stem/headset play?

My road bike has an allen head screw that holds a cap on top of my stem, that is the only screw I can tighten.

My questions is if I hold the front brake and jiggle/rock the bike front-to-back there is a slight and I mean slight amount of play in the headset/stem area.

How much play is acceptable. I assume there is no way to totally remove the play since the pieces aren't welded together but when is too much too much?

There should be no real noticable play in a headset. It sounds as if you have a threadless headset. With this type of headset, you will need to loosen your stem bolts and then tighten the adjusting bolt which is the allen head screw that you identified on the cap. Tighten it just enough to remove the excess play but not tight enough to cause the steering to get tighter. If you cannot get a perfect adjustment, it is better to leave a little play in the steering instead of having it tighter. After the adjustment is done, check the alignment of your stem and tighten it up. The stem is what locks you adjustment.

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When I remove the allen head screw and the cap that the allen head screw was holding down I have an open/empty silver depression. There is nothing there except the hole in the middle for the allen head screw that holds the cap. Is the part I need to adjust/tighten below this somehow?

No. Put the bolt and cap back in. Loosen the steerer clamp bolts on the stem. Tighten that bolt holding the cap on until the slop goes away. Your fork should turn freely when done. If it turns with a bit friction, back off the cap bolt by an 1/8 of a turn and see if the friction goes away. Repeat as necessary.

That bolt you're tightening pulls on that piece you saw inside the steerer. This causes the steerer to shorten and to increase in diameter a bit, thus tightening things up, removing slop. That's exactly the way threadless headsets are supposed to be adjusted.

This Park Tool page is all about threadless headsets.

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I guess I'm going to have to take the damn thing into the LBS. I took it apart like it says on the webpage at Park Tools.

The strange thing is when I took the nut and cap off it also brought the inside piece with it. Then I heard somethig fall. I turned the bike upside down and the nut inside the stem was falling back and forth freely in the stem.

So I took evertying apart, removed the brake, wheel, etc...I was able to get the spacer out and underneath that was the nut. I took it out, started the nut w/ the spacer and inside piece, then tapped it back into the stem and reassembled everything. All went well but now I really have some play. If the bike is sitting on the ground, on the wheels, and I hold the front brake then gently rock the bike back and forth I have a good amount of play in the front stem/fork area.

I took the bolt and cap back off and made sure there is a slight gap inbetween the top of the stem post and the handlebar stem, like the picture shows on Park Tools website. When I replace the bolt and cap and tighten I can see it sucking everything together like it's supposed to but there is still slack. I am getting this bolt pretty tight.

Any clues?

...Any Clues?...

Well it sounds either like the starnut (or expansion nut depending on how your bike is set up) wasn't reseated far enough into the steerer tube when you reassembled the bike or you need a slightly larger spacer below or above your stem.

Bottom line, you should be able to preload the headset appropriately without a lot of force or effort. One way or another it sounds like the top cap bottoms out before you've achieved proper preload.

One other thought, are you absolutely sure both the top and bottom bearings in the headset and any internal headset parts are installed correctly as in the correct sides up? I bought a used frame recently that has an integrated headset and the headset was improperly assembled and wouldn't adjust properly. Turned out the bottom sealed bearing was installed upside down and wouldn't seat properly. Took things apart, flipped the bearing right side up and everything worked like it was supposed to.

Bottom line on headset preload, the top cap preload procedure is just a convenient way to adjust the headset bearings. As others have pointed out the stem bolts actually clamp and secure the headset. You should in principle be able to just apply vertical pressure to the stem before tightening the bolts or use the "french method" of inverting the bike in a workstand and using gravity to preload the bearings. I'm not suggesting either approach, just remember that the top cap is just an easy adjuster and not what really holds a headset together. If the adjuster bottoms out before achieving the proper preload or it simply isn't possible to remove the headset slop even when the top cap isn't bottoming out then something is wrong.

Good luck,
-Dave

So maybe I'm not sure if everything is assembled properly b/c I see what you're talking about if I remove the bolt/cap I CAN apply down-force and remove the slack. Then when I tighten the bolt/cap it doesn't remove this slack.

Now that I've fubared the allen head bolt I'm assuming the bike shop will have a magic trick to removing it?

Also, my other bike, Trek 5200 and my wife's bike, Trek 2200 both have crane creek headsets. My Trek has a very VERY slight amount of play, I would consider this acceptable. My wife's has absolutely zero play.

My TCR C Zero Giant that's having this issue has a Giant Carbon Stem, the headset has no markings so I assume it's stock Giant parts. I've never changed anything. Is it worth getting a new stem? Could this be my problem?

Good news, the Local Bike Shop fixed me up. The headset needed an extra spacer. I recently added a new stem and this was causing the problem. Now all is well.

Nice, the LBS is under rated these days.

My experience exactly! I had a little play - added an extra spacer on my headset to create the 1/8 to 1/4" space required over locknut, and no play!

Eureka!!! The bike shop comes thru again, good for them.......and you.

a bike shop anecdote for this post: as teenagers our father bought brand new road bikes for me and my brother in the 80's. At home I wanted to set the stem (old system) for better confort. I then panic because the long bolt actually seperated from the fork so we head back to the bike shop. The mechanic told us it was a serious problem and sent us miles away to the bike distributor...

as you all now it was a matter of patience until the bolt attached the fork and that was all the serious fix needed

It's good the LBS worked things out for the OP.

Of course, there are other sorts of LBS........today I was in an LBS buying some small bit, and I overheard a mechanic telling a customer with a newly built bike, "...just put two spacers under the stem and cut the steerer down." How's that for advice? FWIW, there was no fitting going on.

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Ain't it the truth. Even here in the cycling rich 'republic of Boulder', we see what other shops do or what other shops say that really makes us scratch our heads. We have opinions and make mistakes but this isn't rocket surgery.