Classic & Vintage - Stem expander bolt question

I'd like to replace this stem expander bolt, because the head is so chewed up that I'm not sure I can get a wrench to fit it securely. It's from a TTT stem, and the bolt head recesses so deeply into the stem that little of the head is exposed once it's tightened. A few more washers under the head would have helped, but the damage was done long before I got it.
One thing that puzzles me a little is the necked-down section of the bolt just below the head. I don't recall having see that before, and can't think of what its purpose is. Is there some reason I can't replace this with just a straight bolt of some kind? Haven't tried to figure out the threading yet, but it's finer than the threading on most of my other stems.

that does look odd. perhaps there was some sort of factory bushing there at one time

http://www.bikeforums.net/attachment.php?attachmentid=226965&d=1321361805

My first reaction was that it has been altered, but, to what end? Perhaps that necked down area was machined in when they formed the hex head? Still, I don't see any reason for it being there. Might be difficult to find a replacement with the proper threading so you can re-use that expander. Have you got a spare bolt that will fit, and with an expander nut with a similar taper?

We have a pretty huge assortment of these at the coop currently
here in Sacramento. In my experience, the length is the only really
critical factor in replacement. i just use an alternate bolt with the
wedge it came with, taper is not usually all that different on most
of them.

But I. too, have never seen one necked down like that.

If you get desperate, PM me your exact length and I'll
look for one at the Bike Kitchen for you.

If it recesses that deeply, are you possibly better off with
an allen head bolt as your replacement ?

It's hard to tell from the photo but I suspect the the head may have been formed from stamping, as opposed to machining, in which case it could be too much pressure during forming.

Alternately, it may be an attempt at stress relief by allowing for a larger radius at the junction of the head and shaft. In such a situation, the shaft diameter would have to be reduced to maintain the necessary bearing area under the head. Undercutting is a common form of stress relief, but this is a long undercut.

Finally, it be a combination of the two (i.e. stamped undercut for stress relief).

The bolt shank looks to be the same size as other stem bolts, but the other bolts don't *quite* fit the drilled hole in the top of the stem.

Bummer. Can you ream out the hole a hair and still maintain a wide enough counter bore for a load bearing surface?

I'd just find one that fits down in there, you should be OK. The threading on the expander nut is really your only concern. I may have a bolt, I'll check, as some Modolos and 3TTT's work fine with Cinelli bolts and nuts.

Yeah. You're right Robbie. After I typed the above I thought ...nah, probably not good practice.

When I was looking for a replacement bolt for a Cinelli stem, I think I remember TTT stems also used the same M7x1 bolt (though I was looking at hex socket).

They are a bit of a pain to find. I lucked into one at a local shop.

I have also downsized the hex head on a difficult to
replace part such as this on a couple of occasions.

It's a little tricky, but not that bad, to file it down slowly
and evenly to the next size down in your socket selection.

Use another hex head of the desired final size as your pattern.