It all started when I wanted to replace the completely functional plastic-coated downtube shift levers on my old french road bike:

With some slick-looking shifters a friend gave me:

Should be easy, right? Just cut the existing cables, replace the levers and string a new set of shift cables… wrong.

The old shifters and threaded bosses on the frame were an unusual, non-standard thread, M5x1.0mm, which as far as I can tell mainly existed on 1970s French bicycles and is no longer made. The standard metric screws near that size are M5x0.8mm or M5x0.5mm (the same diameter, 5mm, but a different thread pitch). I knew from Sheldon Brown and previous work on the bike that old French bikes are “special” and, for example, one of the bottom bracket cups is threaded in the opposite direction from pretty much every other bike ever made. But I didn’t think this applied to even small-size, standard-looking screws.

Do I dig up old screws on ebay? Custom-machine a screw? Braze or clamp on new threaded bosses? Or maybe… convert the old threads to the new standard thread using spiral threaded inserts (e.g. Helicoils)?

Step 1: drill out the old M5x1.0m threads with a 13/64" bit.

Step 1.5: Decide I need to own a bench vice.

Step 2: tap new oversize threads for a helicoil (using the specialty tap provided with a set of heliocoils– not a common thread size). It’s tough steel– plenty of tapping fluid is essential, as is backing out the tap a fraction of a turn every half rotation or so.

Step 3: Using the helicoil insertion tool (line up the ridge on the end of the insertion tool with the “tang” (metal crossbar) in the heliocoil), insert the heliocoil:

Body of the insertion tool shown lifted, but you’d normally have it pressed down against the surface of the part below. Insert the helicoil so it’s slightly below the surface of the part, then unthread the insertion tool.

You can see the new helicoil threads in the hole if you look carefully, providing a new, strong, M5x0.8mm female thread:

Step 4 (not shown): Use a hammer to tap a small rod in the new threaded hole, to break off the tang on the helicoil (it should be scored for easy breaking). This prevents the tang from interfering with a long screw.

Step 5: The first moment of truth… and the shift lever attaches perfectly to the new threaded boss on the band. Success!

The band only had a threaded boss for one of the shifters– the other boss is brazed to the bike frame. Repeat the process, avoiding drilling into the bike frame itself…

Finally– threading the cables ($2 each from Sports Basement – the heads didn’t quite fit and I had another trip to the hardware store to get a file to file them down slightly) through the shifters, frame bosses, and sheaths, and connecting them to the parts that need to move:

After all of that work… the shifters fit perfectly, and the shifting action is smooth and buttery. Success!