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Modification: Old style Shimano downtube shifter mounts to later style

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Modification: Old style Shimano downtube shifter mounts to later style

Old 07-04-13, 10:45 AM
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Modification: Old style Shimano downtube shifter mounts to later style

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:

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Last edited by rccardr; 07-04-13 at 10:51 AM.
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Old 07-04-13, 10:55 AM
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Cool!
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Old 07-04-13, 12:40 PM
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Next up, the De Rosa...
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Old 07-04-13, 02:09 PM
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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.
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Old 07-05-13, 11:22 AM
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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?
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Old 07-05-13, 07:57 PM
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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.
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Old 07-05-13, 10:02 PM
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Good work.
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Old 05-07-15, 09:44 AM
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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.

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Old 05-15-17, 09:19 PM
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Can this mod work to mount a Simplex Retrofriction Shifter Lever?
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Old 09-13-17, 04:41 PM
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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.
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Old 09-13-17, 04:51 PM
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Nice job on that!
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Old 09-13-17, 08:21 PM
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brewerkz, you are correct
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Old 09-13-17, 08:44 PM
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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.
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Old 09-16-17, 11:00 AM
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my results with Cilo SLX frame

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!


IMG_0165.JPG

IMG_0169.jpg

IMG_0167.jpg

IMG_0170.jpg

IMG_0171.jpg

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Old 09-16-17, 11:01 AM
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final result

IMG_0173.jpg
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Old 09-16-17, 11:11 AM
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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.

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Old 09-16-17, 02:31 PM
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Yay!
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