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I have a Continental Road inner tube with a threaded presta valve. I also have a threaded bike pump which I screw onto the valve to inflate the tube.

When I try to unscrew the pump, the uppermost part of the valve unscrews and detaches from the main body of the valve and all the air rushes out.

Presta valve with labels for both parts

Is it normal for a presta valve to be separable into two parts like this? How can I prevent this from happening?

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The correct answer is yes and no. There are in fact two types of Presta valves: One where the core can be removed. Those are often used with tubular tyres so that the inner tube can be filled with a sealant (latex) fluid. Some higher quality tubes also have removable core valves for the same purpose. Normal and cheaper tubes have Presta valves where the core cannot be removed.

There is a special tool to be used for handling the core although like @Michael said the (careful) use of pliers also works.

Another reason for a removable core is the possibility to use valve extensions with high rims.

  • You'll also see these on tubeless mountain bike setups so you can remove the core and add sealant as well. The picture in the question is actually the valve from a tubeless mountain bike setup. – Benzo Oct 15 '15 at 15:41
  • @Benzo, that setup is not specific to MTBs, my road bike runs valves pretty similar to the OP pic. – Klaster_1 Oct 16 '15 at 5:17
  • Agreed that the presta valve is used for road bikes too. However the one in the picture is for a tubeless system used on mtb and cx bikes, as indicated by the tapered base below the valve (instead of being permenantly attached to the tube) which seals tightly against the opening in the rim when the lock nut on the outside of the rim is tightened. – Benzo Oct 17 '15 at 13:16
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Yes it is normal. You can carefully tighten the valve core with pliers or special tools. Medium strength Loctite helps a lot too.

  • 1
    It is also a small wrench size (like 4mm). That is what I use. And Loctite. – paparazzo Oct 15 '15 at 12:01
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It happens to me every time I buy new tubes. You just tighten the valve and it won't happen again.

  • Welcome to Bicycles @JMAR. When yo have enough rep such a short post should be made as a comment. I encourage you to check out the help center to see how this site works. Again, welcome. – andy256 Oct 15 '15 at 22:47

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