55 AMJun 18
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Titanium works wonders

On 6/18/25 16:52, Duane Draper wrote:
> That’s my thinking but the risk of a positioning tool getting brazed
> inside would seem high. Maybe there are tricks I don’t know.
>

John Thompson

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Jun 18, 2025, 11:07:07 PMJun 18
to frameb...@googlegroups.com
> Like Jon and you, I've never seen DT shifter bosses drilled through.
I've never seen drilled-through bosses on steel frames, but I do recall
that the bonded aluminum and carbon fiber frames we made at Trek had
drilled-through bosses to stabilize and reinforce the epoxy bond holding
the lever bosses to the down tube.

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-John (JohnDT...@gmail.com)
Appleton, WI USA
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