I experienced the same tapping of the valve stem against the chassis after changing out my V10f tire tube. I believe it was aftermarket, as the original OEM tube has the properly angled stem.
I guess you could possibly shim the valve stem with a couple of washers before putting the stem into the wheel rim hole.
What I did on mine is glue washers, 8 of them in all, over top of the axle bolt holes and slip the chassis back over the shims, essentially splaying out the chassis so it doesn't hit the valve stem anymore.