Convert 1" threaded headset to threadless?
 

In general, has anyone ever reamed out the upper threaded cup on a one inch headset and used it on a threadless fork? Is there any rational reason this cannot or should not be done?

Specifically, A Stronglight A9 alloy headset, to be inserted in a 1974 Viscount Aerospace Pro (remember those, anyone?). The OEM aluminum and Tange replacement forks have both been removed and are being replaced with a carbon fork with aluminum steerer. This headset is nearly weightless, and aside from the fork and stem the build will be very old school, down to the OEM sealed cartridge bearings. It would be cool to make the mod and use this headset.

The steel races are removable and will not be affected by the reamer.

The locknut and keyed washer will not be required, obviously.

Threadless top races aren't just laid over the steerer tube.The central bore isn't cylindrical but conical, with the pointy end downwards to take a compression ring, which is likewise conical. Then the spacers and stem comes down to push the compression ring into the top race, and simultaneously preload the headset.

If you want to do this, you'd still need to get a compression ring somewhere, and it'd be a lathe job, not a reamer, to get the conicity right.

I'd just use the complete top half of a threadless headset in the right dimensions instead.

I think an adaptor would work best.

from what i see the threads simply provide a method to tighten the upper cup. without them something would be required. if that something is simply a top cap and expanding carbon steerer plug (or just your hand), i don't see why it wouldn't work. the reamed cup would need to be a tight sliding fit.

i don't see why anything like a compression ring would be necessary. the stem will be in contact with the upper adjustable cup, which in turn will be in contact with the bearings just as it should.

in essence you will be using the top cap (or hand pressure) to provide temporary security and adjustment to the headset during assembly and the pinch bolts on the stem to maintain it after assembly rather than relying on the threads on the adjustable upper cup and steerer tube and a lock nut.

IMO, the idea is good enough to take the time and effort to give it a try. good luck. if you decide to do it, let us know. i, for one, would be interested in the outcome. :)

BTW, i would think that as soon as the adjustable cup slides over the steerer tube the reaming can cease. :lol:

OTOH, i don't know how many times i've sat down to do something slightly unusual on my bike, thinking "this should be easy" and a couple of frustrating hours later thought, "dammit! this is not going to work at all", but this caveat may be unnecessary, for i think you may be a member of the choir. :)

True, get a good enough fit and the steerer will stay centered and not slop around w/o the compression ring.
The purpose of the ring is to cover for production tolerances and to center the steerer. Which is doable through other means, if you put the effort in.

Exactly. The key here is that the upper cup must be reamed out to be a "perfect" fit to the steerer tube in the absence of a conical compression ring. I think it's definitely possible if you're careful enough.

If the threads will not fit over the steerer tube; just thread the threadless steerer tube to get a tight fit for the upper cone, then but standard threadless stuff above it. The threads on the aluminum steerer tube will not be strong enough to clamp everything together, but will be more than sufficient to hold things correctly until you clamp everything up. This is approach is a lot simpler and cheaper than the others proposed.

Years ago when threadless steerers and headsets were first introduced, Shimano didn't make (and still doesn't make) any threadless headsets but their sponsored Pro teams wanted to use threadless forks. The team mechanics lathe turned the adjustable cups of Dura Ace headsets to remove the threads and allow their use on threadless steerers. It had to be done carefully to keep the race concentric and a very precise fit to the steerer tube but it apparently worked. This is not a job for a file used by hand.

If I understand your plan correctly a lot of the steerer tube would be threaded, including the area where the stem clamps. That doesn't seem like a good idea.

As Dabac mentioned, it's not just a matter of sliding on, but having a secure fit equivalent to what a press fit would produce. The conical race and tapered compression ring do this and are key to the threadless design.

An alternate "poor man's" method if the OP wants to save his headset would be to carefully ream the cup to the closest fit which still allows it to slide on the fork. Then using a half round or rat-tail file, carefully filing an angle into the top corner to accept an O-ring which will be crushed into it. You want something that goes deeper than 45°, so that most of the wedging force will be radial vs. axial. The O-ring needs to be thick enough that it cannot be pressed entirely into the recess. This provides the poor man's compression ring effect and press fit analog needed for the headset to work.

I didn't see anyone else suggest to get a threaded carbon fork. Nashbar had them last I checked.

Unless I'm misunderstanding, this is a recipe for disaster. Any threading above the upper bearing is a stress riser (especially in aluminum) and will snap under the stress coming from a threadless stem.

Threaded forks, without exception call for a quill stem that extends below the last thread to buttress the threaded area, and carry stresses down to the non threaded section. Even non-threaded steerers have been known to snap from excess stem stress, and a threaded one with an external stem is doomed from the start.

re my post, 7th... Dont want to pay machine shop time? maybe just use the A9 Bottom race , & use a 'What ever', 1" threadless top race,
ball bearing types will be fine..

That reminded me of something I read about Jobst Brandt's bike a few years ago: bikecult.com > bikeworks nyc > archive bicycles > peter johnson road touring

I'm not quite sure I understand why the adjustable cup still needs to be a precise fit to the steerer tube, though -- I can see the problem if it were a tight fit and off-center, but I'd think that if it had enough clearance, it could self-center while adjusting the headset bearings, and then it would hold that position after the stem clamp bolts were tightened.

But threadless headsets aren't in my wheelhouse. :)

i agree TheromionicScott...

centering shouldn't be a problem. the two upper races, the one on the adjustable cup and the one that is pressed in, in concert with the bearings should center everything, taking its alignment from the pressed in cup. it would help as mentioned if the reamed cup, formally known as the "adjustable cup" :lol:, was a tight sliding fit with the steerer tube though.

Yes, it's not about centering. The bearing will float to center and ride squarely unless pressed unevenly.

But it is about a fit tighter than you can achieve pressing down the length, which is why crown races are always pressed onto a raised set. Without a truly tight press fit, or a compression ring, the headset will click and creak until you can't take it anymore and go back and do it right.

Thank you all for your input. I am now in the hunt for a 1" threadless set or just top set, because:
1) Paying a machine shop to lathe it out properly is more costly than a new one.
2) Doing it myself, however carefully, is likely to wreck the A9 adjustable cup, making it worthless in either application.
3) I already own a threadless carbon / aluminum fork, and using a threaded one, with quill stem, puts the project overweight and over budget. (even though I'd like to use the branded OEM stem, it's just an anchor)
4) I will not thread an aluminum steerer, designed and installed for a threadless application. A loose, creaky headset is annoying, a cracked, broken or otherwise compromised steerer is potentially fatal.
Great help, everyone, thanks again.

I say do it. Just get the wedge ring from a threadless headset and match its angle when you carve up the threaded part.

I reckon you could do a good enough job with a hand file, with enough care... if the widest dimension between the bottoms of the threads is any greater than a threadless steerer's OD, you'll have a nice guide to work to.

Kimmo, I would consider it - after all, Chris King makes a conversion kit with what I'd need after I'm done with the ream job, BUT, for the same price I can get a new Miche complete on eBay.

FBinNY, I'm too new to the forums to send personal messages. Thank you for your offer, if it still stands, please contact me - fbrichetto@yahoo.com

Thank you!
Frank