Charles makes a good point intuitively (about not clamping in the threaded area of the steerer), but I'm not sure that it matters. The clamp will put the steerer in tension around its circumference, and it might be ok (if not slit like some are). But, let me make several other suggestions, since our goal is to ride the bikes, and to ride them as nearly in original spec as possible. Here are three other options:
1) Find one of the old TTT adjustables with the "horizontal" part sliding along the "vertical" part and a fixed mount for the HB. Reverse it and you suddenly have an upwardly inclined "horizontal" that gets you up an inch or more. Good luck. :-) Will bring beat up example to Cirque.
2) Set up a "hack" set for riding, using a modern "aheadset" stem and adapter to "our" type of steerer. Now you can get much more lift upward, using the adapter and upward inclined (10 - 15 degree) stem pieces of any length you want. Keep the old bars/stem/brake lever for "show," if you aren't using HB end shifters it takes little time to change over the brake cables. That's what we're doing with Beloved Spouse's Orphan. "Show" stuff with Mafac Plastic levers, Cinelli bars, and "go" set with smaller Shimano levers, etc.
3) Step up to the plate and order a custom stem of any length, angle, or whatever. I'll bring an example of ancient British provenance to Cirque. This would be real KOF. I will admit, though, that if I were doing this I would be heretical enough to specify a modern-style HB clamp with bolts up and down, so the HB can be changed w/o removing tape, bars, and the whole deal.
Just some alternatives to the Nitto, to help keep life complicated. :-)
harvey sachs mcLean VA
The issue is not the stress you put on the stem, it seems to me.
The issue is that if you clamp the stem too high in the steerer, you're clamping in the thread area. The stress will come from the clamp-pressure itself. If the stress is too high, the steerer will crack in the thread-area, and if it happens at the wrong time, goodby asphalt, hello traction, if you're lucky...
But then, you all know this, right?
I've measured a lot of steerer tubes, in the interest of raising a conventional Cinelli stem as high as possible...and I find that while it's sometimes possible to raise a stem a little beyond the limit line, it's not usually possible..unless you want to clamp in the threads, and I assume no-one wants to do that.
Seems to me the only way to do this is on a case-by-case basis, measuring the threaded area, and clamping the stem just below it.. or well below it, depending on how much of a risk-taker you are.
I'm not real thrilled with the look of a Nitto Technomic either, although I use them...because I have a long-standing neck injury that likes to irritate itself at the slightest opportunity. Higher bars help me avoid that annoyance.
I'd love to be able to ride with a lower stem and higher saddle, this posture is ideal for accelleration, consistent speed, and stamina, since it gets the big muscles into the act. I rode like that all the time when I raced, of course...I was younger then, too. All other things being equal, and in perfect condition (like your neck and back), a higher saddle and lower bars will make you go faster.
The more upright we sit, the less we use the big muscles....
One solution to this problem that I like a lot, but that is tough to implement with most vintage bikes, is to have a longer steerer tube, cut it off long, slit the top inch or so with a 1/16" slit, install the headset, and install one of those old-style steerer clamps that used to be fairly common (I'm told). You see them often in pics of bikes from the 50s and before. *Lightly* clamp your vintage stem to suit, then tighten down hard on the external clamp. Works great, and it gets the stem right up there. I have one of these on our old Pogliaghi tandem and it works great. It's been set up this way for 35 years, and many miles, and hasn't broken yet (knock on wood...if it ever cracks the steerer on a down-hill, we're toast).
The other solution was described in an earlier post...keep your shoulders down and elongate your neck when you're on low bars, to keep your cervical spine stretched out. If you let your shoulders creep up and put your C-4 joint under a lot of stress, you'll regret it, sooner or later.
Charles "master of the obvious" Andrews SoCal