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I found the question How to keep Presta valve insert from detaching after inflation? but none of the answers are helpful because this happened to me in the middle of a ride on a rental bike. All I had was a pump, a hex wrench (allen key) for the axle and a tire lever.

I got a puncture, and without a patch kit I swapped on the spare tube. No problems thus far; I proceeded to screw on the pump and inflate the tyre. When I went to unscrew the pump, the valve core unscrewed and the tyre immediately deflated.

(This is when I discovered the tube had a removable valve core.)

On my fifth try (I had nothing but time) managed to make sufficient seal by main force (without fully screwing on the pump) to get a couple bars of pressure in the tyre (fortunately it was a 32, not my normal 25) and it sufficed to get me to a proper pump.

Is there any trick or technique that I could have used to deal with this more effectively?

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    One thought I had was to try to gum up the threads of the core with a gel, but I was afraid it might only make things worse.
    – DavidW
    Sep 20, 2022 at 15:19

6 Answers 6

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I once saw someone riding out of a bike park with their minipump wedged in the front wheel and tied to the hub between the spoke planes with a clean poop-bag.

They'd had exactly this problem where the valve core would not stay in when the thread-on pump was removed, so simply tightened it all together for the trip out.

If it works... and it did.

Clearly this was a get-home (or get to the carpark) fix only.

Update - Coincidentally, this happened to me on my work commute today.

The nozzle would not come off the presta valve and moderate force was not sufficient. Excessive force seemed like a bad idea given I was a long walk from anywhere.

So I unscrewed the end of the topeak pump and used some duct tape to secure it between adjacent spokes. This minimised the off-balance mass.

Surprisingly the bike rode perfectly other than having only ~60 PSI in the tyre and resulting squirm feel on corners.

enter image description here

Once lunchtime arrives, I will attempt to save the pump. Update to come.

enter image description here

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    I once saw someone ... So how'd you know exactly what was used to secure the pump? Did that "seeing" involve a mirror? :-D
    – Andrew Henle
    Sep 21, 2022 at 21:02
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    @AndrewHenle I stopped and asked the guy - he was going super slow and I wondered if he'd tied it on with a sock or glove. In this city, dog owners are required to pick up their pet's poop on pain of fines, so many owners always carry some ~A5 sized plastic bags, also known as nappy bags. It's a good idea for the mental toolkit.
    – Criggie ♦
    Sep 22, 2022 at 1:08
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    @AndrewHenle not 24 hours later - it really did happen to me
    – Criggie ♦
    Sep 22, 2022 at 23:20
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    The good thing is that even if the valve shot somewhere far into the grass and cannot be found, this will still work.
    – Vladimir F Героям слава
    Sep 23, 2022 at 9:09
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I know you weren't carrying one, but there are multi-tools that include valve-core wrenches. There are also standalone valve-core wrenches, although that would be a bit fussy to carry on a ride.

Barring that, I'd look for something in my environment that would engage with the flats on the valve core and serve as a makeshift wrench. You might be able to pinch it between a pair of keys, for example (I've never had to try this).

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Most often I am fine with tightening the valve with my fingers, but I must admit that the risk of unscrewing the valve when detaching the pump is there. Or, more likely, when detaching a valve adaptor for Schrader pumps on Presta valves.

Otherwise you just have to improvise and try to find something narrow wth the right width. Perhaps a pair of keys? It should allow you somewhat more force than just bare fingers if you just hold them on the flat parts of the valve. Or some coins...

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Some answers and comments advocate tightening the valve core to the point that it does not come undone with the valve head. However, you should be careful not to over-tighten it, as you may snap the valve core – they are not very strong.

There is a "Goldilocks" zone of tightness that is neither so tight that it damages the valve core, nor so loose that the valve core comes out with the pump head. Unfortunately, you never really know when you're in this zone until it's too late (speaking from experience).

You can avoid this problem altogether by getting a pump that has a locking attachment rather than a thread-on attachment.

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  • Very few pumps are thread-on, the problem is usually in adaptors to compressors. They are always screw-on. (Or the Schwalbe boost bottle, but that is done at home.) There is enough of this safe zone, just don't be crazy and don't use a wrench. And be gentle with the usuall small plastic spanner, if you have it.
    – Vladimir F Героям слава
    Sep 21, 2022 at 6:10
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    @VladimirFГероямслава Many mini-pumps have thread-on attachments – particularly those that attach via hoses rather than directly at the pump body. Besides, this question is specifically about pumps that are thread-on :-)
    – Will Vousden
    Sep 21, 2022 at 7:09
  • Yes, some do, but I had these problems typically with Presta to Schrader adaptors. I'd argue that the question is not about pumps at all, but how to screw a valve so that it does not come off when unscrewing. Whether it is a pump with a Presta screw-on hose or a pump with an adaptor on a Schrader hose is really secondary (I own the latter).
    – Vladimir F Героям слава
    Sep 21, 2022 at 7:35
  • Many main message is that the risk of snapping the valve is not huge if you do not go crazy with a wrench or spanner. With improvized means it is very unlikely and you are happy that holds in place at all.
    – Vladimir F Героям слава
    Sep 21, 2022 at 7:37
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I think the removable valve cores are mostly removed and installed with a wrench.

So carrying the smallest possible 4" adjustable wrench helps with that, you can screw the valve core so tight it won't be removed by removing the screw-on pump. This may be advisable in any case too, because I once had a new tube (just bought from bike shop) that lost air slowly but faster than the other tube, and noticed the valve core wasn't fully tight -- fully tightening it repaired the slow air loss.

The adjustable wrench has also other uses too:

  • You can straighten bent rims with an adjustable wrench (note: bent rims, not bent wheels; a bent wheel requires a spoke wrench)
  • You can straighten bent disc brake rotors with it
  • If you ride with someone who has nut-type wheels (even if you don't yourself have these), the 4" adjustable wrench is usually just barely large enough to allow unscrewing the wheel nuts, and also nearly any hex head bolts and nuts on a bike apart from headset nuts and recessed bolts like crank bolts can be tightened and untightened with the adjustable wrench too
  • If you use Shimano chains and carry a mini chain tool and reinforced connecting pins, the adjustable wrench is handy for removing the removable end of the reinforced connecting pin after pushing the pin in after a chain repair
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  • Might be handy (or not, I do not carry it), but if the rented bike does not have it...
    – Vladimir F Героям слава
    Sep 20, 2022 at 15:46
  • I only had a hex wrench; I was not carrying an adjustable wrench, so this is not helpful. I agree this wouldn't have been a problem with proper tools.
    – DavidW
    Sep 20, 2022 at 15:59
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    Please don't recommend straightening bent rims with and adjustable wrench. People out there might actually believe you.
    – ojs
    Sep 20, 2022 at 16:38
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    Bear in mind that you can over-tighten a valve core and snap it. This has happened to me before (after having exactly the same issue as the OP) – and it's not fun. The result is the bottom fragment of the valve core being threaded into the valve stem, while the top part (the only bit you can grip) is spinning freely. Good luck fixing that by the side of the road.
    – Will Vousden
    Sep 20, 2022 at 19:04
  • @ojs Adjustable wrenches actually work pretty well for straightening dings in rims in my experience. Of course, trying to true your wheel with an adjustable wrench would be a pretty poor idea.
    – MaplePanda
    Sep 20, 2022 at 19:12
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I just buy tubes without removable valve cores. I patch my tubes after punctures so I get a lot of use out of them, I've never had to throw away a tube because the valve core needed to be replaced, its always been an irreparable puncture/tear.

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    But the question is about a rented or borrowed bicycle. You don't get to choose what kind of tubes are installed there.
    – Vladimir F Героям слава
    Sep 23, 2022 at 8:52
  • My bad - I missed the rental bit
    – user3413340
    Sep 23, 2022 at 11:51

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