OK, disclaimer: I did this to an '85 Centurion Ironman frame. Bought the bike very cheaply for the groupset but it was damaged in shipping. Frame was OK but the fork was toast. It's a 60cm frame and I usually ride a 57-58. So, bottom line is that I didn't have much to lose by experimenting.

My intention here was to put together a riser bar bike using this frame, a replacement NOS Tange fork (originally for a very tall pink Schwinn Prelude) and either trigger, grip, or indexed downtube shifters- all of which require the later-style universal square downtube mounts. '85's were originally equipped with friction 6207, so the frame had the older style Shimano-specific round mounts with a tab in front that don't mate with the newer parts. I could have modified the downtube cable stops or shift adaptors, but instead decided to try modifying the mounts themselves without damaging the frame anywhere that could be seen once the shifting mechanisms were in place.

I work a lot with radio control stuff and am used to working in tight areas and to exacting dimensions with a Dremel, but you could do all of this with a flat file, too. Might be harder to avoid damaging the finish with a file, though. I measured the external dimensions of the old-style mounts and the internal dimensions of the new-style adaptors and found that by judiciously removing a little material, the new mounts would fit. And it was pretty easy.

First I carefully removed the front tab on the mount using a fibre-reinforced cutting wheel, being careful to remove only the tab and not scar the frame past the level of the paint. The frame had some minor damage from the PO around the shifter mounts, but I wanted to avoid having to do too much touch up when I was done.

Then I squared off the large round base to 12.3mm- it doesn't take much since they start out at 12.5mm. Then I cut down the height of the inner round column by about 1.5mm, or until the adaptor and cable stop would lie flush against the downtube.

The mount after carving:



Remember, the old style mounts also used 4.5mm mounting screws. They were too short to use with the new parts, so I made a "poor man's tap" from a 5mm hardened steel screw by cutting a channel in each side for the steel bits to float into as the new threads were cut. A couple drops of oil and removing the tap every two full turns to clean it off did the trick:


End result: now the frame can accept either cable stops or indexed downtube shifters and it looks like it came that way from the factory:

Cool!

Next up, the De Rosa...

That's the way I did mine. I felt bad taking a grinding wheel the it, but figured what the heck. Its not like anyone is ever going to want to go back to the odd ball early shimano DT shifters any how. I haven't tried but I think standard DT shifters could be made to work.

good on ya, I don't think this qualifies as "Drewing" but rather is "improving" or "correcting" a bad design decision foisted on us by Shimano.
I'm surprised that your "poor man's tap" was good enough to cut new threads, I've only use this type of homemade tap for thread clean-up...was this done just cause you didn't have a proper M5 plug-style tap?

I figured I didn't have anything to lose. The braze-on metal was soft enough that I had to be very careful while reshaping it with the cutting wheel, so I assumed it would take a rethread easily if I had a hardened steel hex head bolt to work with. Done it a zillion times with 3mm and 5mm threading into aluminum.

Actually was minutes from ordering a bottoming 5mm tap in eBay for like $4 and then thought what the heck, give it a try. I've worked with a lot of soft metals before so have the 'feel' for when I'm possibly pushing too hard and reaching the galling/seizing/unhappy point.

Good work.

A 1/2 inch outer diameter ceramic hole saw with a 9.2 mm inner hole diameter will do a much cleaner job of reducing the outer stud flange height to allow the shifter adapter to set flush against the frame down tube. A ceramic hole saw is not meant to cut steel/metal however the shifter studs on most older frames are easily cut with these diamond bladed saws and a bit of oil.

Can this mod work to mount a Simplex Retrofriction Shifter Lever?

just to confirm, the height of the shoulder the shifter cover sits on is shortened and squared, and the threads are tapped. The overall height of the boss doesn't change.

Nice job on that!

brewerkz, you are correct

thanks! I am going to take the plunge. Last month I had to cut a rear control arm out of a Honda CRV with an angle grinder.....now I will switch to a delicate operation.

My Cilo 54 CM SLX Frame with Shimano Type A bosses
Following instructions provided I used a dremel w/composite wheel and a 1/2" hole saw as recommended - with a 9 MM ID as stated. Here are my results. I kept my dremel at a medium/low speed to avoid cutting to fast. Also used a 5x.8 MM tap bought at the local hardware store, not shown.

Thank you Dr. Cannondale!


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Yes, thank you Dr. Cannondale! I did the same a couple of weeks ago on this frame, following your lead. 10 minutes with a file was all it took.

Yay!

I'm bumping this, because it's awesome. Thanks a lot, guys.

But ... I don't think this great info will help me, because I want to use Campy Triomphe shifters on my Shimano A type bosses. The bike is an '82 Miyata 912. I've added the boss measurements to the diagram below:


Notice how the sum of C and D are 3mm shorter than the Campy boss. Unless someone can chime in with a hack, I'm pretty sure that won't leave any room for the plastic outside shifter washer to effectively bite on the square part of the boss. This is true even if I could shorten the height of E (a measurement not shown for either boss).

​​​​​I'm sure someone like Jim Merz could make this work.

For those without a hole-saw (or the dosh to get one for a single job) -

Take a 9mm socket, grind the open end flat, cut some teeth into the now-flat edge.
Now you have an end-cutting hollow milling bit - put a hex-to-square adapter in your drill-motor and bear down.
Works a treat.
(It helps if you grind away the tab first.)

I’ve also recently had luck using later Shimano 6207 shift levers on a Shimano specific braze on using 4.5mm D-ring bolts and the small, thin boss.
Bet it would have worked without the boss.

Look at the face on a standard Shimano mounting boss. Now look at the Shimano braze on.