Mountain Biking - Schrader/Presta

If im using the Alex DM-20 rims that came stock on my hard rock, can I use presta instead of schrader tubes next time i get a flat? i didnt know if they were interchangeable or not

You should be able to as the presta valve has a narrower diameter than the shraeder valve. You can even get a rubber grommet or aluminum tube that is made to fit the cutout so your valve isn't swimming around. A bike shop should have them or a hardware store should have a grommet that will work. It'll prob. even work OK without one but my LBS gave me a bunch for free. I'm using presta tubes in a cheapo set of shraeder rims for my snow tires.

Jay

Just ensure that the hole does not have a sharp edge. Run some emery paper around the edge.

i had to run presta valved tubes for my schraeder valved rims before... i just stuck a small washer in there to keep the valve from moving around.

All of my wheels are set up for Schrader valves, except for 2 that only take presta. Two options, drill out the hole to take the larger size, or for convenience, just buy Presta tubes. As I like to run higher pressure than normal, I now only buy Presta valved tubes, and they will fit all the wheels. Only one pair of wheels came with the grommets to reduce the valve size, but I do not use them. Only point to make is that the hole where the valve enters the wheel, should be rounded, as a sharp edge can damage the tube by cutting into it( But that goes for schrader valves too)

All of my wheels are set up for Schrader valves, except for 2 that only take presta. Two options, drill out the hole to take the larger size, ...snip...)
He already has Schrader size holes so he doesn't need to drill.

He already has Schrader size holes so he doesn't need to drill.

correct