I appreciate this is supposed to be a mountian bike forum, but answers from here are usually quicker and make more sense. My bmx headset is FUBAR'd and I need to fit a new one. it is an Aheadset type and has a hollow bolt so a cable can go through it (though I don't use this feature). The hollow bolt has a nut type head and there is a topcap type structure underneath this. I undo this bolt, it turns and turns and turns but nothing comes any looser or tighter. it doesn't appear to screw into anything (that I can see clearly anyway) Getting cross, I tried cutting into the top cap to remove this and see what this bolt is screwed into, but the top cap appears to be "mushroom shaped" if that makes sense.

so, the question is, how the hell do i get this off? I've tried turning it, turning it whilst stopping the top cap turning, undoing the stem pinch bolts and hitting the stem with a hammer, putting a screw driver into the bottom of the forks and hitting it with a hammer and bashing the bolt from above.

any ideas?

I tried cutting into the top cap to remove this

hitting the stem with a hammer

putting a screw driver into the bottom of the forks

hitting it with a hammer

bashing the bolt from above

don't be soft with it-its laughing at you that bike is, try harder, use more force, make it stop laughing.

I'd imagine that it screws into a star nut just as with an mtb headset.

Could it be that the threaded part of the star nut has come loose and is spinning? In which case I reckon your best bet is to somehow cut the top cap off and then hammer the bolt and star nut down into the forks.

seems most likely. I assumed there would be some sort of star spangled nut in there somewhere! Time to dig that top cap out! thinking drill might be the answer for that task?

Maybe of help – but some BMX forks have a "welded nut" inside the steerer instead of a star fangled nut.