I would like to understand how Presta valve cores work.
A Presta valve core contains all of three screws:
- the top nut screw (brass part, right side of the image),
- the valve cap screw (with notches to enable grasping with nose pliers or with a dedicated tool), and
- 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?
Here is another design.
- 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.
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:
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.
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.
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.
A valve core is valuable for the regular addition of sealant, but why are inner tubes ever manufactured and sold with a valve core?