In case you really want to drill, the only thing you need is a 10mm drill bit, and a sandpaper to give a smooth finish.
It is true that the rim gets weaker, and very narrow rims should not be drilled, but I have performed this enlargement a couple of times and rode the wheels some honest hundreds of km in every kind of terrain, with no problem.
I did this because I like the ease to get at any gas pump to top tire pressure after trails, and also because schraeder tubes are more avaliable and less expensive where I live.
bicycles.stackexchange.com/users/2355/heltonbiker
Drilling out your rims will reduce the strength of the rim and increase the likelihood of cutting the valve stem on your tube.
For a few dollars you can buy two of these:
This adapter threads onto a Presta valve and effectively turns it into a Schrader valve.
Presta valve tubes are the same price and are as widely available as Schrader valve tubes, so you shouldn't need to drill out your rims.
(Also, depending on how deep your rims are, finding Schrader valve tubes with valve stems long enough could be a challenge.)
For a discussion of the virtues of Presta v. Schrader valves, check here .
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With MTB tires, there's really no issue in drilling out the valve hole to a larger size since the rims are so wide to begin with.
You can do it yourself with a drill bit (3/8" or 10mm) or have your LBS do it.
Sanding or reaming the hole afterwards is important, as well as making sure there aren't any sharp metal bits floating around afterwards that might puncture your tubes.
If you ever want to go back again (after having blown through your 10-pack of schraders), you'll find that you have the opposite problem -- the hole is now too large and the presta valve will fit loosely.
You can get "presta schrader rim adapters" that fill in this gap and prevent the tube from bulging through or the valve rattling: search term: https://www.google.com/search?q=schrader+presta+rim+adapter
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It depends on the width of the rim, and how it's constructed.
If the rim is over about 32mm (about 1.25", measured from inside to inside)
then it probably has sufficient "meat" in the rim that drilling will not significantly weaken it.
But of course it would be silly to do this simply because you have a Scharader tube -- tubes are cheap.
bicycles.stackexchange.com/users/1584/Daniel R Hicks
Tubes may be cheap, but good luck filling a presta valve when you are on the road in the USA.
NO gas station has presta chucks.
And if you are living in the USA, it's a 3/8 inch drill bit to convert to schrader valve.
I have yet to see anyone have a wheel collapse due to converting to schrader valves.
The bike shops warn against it because they gotta sell those presta tubes they have in stock.
Only reason I can see for NOT converting is there may not be a schrader tube available in your size rim.
That's when you use the adapter shown above.
Look on YouTube and you will find videos on how to change from presta to schrader valves.
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Actually, correct drill bit size is 21/64", not 3/8" http://sheldonbrown.com/gloss_sa-o.html - search for Schrader
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21/64" is the proper size drill bit.
I found one of this size at Autozone.
If you drill from the tire-side in toward the hub, most of the sharp edges will end up on the outside of the rim and will never even impact the tube.
Just scrape off the worst of it
so you don't cut yourself on the edges.
In my case, sanding was not even required.
I have ridden on a drilled out rear wheel for the past six months and treated it to some rough cross country sections including small jumps (at most six feet of distance spent airborne), tree roots and steep rock gardens and there is no sign of metal fatigue around the drill site.
I am not sure how this sort of modification would impact a bike used for downhill biking.
I am so happy with my results that I just drilled out my brand new set of wheels because I appreciate the convenience of the schrader valve.
bicycles.stackexchange.com/users/34018/Aaron
My 26" mtb rims support presta sized tube stems, however can I drill the diameter of the holes in the rims bigger in order to support Schrader stemmed tubes? what is the proper way to do this, is there a kit?
Schrader tubes in Presta configured Rims?
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<rims><presta><schrader>
bicycles.stackexchange.com/questions/12992
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iT is perfectly possible to drill out Presta rims so you can fit Schrader tubes. The correct drill size to do this, according to Sheldon Brown is 21/64", not 3/8"  http://sheldonbrown.com/gloss_sa-o.html.  Schrader valves are more convenient, especially for inflating tire at gas stations in US. However, there is no price saving as Presta and Schrader valves cost about the same. Before drilling, it is a good idea to check if a Schrader valve is available to fit the Presta rim. This is easily done with a 10mm or 21/64" drill bit. There are warnings that this can weaken the rim, but especially with thicker MTB wheels this is not a great risk. Care should be taken to sand the hole and remove sharp metal bits before replacing the tube. Tubes with both valves are readily available, but some sizes may not be available with Schrader valves and it may not be possible to fill a Presta valve while on the road.
This is easily done with a 10mm or 21/64" drill bit. There are warnings that this can weaken the rim, but especially with thicker MTB wheels this is not a great risk. Care should be taken to sand the hole and remove sharp metal bits before replacing the tube. Tubes with both valves are readily available, but some sizes may not be available with Schrader valves and it may not be possible to fill a Presta valve on the road.