Originally Posted by
Milton Keynes
So what I did was screw one of the flat Presta nuts all the way down on the stem before inserting it through the hole, then another one on the outside like normal.
Bad idea, if the flat Presta nut prevents the rubber at the bottom of the valve stem from sitting flush on the rim's inner surface. The inflated tube will try to fill that gap, possibly stretching the rubber thin enough to fail.
You can pick up various diameters of rubber grommets for electrical wiring at almost any hardware store. Chances are you can find a size that fits the rim and valve perfectly.