Mick F wrote:…... Mercian could possibly be fitted with the Moulton system. Next time I'm tinkering with nothing better to do, I might strip both bikes down and see if the Moulton one will fit Mercian. I don't see why the non-threaded cups and fittings wouldn't go over the threads on the steerer. It could be that the head tube is a different gauge, but I doubt it as the headset isn't Moulton-specific.
Ahead parts will go over the threaded part of the steerer, fit badly and be a PITA for ever. Worse yet they may ruin said threads and cause it to be impossible to fit a proper headset ever again.
Ahead headsets have pluses and minuses. On the plus side the typical construction (with cartridge bearings that just drop in and sit on tapered seats, and which BTW isn't the exclusive province of threadless) allows some articulation with is potentially useful if you have a long and flexy steerer. On the minus side the same construction means you need more preload for the bearings not to rattle horribly, and the cartridge bearings are almost invariably nothing like as strong as a well-designed loose-ball assembly.
Given that a good threaded headset will (quite predictably) work for years without being touched, and many A-head ones I have owned have not been like that, I'd favour a threaded headset from that standpoint. On a road bike I think Ahead stems (even in 1") are lumpen and ugly things. They usually also have bolts and sharp edges placed in exactly the right location to eviscerate you in the event of a prang.
A major black mark against 1" threadless is that the standard is already nigh-on orphaned; plenty of road bikes are made in 1-1/8" threadless and plenty are made in 1" threaded, but 1" threadless is now very rare in new bikes. Already you can't easily buy 1" Ahead stems (most folk end up shimming a larger one) and whilst you can buy 1" threadless headsets still, the choice isn't exactly burgeoning and there is no reason to expect it to increase any time soon.
I've owned (and still own) both types and I know which I prefer. I've still got 1" threadless in the Brucey fleet and that stays as long as I trust the (all carbon) fork in that frame. When that fork goes, so will the headset too, I expect. I certainly wouldn't convert from threaded to threadless in 1", not without a very special reason to.