@Woz
That's the neco. I've had a look at the bog standard Cane Creek and it works too, however, it has a steel 26.6 race, so not as good as a split alloy ring made for 27.0 to fit a 27.0 fork.
The only difference (assuming you can sort out the preload on parts 2,3,4,5) to running this with a threadless fork is that French fork is 25.0 instead of 25.4mm. That's a shim job in my mind, as part 5 is a compression ring and it doesn't care whether it goes round a 25.4 or shimmed 25.0+0.4.
Part 5 is doing the centering of the steerer in the bearing.
Part 2 only does the pre-load from above.
The pre-load can be achieved via Sheldon's hack (two locknuts threaded onto steerer, pressing down on part 2). I'm trying this.
Alternatively, a ahead-conversion quill can be inserted into the steerer, fixed in place, and then modified to be able to generate the preload (Mickey's idea). Which was what I was getting at in my proposed approach. The star nut will probably need re-thinking and I do not like that particular adapter, but the concept would be the same.
None of the parts are fundamentally working in a way they were not designed to, but the second approach hinges on the assumption that the ahead quill never moves. And quills do move unfortunately.
You are missing the important point, that Part 5 must be driven lower, and potentially out of it's manufactured tolerance regardless of what mechanism you choose to do the pre-load - via x 2 metric nuts or a quill a-headset bodge.
EDIT: You are right. French fork is 25.0 instead of 25.4mm. That's a shim job in my mind, as part 5 is a compression ring and it doesn't care whether it goes round a 25.4 or shimmed 25.0+0.4.
This shim job would need to well made and of a hard material like steel. It's a 0.2mm wall thickness so some rolled steel for that section may just work, putting the splits at opposing sides. The threads could all keep it in place. It may need regularly adjustment till it all settles.
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