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I have a rim with a Presta valve hole, but I would like to drill it out to fit a Schrader valve. Is there a specification for how large the hole should be? I want to be sure the hole isn't too big, but I also want to make sure it will fit all varieties of inner tubes I might run across in the future.

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    Practically speaking, schrader valves are all the same diameter. Don't take that as an endorsement of the idea of drilling out a rim, though. – Paul H Oct 13 '20 at 19:20
  • Answered here bicycles.stackexchange.com/questions/12992/… - Sheldon Brown says "If you want to convert a rim drilled for Presta valves to accept Schrader valves, drill it out with a 21/64" drill bit." – mattnz Oct 13 '20 at 23:28
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    Not a good idea, the rim will be weakened. – Carel Oct 14 '20 at 7:25
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Short answer: 8mm.

The Wikipedia page for Schrader valves gives the thread size for the Schrader variant used on automobiles and bicycles.

8V1 thread: 0.305 in (7.747 mm) x 32 TPI

Be aware you need to make sure the rim has enough width to accept a larger valve hole without becoming weakened.

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    additionally, the enlarged hole needs to be deburred so its a smooth rolled edge, and has no way to frett through the tube's material over time. – Criggie Oct 13 '20 at 21:57

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