How to tighten presta valve?

[sgcyclistsg]

Changed my front inner tube a few weeks back. Everytime I try unscrewing the black cap after a ride to pump the tyres, the top part of the valve flies off. Apart from screwing it back by hand, I've tried using pliers, a spanner, pliers with cloth wrapped around the valve... but nothing seems to work. Kind of scary to think that the only thing holding the valve in place while I cycle is the flimsy black cap.

Any advice on how to tighten the top piece? Not sure of the exact term for the part. The picture probably describes the problem better.

 

[dhk2]

I believe your presta valve stem has broken. The only fix I know is to replace the inner tube. With the new valve, remember that the knurled nut only need be loosely finger tight. In a presta valve, the air pressure in the tube holds the valve closed, the knurled barrel nut is only there for extra security...... After you inflate the tube, no air should escape from the valve, even with the stem nut loose. You could ride with it that way. So, just lighty snug the barrel nut to avoid damaging the valve (or snapping the stem in two).

The plastic cap is only there to keep dirt off of the valve top, where it could be forced into the valve by the pump. Most riders I know don't use the plastic cap (or the nut at the base of the stem either).

 

[sgcyclistsg]

I believe your presta valve stem has broken. The only fix I know is to replace the inner tube.

Oh no... I was thinking that it was just a matter of tightening it better. Here it is after tightening it back using hands and re-inflating the tube to optimal pressure. It can handle the current 6 bar; but if I ride the bike, it pops off only held by the black cap.

 

[CAMPYBOB]

The internal (female) threads of the Presta stem may be stripped out. The picture lacks enough detail for me to tell.

The external (male) threads of the Presta core look OK in your picture.

As dhk2 has suggested and if the threads are damaged the fix is to replace the tube.

 

[sgcyclistsg]

Thanks both for your advice. Was a new conti tube too

 

[dabac]

There are plenty of tubes that don't have removeable valve cores.
If the core coming out is the only problem, why no use some super glue, epoxy, or pretty much anything gluey of your choice on the threads before putting it back in?

The tube is useless to you as it is now, glueing the core in might fix that problem.
Only losing you a feature you might not ever need.

I'd use a q-tip and some rubbing alcohol, Clean out the inside of the stem. Give the valve core a wipe, then use either epoxy or super glue to get it to stay put.

 

[dabac]

...if I ride the bike, it pops off only held by the black cap.

I don't understand.
Does it pop off if you ride?
Or does it stay on until you undo the cap?
Or does it come off with the cap on?

The cap isn't meant to act as a core retainer.
AFAIK they don't extend far enough to engage the stem threads, only the core threads

 

[sgcyclistsg]

I don't understand.
Does it pop off if you ride?
Or does it stay on until you undo the cap?
Or does it come off with the cap on?

The cap isn't meant to act as a core retainer.
AFAIK they don't extend far enough to engage the stem threads, only the core threads

1. I screw in the core, pump the tyres, screw in the black cap. Everything seems fine.

Scenario A: If I do not ride
2. I unscrew the black cap, everything is fine. The core is still in place, screwed on correctly.

Scenario B: If I ride
2. While riding everything seems fine.
3. When I unscrew the black cap, the core flies off the stem.

Thanks for the glue suggestions, but didn't think of that, and replaced the tube yesterday. Only consolation for me is that my new schwalbe tube I ordered from CRC came with sexy see thru cap haha.