Bike: 1988 Trek 360
Problem: Stem will not tighten securely in steer tube

No matter how I try to tighten the compression bolt in this Cinelli stem, the stem does not stay tight in the steer tube, and even when the bolt is tightened to feeling on the verge of stripping out, the bars can still be pulled from side to side with a little force if I am holding the wheel straight while pulling. Nothing appears damaged on any of these parts, but I've only dealt with the "wedge" type compression bolts in the past so maybe I am doing something wrong? Also, the steertube seems to have some sort of a shim, or an area where the diameter narrows towards the bottom of the tube, does this have something to do with my problem? I do not have a set of calipers, but as far as I can tell using a ruler, the OD of the stem and the ID of the steer tube are compatible.

Is that the proper wedge? My suggestion is to measure the stem,with the wedge tightened down (out the steer tube) and varify that is slighty larger than the steer tube. Post your findings please.

the steer tube is 22mm, and the wedge spreads the stem to about 23mm before it feels like it will strip. wedge itself is about 20mm wide and seems to sit into the stem pretty snugly

Yeah, thats not nearly enough. (IMO) Your getting a mm of defection into the steer tube and you have to a assume that the stem is able to flex back a little to some degree.
Another question. How much thread is available on the stem bolt? Is it able to pull all the way up the length of the slot in the stem?

the stem bolt is about 135mm from top of the shaft underneath the head to the bottom. theres about 4mm of unthreaded tapering, then about 40-41mm of threading. looking at the bolt, only about 15mm of threading looks like it has ever seen anything threaded onto it. So what do you think the problem is? Wrong wedge nut? Can these things even be replaced? If so, what size and where? Or should I just accept I need to buy a new stem?

If you insert the stem too far it may be trying to tighten on the tapered lower part of the fork steer tube which is normally internally butted. Try inserting it less, though to or below the minimum insertion mark, and see if that helps.

Otherwise you might have split the steerer tube at the groove that is machined into threaded steering tubes. Seen photos of it happening with aftermarket forks which are threaded and grooved too far down in an attempt to allow fitting to a wide variety of frame sizes. I would remove the fork and check the steerer for damage.

If the steerer tube is swelled or split then you will need a new fork.

a couple of good suggestions, tatfiend, but negative on both counts. I tried the stem at all different heights, from slammed and at the min. mark and everywhere in between. The fork is definitely not the problem, it is original and this bike is small 53cm, the threading is only maybe 3-3.5cm of the steer tube, and I checked to be sure, and the tube and threads all look like brand new. I think I will just try a new stem

Thoroughly grease the wedge surfaces and grease under the head off the bolt.

It only needs to be tight enough so it doesnt move when you torque on it while ridding.

On my mountian bike everything up front is relatively loose. If I crash things move versus break.

its not tight enough to stay in place while riding. a decent bump or pothole would knock it loose

Did you grease everything yet including the threads and under the bolt head?

everything is greased up. Just for fun, I threw in a coda stem with a standard wedge bolt, and it tightened up just fine, I don't know what the deal with this cinelli stem is

Two things come to mind, one is that it may be a French diameter stem, its just .2mm less but a world of difference. Can be hard to measure without a very accurate caliper.

The other thing is what happened to me once, on a very nice bike being built up, the steerer was TOO smooth inside, a ******* rat tail file roughened up the interior surface and the problem went away. A few file scrapes down the stem well below the insertion line helped too.

(hopefully you guys can understand the coarse file type I was referring to, and I'm not the swearing type either)

The other thing that came to mind, but was probably addressed was that a steerer can develop a bulge, yes some gorillas tighten a stem THAT hard, the way to diagnose without a feeler gauge ir dropping the fork is to tighten the stem just enough to reduce play and allow it to be moved up and down in the fork, then watch for changes in the amount of play if the play increases as it goes lower, you have a fork problem. Another indicator is where the headset binds or gets loose at some point in the rotation of the fork, be sure to remove the brake first for this test.

my best guess is it a French stem that came on the bike(who knows what the P.O. threw on there), I pulled the whole front end apart, and there is no damage of any kind. French diameter is the only thing that would make sense at this point. I guess it didn't occur to me at first, since I have never messed with a French bike. Did Cinelli make French diameter stems?

Yes. Cinelli made 'em all, at one point or another.

You need to get a caliper. I couldn't do any bike stuff without one.

I agree, I had a digital model, which somehow got a damaged LCD screen, new one coming soon, just not at this moment when it is needed.

I figured this was the case, I'm just not very familiar with Cinelli items, it just happened to come with this bike. I think the only original components that were on this frame set were headset, BB, post and seat.

+1 Wrong Size.

+1 Use a micrometer. I picked up a "decent" digital unit at Harbor Freight. OK, I know they sell junk, but it is good enough, and the price was low. I use that tool all the time, I should have gotten one years ago.

It would figure that it's F'n French, they had a better way of doing most everything.

Should be marked below the insertion line..

lower part of stem is kind of worn, all i can make out is "100" and just barely see the little "C" logo. eh, I'll just have to order up a new one.