STEERER EXTENDER: The report back: well all back in one piece (much to my family's relief). headset still nice and snug zero play etc. despite taking in a couple of sets of steps and some pretty bumpy singletrack. (I won't intentionally be getting air, or racing downhill – it is a xc singlespeed after all)
just for the record: yes its a bodge,not ideal – but it works. its set into a steel steerer. the steerer is only about 20mm short so i guess its about 50% steerer and 50% extender inside the stem.
how it works – you insert the extender minus the top cap into your steerer so theres enough poking out to take the stem. pop the spacers on to create the correct diameter. put the stem on.tighten the top cap into the threaded head of the internal bolt (that holds the quill in place) as this tightens it pre loads the bearings (which is its job) and tightens the quill a bit more. (At this point I checked for any paly and had a right tug around to see if i could pull it off (ffnar ffnar) it being the fork ) Tighten up your stem and your done.
I did make one slight alteration – I replaced the BBB top cap with a spare Chris King I had lying around.
Now back to how all this started – the bike 'mechanic' i was refering to did not say he thought the extender was a pile of sh#te and in his opinion it wouldn't work – he told me it wasn't for extending steerer tubes, it wasn't to be used for threadless headsets it was for converting aheadset stems to quill type.
@rocketdog – I've burnt the note: somethings are best left unsaid until after you have departed this life.