French stem
 

Can anyone here rethread or shorten a French steering tube for a Peugeot? Or know someone that can? I have a fork for a PX10 that needs to be 93mm shorter than it currently is.

Don't you just need to saw the top off in that case?

Probably needs more thread. That's a lot of thread to put on by hand, I would keep searching for a fork about the right size.
There might be someone out there that has a French die, but I'm not sure using it up on a too-long steerer is a good way to dull it.

Some steerers are of a gummy metal, 531 as example. A very sharp cutter and plenty of cutting fluid with best technique is advised to reduce the chance of the threads just pulling off the steerer as you cut them. For this reason I would pass on this job. (Having said that many French bikes did use other then 531 in steerers and stays...)

93mm is a VERY lot of thread cutting, independent of the material involved. As mentioned I would search for a more suitable fork before risking the current one. Andy

That's interesting you say that about 531. I pulled a thread off of a 531 steerer doing this. I put less than 50mm of thread on it and I really never want to do it again

Another option is to shorten the steer tube via splice and welding. I'm not sure if 531 can be welded. Costly if you can't do it yourself.

Except for the paint/chrome damage I'd rather replace the steerer, and consider installing a Eng 1" one instead. Andy

You're not supposed to weld 531.

French steer tubes
 

I have a stock of French steer tubes if you want to replace it.

Or, replace the entire fork with an English/ISO thread aftermarket fork. A lot less work.

What's the length of your existing steerer? I know it's a longshot, but maybe we can swap. I'm looking for a PX-10 fork for a tall frame, needs to be 240 mm from the crown race seat up. I have three PX-10 forks with shorter steerers, maybe one that could work for you?

I also have a French (M25 x 1) steerer die, but like a couple folks who have posted already, I've found threading steerers to be risky. I've had success (did one just last week), but also failures, where the new threads are just torn out at the root as you screw the die back off. I find a strong correlation between success rate and how many threads you cut: The die comes off clean, threads perfect, if you keep the number of threads (threaded length) to a minimum, somewhere around the height of the die itself. Long threaded length = almost guaranteed failure.

I have cut steerer threads on a lathe, using a single-point tool. That's the best way of all, very reliable, but I don't have a lathe at the moment. Maybe bring your fork to a machinist and get a quote? Might cost more than a PX-10 is worth. No insult intended, I like them, but they are not super valuable, you can just buy another one. That might only be feasible if you have a buddy who has a lathe and likes a challenge. Due to the offset (rake), the weight of the dropouts makes the lathe shake when spinning a fork, so this would not be a project for a mini-lathe. It needs to be a heavy enough machine to do it well. Also some older American lathes can't cut metric threads, or can do it but only after partially disassembling the gear train and installing "change gears" with different numbers of teeth, that your machinist might not even own.

Another option that I'm fond of is a "thread-less" headset. Not threadless, you still need some threads, but only a few, 10 mm or even less. You still use a quill stem. The trick is to use the upper cup of a threadless headset, and it goes on the unthreaded part of the steerer. That can be made from an existing threaded headset by simply removing the threads. Best done on a lathe for precision, but a few people have reported success with just grinding out the threads with a Dremel. The existing threads serve as a visual guide; just grind away metal until the threads just barely disappear. This method takes patience and good hand-eye coordination. Not for everyone!

Then you need some way to lock in the headset adjustment. A single, normal threaded headset nut at the top can work with Loctite, but I prefer a two-nut method where a thin jam nut is tightened against the top nut. In your case both nuts would have to be M25 x 1 and such nuts aren't common, so you might have to make one yourself by thinning a vintage French headset locknut. At a minimum, you need to remove the flange at the top of the nut that keeps it from threading down past the top of the steerer.

Here's an example of a two-nut solution, in English thread:
https://cimg6.ibsrv.net/gimg/bikefor...31c10a70bc.jpg

The bike is an indoor trainer, a small frame with a way-too-long steerer and an adjustable Look Ergostem that we had lying around. The stem is quill style, which is why we didn't just use a threadless stem, needed that adjustability. The Ergostem allows one frame to fit multiple sizes of people with just a quick adjustment.

There's only maybe 12 mm of threads there, well within the range of what I feel I can cut safely with a die, and if you keep the two nuts thin, you can use even fewer threads. That jam-nut, that takes the hook spanner, is Japanese, came on cheap bikes in the '70s — uncommon to find one nowadays, and definitely unobtainium in French thread. You'd have to make your own jam-nut. Leave out the washer that you can see in the photo, not necessary if you're trying to minimize the number of threads.

There are also single-nut solutions that use a slit and a pinch-bolt, or a setscrew (grub screw). But again hard to find in French.

Mark B

Mine is 230mm, I need one that is 155mm, do you have one?

My understanding is you have a sheldon. Okay, I see you were going to look at it, did you decide not to buy it?

How do you mount the fork in the lathe for this, with a mandrel? I guess the OP's fork is long enough you could chuck the existing threads.

Yep still without a lathe. Lathe shopping is on the back burner for now. Hot leads happily accepted though. Seattle area, or willing to ship at fantastic (for me) rates.

I've done it two ways.

1. When the steerer was long enough: a 7/8" internal expanding collet in the headstock to hold the top of he steerer, and a steady-rest to hold the bottom of the steerer just above the crown. That requires enough steerer length for the carriage to fit in between the head and the steady, plus whatever thread length you want.
2. When the steerer was shorter, I made a mandrel that started at 7/8" and then stepped down to clear the ID of the butted end of the steerer, and then was long enough to allow a live center in the tailstock to clear the dropouts. Or was the reason for the mandrel just because that shop didn't have a steady-rest for their lathe? I don't remember. Two diff. shops tho.

Neither setup is super rigid, but both were plenty at the point where the threads were happening, which was close to the headstock. A little flailing about at the dropout end was tolerable that far away from the threads.

-mb

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