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I have to keep tightening my stem cap onto my carbon steerer Rock Shox forks after every ride as it develops play. I'm currently using an M-Part steerer/stem cap arrangement.
What's everybody else using?
I wouldn't worry about it, now if your stem was coming loose...
assuming the M:Part piece is a bung arrangement, and that if you "keep tightening the stem cap" it implies the bung is not moving upwards in the steerer, then the top cap bolt must be loose and precessing/unwinding. Which I wouldnt expect to happen often.
Dont forget, once you have preloaded the headset bearings with the top cap bolt, you should be tightening the stem bolts tight and then usually backing off the preload bolt a quarter turn - it's done its job.
Also check that the stem cap isnt tightening down on the steerer top, rather than the stem itself - otherwise you will simply be locking the stem in place with play left in the headset.
How much gap is there between the top of the stem (or any spacers you have on top of the stem) and the top of the steerer tube ?
I've checked to make sure the stem cap's tightening onto spacers and not the steerer tube (there's no gap).
Didn't know about back off the load on the stem cap bolt after I've tightened the stem bolts so I'll give that a whirl.
Stick a 5mm spacer in there and you should be right as rain.
You don't need to back off the preload bolt, it doesn't improve anything. Might as well leave it alone. If you leave it alone then if you need to tighten the headset you can just add an eighth of a turn from where you left off.
I had this with the WC Revs on my trail bike, fresh carbon paste between the steerer and stem sorted it.
Once the stem pinch bolts are tightened the pre-tension bolt is redundant, you could take it out and the headset wouldn't know about it.