Since this thread was in danger of ending peacefully, I'll throw in that Jobst Brandt reckoned chainstay bridges were to prevent the tire from getting jammed between the chainstays on removal.

Chainstay bridges (Jobst Brandt)

Thanks, Therm for your attempt at a constructive ending that continues to stimulate!

In this case, Zandoval made the same point in Post #2, but did not attribute it to Brandt. I'm glad to see he's remembered!

Or should I just say, "No, it CAN'T be!"

Feel free to bring in more (and more contentious!) Brandt-isms.

Sheldon fender nut! (I think also through Problem Solvers)

Problem Solvers Sheldon Fender Nuts Set, includes 32mm Front and 10mm Rear in Tree Fort Bikes Fender Parts

I was assuming this was for standard rim caliper? Replace the bake caliper nut with one of these, and you're done.

You can drill a small hole in the bottom of the brake bridge and use a short sheet metal screw to attach the fender.

That won't help me. I don't have a problem with the caliper bolt, since the caliper is an ancient Mafac center pull with nutted brake bolts, and the brake has the right kind of bolts.

I need to see what mounting setup will move the fender down from the crown to be about 10 mm above the tire. I think I can do it with the little L-bracket V-O put in the fender kit. The daruma bolt will not give me the correct spacing, and the frame is not set up for a bolt going up into the bottom of the fork crown.

My fork has extra-long blades since it was custom made to give low trail (the 1984 Trek 610 was not low-trail, in my opinion), and the fender needs to be mounted down-low from the crown to give good coverage to the tire. That is not the problem addressed by the Sheldon nut.

Well, true! I hadn't intended to get that deep into frame mods, but actually, why not? I've already got a special fork on the bike.

Just plain steel twist drills with a variable speed 1/4 " electric drill, or would you suggest anything different? I guess I need a 5 mm drill, then to tap the hole for the standard M5 threading?

It would certainly be a quieter installation than a hand-formed sheet metal bent bracket - I find those rattle a bit with a plastic fender, so I'm sure to hear them with an aluminum fender.

I should probably use a punch to initially locate the hole, then a small drill with the pin vise to make a pilot hole, then use the M5 drill with the electric drill to carefully start the hole, then pray for alignment as I drill and tap the hole.

It won't be a blind hole, so it should not be too hard to clean out all the shavings before fender attachment.

I'm open to any advice on this point - how hard is it to drill a radial hole squarely? On one of the forums, one of us built a fixture to create a similar hole.

Don't use a 5mm drill for a 5mm tap. 5mm is the OD of the threads. A 4.2 or 4.3 drill is what you need. Google "drill size gor 5mm tap".

Odds are the OP wont be able to tap the holes. Hes drilling though a thin walled tube with walls not much thicker than the pitch of a single thread.

The right way to do this is either with a Riv-nut, or by drilling a larger hole and gluing in a threaded inset.

I suggest the OP take the time to analyze his options, buy the necessary and drill the hole with the diameter matching what his solution calls for.

Or he can simply drill a hole and use a screw and nut running through the bridge.

Or he can buy a flange nut (looks like the recessed nut used for brakes), pop it in from the front with a bit of glue to hold it steady. The tightened fender bolt will keep it home, and the flange will keep it from pulling through.

You're right, my apologies to [MENTION=194136]zandoval[/MENTION] for not reading to the bottom of his post.

Now we're talking about the brake bridge, which has a solid steel cube suspended between a pair of thin tubes such as you describe. The brake bolt runs through it. John is suggesting to run a short machine screw up into that block to attach a metal fender solidly. If I was dealing with a thin tube like the chainstay bridge, correct, I could not tap it.

I wouldn't bother trying to tap it. I've successfully used a plain sheet metal screw in an untapped hole.