Stem change problem

[NickTB]

Hi all

I’m in the process of changing a stem back for a friend to borrow my bike and whilst tightening the steerer tube back I managed to push the collar and nut through to the forks. I’ve retrieved the parts but now every time I attempt to tighten the bolt it pushes the collar back through.
Any ideas what’s going on?
I’ve attached some pics to hopefully show what I mean

 

[Ajax Bay]

Thread on nut other way round?

 

[raleighnut]

Don't you assemble it as per the bottom photo* but with less thread sticking out the bottom then insert and tighten the whole assembly

* checking the taper is right so the bung expands as you draw the wedge nut into it

 

[NickTB]

The bung is sliding down the tube and not gripping. I’ve tried the nut both ways round, it doesn’t seem to be expanding

 

[Racing roadkill]

That looks like one of Decathlon’s oddities. I’d bin the weird bits and use a star nut instead. Unless it’s a carbon steerer, then I’d just get a standard expander bung.
The bit on the right of the first picture sits underneath the splined bit, the opposite way round to the way you’ve got it in the third picture ( you’ve got the smooth insert upside down ). Then as you tighten the top cap, it pulls the smooth bit into the bit with the splines, which expands. The splined bit has to be in the steerer (confined by it) or you won’t get the expansion reaction ( you won’t get it to work outside of the steerer).

 

[Sharky]

With the bolt out, can you give the nut a tap with a mallet to push it further into the splined compression thing, to make it expand slightly?
Then, does it grip a bit better when you insert it into the steerer? Just needs to be a push fit, but not loose.
Is the nut tapered?

 

[davidphilips]

Think one of the best expansion plugs are these type on Ebay cheap and give good support, Any of my bikes with carbon stearing tubes have them fitted. https://www.ebay.co.uk/itm/Top-Cap-...var=692001111263&_trksid=p2057872.m2749.l2649

 

[NickTB]

With the bolt out, can you give the nut a tap with a mallet to push it further into the splined compression thing, to make it expand slightly?
Then, does it grip a bit better when you insert it into the steerer? Just needs to be a push fit, but not loose.
Is the nut tapered?

The nut is tapered, and I tried that but then it’s too tight to fit in the tube. I’m concerned about damaging the steerer

 

[Venod]

These are good if you can't get yours to work, the star nut used on a lot of Aluminium forks has always seemed a bit of a bodge to me.

https://www.wiggle.co.uk/fsa-compre...0FCEeMPoorfXN-JTVhbO3pa_ZwkvwrHUaAn8lEALw_wcB

 

[Sharky]

The nut is tapered, and I tried that but then it’s too tight to fit in the tube. I’m concerned about damaging the steerer

I think if you can get the tension just right so you can push it in by hand, it will probably tighten up OK with the bolt.

 

[NickTB]

I think if you can get the tension just right so you can push it in by hand, it will probably tighten up OK with the bolt.

That’s what’s fixed it. Keeping the tension.
Thanks for all the replies