Clydes Dale

Installed a new fork, headset, and stem on my Kestrel with Integrated Headtube.
While trying to load/tighten the headset, I noticed that the Top (bearing) Cap of the headset was not sitting flush with the Upper Bearing Cup. As I rotated the fork, I could see the Top Cap "wobble" (i.e. the gap between the cap and cup would grow and shrink.

I found that it was my stem that was causing the problem. The bottom (and top) surface of the stem fork clamp was not perpendicular to the steerer bore axis. I don't believe this is a manufacturing defect, but a design flaw.

Can a stem be "faced", so that the surface that mates against the headset is perpendicular to the steerer axis?

FBinNY 

Yes, the stem can be faced, but you shouldn't have to. Stem manufacturers know (or should know) that bottom squareness is critical to proper performance of the system, and I find it hard to believe that a decent stem would be off square.

To test properly for squareness assemble the fork, headset and stem with a bit of course lapping compound between the top washer and bottom of the stem. Tighten the topcap system normally but only firm up the stem clamp so the stem is free to spin. Spin the fork, including the top washer a a bit while while holdijg the stem.

If all the wear is on one side of the base of the stem, it's out of square. If wear is all the way around, something else is off square - possible the headset isn't properly seated in the frame.

If the stem isn't square, replace it because properly machining it would probably cost more than it's worth, and also because if the manufacturer got that wrong, it doesn't speak well for the stem in general.

DaveSSS 

If the bottom of the stem is not perpendicular to the steering tube, it is a manufacturing defect, not a design flaw. Unfortunately, it is not practical for the average person with no precision measuring equipment to determine if the face is perpendicular to the steering tube.

Of course a machine shop could face a stem, but unless you find a real friendly shop willing to do this as a courtesy, the cost would probably make this an impractical option.

If the stem is new, it should be returned, but you might want to try another new stem first, just be sure that it cures the problem.

Clydes Dale

Thanks for the input.

DaveSSS, It is a design flaw if all the stems are similar (which I believe is the case). Yes, a machine shop would be cost prohibitive. But I was hoping a headtube reamer/facer tool could be used somehow?

FBinNY, the bottom surface is so far off, I can easily see it when I install the 1inch adaptor shim (that came with the stem). The shim is "sqaure"

LWaB

JA Stein do a stem facing tool, scroll down https://www.jastein.com/Html/Tools_for_Frames_Forks.htm

Clydes Dale

Update: The problem is unique to my stem (lucky me). So it was a manufacturing defect after all. Now all I need to do is find an exact replacement.

Thanks for the link to the SF-1 Stem facing tool. It's the perfect tool for my problem. But I can't justify the $160+ price.

DaveSSS 

I worked as a machinist for 10 years and a manufacturing engineer for another 10 years. It doesn't matter is every stem you can find is not square, it's still a manufacturing defect, since the top and bottom surfaces of all stems are supposed to be square to the ID of the stem. No one would design a stem to be out of square. The design is what's on paper, not the finished product.

The headtube facer is the right idea, but all it does is slide over a guide shaft to square it up to the head tube. The shafts would be made to fit much larger head tube diameters, not stems.

If you have a 1" reducer bushing with a shoulder on it, you could use feeler gages between the stem and shoulder to identify the high and low areas. With enough skill, the high spots would be filed down by hand, but it takes a very skilled person with a large file to do this successfully.