I am a frame builder AMA

Nope. 41mm for a standard integrated headset bearing. You only need the extra 3mm if the bearing's going to sit in an internal semi-integrated cup rather than directly in the headtube. Integrated rather than integral on the diagrams in this manual from Hope.

The only PMW headtubes I could find in steel are 44mm internal 50mm external. Got a link for the 46mm ones?

So yes I was being stupid. I was sure you'd said semi (lolz) but reading back you hadn't. Ok, that makes a lot more sense, at least for a personal bike. I personally find this style of headset a total 'mare with customer bikes, as it's not too hard to ruin a frame. Will be a fun machining job though.

Yep, it could all go horribly horribly wrong. But if it does, I'll just rethink my plans and design some other monstrous doomed folly instead.

?

So to keep things simple, what I actually want is a 36mm OD, 1.1mm wall plain gauge tube?

I can weld myself into a big mess then get someone to ream & face the ends.

Just had a thought while on the toilet. When I was sub contract building frames for someone they used those columbus tapered head tubes where you brazed the cups into the head tube. The top one would be perfect for this. I could see if they would sell me some singles?

Will these cast drop outs from ceeway be TIGable, or will they bubble like manky zinc plate?

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Just buy the cnc equivalent PMW ones, Ceeway stock them as well, and you won't have to worry about it. My guess is yes.

Oh nice! I didn’t know ceeway stock PMW stuff. They’d probably sell a lot more if they actually listed things properly. Thanks.

Are 12.7x0.9mm straight gauge 4130 tubes ok for pencil seat stays? I’m only 60kg and want a plush ride, but I also don’t want the back end flopping around.

Chainstays do the most to dicate rear end stiffness, the seat stays just stop the frame collapsing BB down to the floor. 12.7 x 0.9mm is more than sufficient.

What size are are T45 tubes you use on those pretty little wishbones of yours?

Is 0.9mm overkill?

10x1.5

I don’t have a tube bender capable of the tight radius for a wishbone, but I do like the way the tube flattens when I use my bar bender. I’ve used the technique in my furniture making so it’d be cool to carry over into my frame - crediting @coldharbour of course.

Is this an awful idea that will compromise the tube strength too much? This is 12x1.5 mild steel ERW, not 0.9mm 4130.

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I think it would be fine. Build it, ride it like you stole it and report back. As mentioned already, it's the chainstays that really dictate how stiff your rear end is going to be (within normal parameters) so just go with something beefy 'down there' ;)

Forgot I need a hole in there somewhere to mount a brake...

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How do I work out if there is enough room to add a disc brake mount on the CS?

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Put it on the seatstay?

Could do, but vanity etc.

Hole would need to be much lower to get any tyre clearance, if using standard short drop calipers.

Yup, I thought that. I’ll be using long drop TRP r976 or whatever they are, but I want to fit 28mm plus ‘guards.

Was either thinking a solid billet / thick wall tube that everything joins onto, or a thinner wall tube welded onto the underside of the curve - ugly, but only when there’s no brake mounted. I don’t think I can allow it past my aesthetic filter.

Cut wishbone in half. Weld brake boss into Centre. Job done.

👍🏻