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when used to extend a threadless steerer tube do they have any provision for preloading the aheadset or do you just have to press down on the stem before tightening in a trial and error manner?
btw before anyone mentions it I am aware that they often don't fit inside alloy steerers!
Yes, I've used one. They do come with a top cap but you can use your existing headset one if you wish. The quill bolt has an 8mm socket iirc which you tighten to secure the extender. The centre of this bolt is threaded to accept a standard 5mm bolt and headset cap which you then use to tension the headset. Hope that makes sense.
Following mamadirt's advice I got one for the wife's bike. She's still fine and hasn't filed for divorce or reported an attempted murder to the authorities so the expander must be all right.