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I would like to understand how Presta valve cores work.

Giant Replacement Presta Valve Cores (1 of 10)

A Presta valve core contains all of three screws:

  1. the top nut screw (brass part, right side of the image),
  2. the valve cap screw (with notches to enable grasping with nose pliers or with a dedicated tool), and
  3. the screw fitting into the valve stem (just to the right of the white plastic part in the image).

The proper operation of a valve core means that air should not leak through the third screw, which suggests that something (perhaps loctite), must be used to make the connection airtight (as well as the first—see below).

But if that is the case, and unless these screws are manufactured to incredible tolerances (they don't seem to be), then loctite would not be optional.

How does a Presta valve core work? In other words: what stops air from leaking out of a valve core?

In particular, we can note:

Tighten #1- When we tighten the top nut (at screw #1), we are squeezing the black plastic part. We must tighten hard enough, and the black plastic part must still be supple enough, to avoid air leaking there.

Tighten #3- When we tighten the valve core in the valve stem (using pliers or a dedicated tool), we may be squeezing the white plastic part, fitting it into a resting place in the valve stem. As before, we must tighten hard enough for this connection to be airtight.

The cap (#2) is not meant to be airtight, and is occasionally even discarded by some.

Either way the two (black and white) plastic parts are so tiny we do not want to tighten either of them so hard to crush them. Can you elaborate or explain how this part works?

In particular, Loctite at #3 may be advisable not just to avoid inadvertent unscrewing of the valve core along with the chuck of a hand pump. Loctite would avoid problems with leakage at the white plastic part. Thoughts?

Another Design

Here is another design.

Muc-off Tubeless Valve Core

We note:

  • There is no standard for the size of either plastic part.
  • There is (presumably) a standard for the #3 screw, but the threads on screws #1 and #2 could perhaps be chosen by the maker of a valve core.
  • The white plastic part is not sufficiently wider in diameter than the part to its left for it to hold air against a part inside the valve.

Summary

Metal-against-metal does not provide a seal. For the barrier between the valve core and the valve stem to stop air, the white plastic piece must be acting as a seal, with one of three methods:

Method A1

The white plastic piece (WPP) is a truncated cone. When the valve core is tightened, WPP rests on a flange in the valve stem and creates a seal.

Method A2

The metal part lying between the two plastic pieces has distinctly smaller diameter, enabling WPP to rest, and hence create a seal, on a flange.

Method B

When the valve core is tightened, WPP expands in diameter. It then rests on the (smooth, cylindrical) interior of the valve stem, creating a seal.

It's also possible for the top nut to tighten both plastic pieces simultaneously, though that would be a singularly poor design choice.

Understanding whether it's A1, A2, B, or some other method would inform how tight we should screw in the valve core, and how carefully to inspect WPP for damage or irregularities.

Bonus Question

A valve core is valuable for the regular addition of sealant, but why are inner tubes ever manufactured and sold with a valve core?

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  • I’ve never needed need to use loctite on the core to get any of my tires to hold air
    – Paul H
    Aug 28, 2023 at 21:23
  • @PaulH That's alright. Let's first try to understand what's happening (how it works), and then we can talk about how to hold air.
    – Sam7919
    Aug 28, 2023 at 21:29
  • The core itself works in partnership with the "tube" and specifically the inside shape of that tube. So its not guaranteed that both cores will work the same in different valve stem tubes.
    – Criggie
    Aug 28, 2023 at 21:35
  • @Criggie Are you sure? The first is a Giant product. The second is from Muc-Off. Giant may manufacture inner tubes, and hence sells valve cores for its tubes, but Muc-Off does not seem to make tubes.
    – Sam7919
    Aug 28, 2023 at 21:45
  • 2
    FYI: you don't have to tighten the top nut, #1 at all, for the valve to hold air. Air pressure within the tire will keep the black gasket seated. Aug 28, 2023 at 23:59

3 Answers 3

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Working from left to right of your first example:

  1. The black rubber gasket, when the brass nut is sufficiently tight, prevents air from passing through the core.

  2. The white rubber gasket, when the core has been tightened sufficiently into the hollow valve stem, prevents air from passing around the core.

No Loctite is needed to seal up a valve core + stem combination provide both are in reasonably good condition.

What your question is missing is the second piece of this puzzle: the valve stem.

There’s no way I could possibly take a photo of this, but both valve stems I have sitting at my desk appear to have to separate bores. The threaded end of the stem where the core goes (away from the rim) has a larger diameter bore that steps (or tapers) down to a smaller diameter, smooth bore. That’s how the white gasket forms its seal.

Most common issue for me is that the valve core clogs with sealant and I have difficulty getting air to pass through the core intentionally (in or out)

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  • I have not had any problems with valve core filling with sealant. Usually, if it clogs, the valve stem is clogged. Even if I remove the core completely, it is still clogged. Then I have to use a hex key or other hard object to poke a hole in the clog. Aug 29, 2023 at 12:56
  • Regarding point 1, what holds the air in between removing pump and tightening the valve nut?
    – ojs
    Aug 29, 2023 at 14:33
  • @PaullH All cool, but I am still missing a piece of the puzzle. I added a "summary". Do you see the solution to this last piece of the puzzle?
    – Sam7919
    Aug 29, 2023 at 14:36
  • @ojs as mentioned elsewhere in comments, the air pressure in the tire closes the valve, but you get better air retention with the brass nut tightened down, especially if the valve is partially gummed up with sealant and doesn’t move as freely as it should
    – Paul H
    Aug 29, 2023 at 15:04
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    @Sam - the white rubber will deform to the shape of the surface it encounters so the seal is a combination of A1 and B. Removeable valve cores are useful in road bike applications to allow the use of valve extenders on deep rims. Also the brass screw is soft and vulnerable to damage so being able to replace the core is an advantage.
    – Noise
    Aug 29, 2023 at 22:46
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To answer the "bonus question", there are two main reasons for making inner tubes with removable cores:

  • If you don't cover the valve core with a cap, it can clog up from riding in dirty and muddy conditions. Unscrewing and cleaning or replacing a core is easier and a bit cheaper than replacing the whole inner tube which may be in perfectly fine condition.
  • You can put tubeless sealant into the inner tube to try and patch small holes. There are also dedicated sealants designed for inner tubes.
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This is a thought towards an answer. It is not an answer yet.

The following cross-section of a Presta valve with a nonremovable core suggests that circular expansion might be at work to seal a regular (i.e., not one with a removable core) Presta valve.

cross-section of Presta valve with nonremovable core

In other words, when we tighten the top nut in an—again, regular—Presta valve, the black plastic expands diametrically, touches the narrow interior of the valve stem, and subsequently seals the valve.

Why does it matter? It matters because it informs how much we should be twisting the top nut. We are not merely "keeping the central rod in place". We may indeed be sealing the valve. This becomes more important as we lose air. For then the remaining air pressure may not be sufficient to keep the central rod (name?) in place.

To truly answer the question we need to see a cross-section of a Presta valve stem with its core removed, and to see whether the stem narrows considerably in the area adjacent to the white plastic part, which would expand diametrically to seal the valve, but when we tighten the core itself, in addition to the top nut.

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