A little while ago I replaced the bars on my pug with some wider ones; to make this easier I also bought a quill-to-threadless stem adapter. (I then set about taping the bars with a moderate degree of success.)
The finished product looks like this, including the stem adapter:
Now, I also submitted this picture along with a review of the adapter to the website where I bought it from. I then received this email from them:
Thank you for reviewing your Profile Quill to Ahead converter. I noticed on your submitted image that you have a gap left just below the stem. Normally you would insert the quill section into the frame completely so that your stem rests on the top of the headset. This prevents any chance of the stem slipping down the ahead mounting point or the quill slipping down the inside of your steerer tube. I may be looking at your set up incorrectly but just wanted to be sure that you are aware of the risks. Do let me know if you need any help with this.
Now, I'm not sure about this; I actually don't see a problem with the way it's set up. So I sent this reply:
Many thanks for your mail, I appreciate the time you have taken to write it.
However, I'm not sure your concerns are justified. The quill is no more likely to slip inside the steerer tube than a regular quill stem of the kind that was on the bike before; one of the advantages of a quill stem is, after all, that the can be raised or lowered, and then tightened into position.
I suppose it's possible that the stem might slip on the converter, but this too seems unlikely. Whilst we are used to seeing such stems resting on spacers, the primary purpose of the spacers is actually to prevent the steerer moving up and down with a threadless headset - it is the clamp action of the stem that holds the forks onto the bike (something that is done by the locknut on a threaded headset). The stem is no more likely to slip down the adapter than, say, the bars are to slip round in the stem (in fact it is somewhat less likely, given that the turning moment of the bars is on axis with the clamp, whilst for the stem would be against it, thus pressure on the bars would tend to jam the stem against the adapter).
As such, I don't think there is a problem. The quill is inserted to the 'minimum insertion' mark, so it seems it was designed to be used in this way (otherwise the 'min insertion' mark would be just under the lip of the top part of the adapter). It does look a bit odd, I agree, but that's simply because we're not used to seeing an oversize stem fitted to a narrow quill. However, I'm confident there is no mechanical risk from this arrangement, provided all the bolts are tightened to proper tolerances.
So - what do you think? Am I correct, or does the stem need to be resting on the headset locknut? It's worth noting that I actually doubt I'd be able to get the quill adapter any further into the steerer; it was a tight fit and difficult to get down as far as the 'min insertion' mark. And if I did get it lower, then the bars would be too low, unless I then bought a stem with a huge rise angle.
Anyway, I await the superior skill and knowledge of the SydneyCyclist community!