Punctures that happen because some sharp thing has worked its way through the outside of your tyre and punctured your tube are annoying but fairly inevitable. On the other hand, those that happen because your tube has been punctured by your rim wall, or because your valve has shifted around in the rim and torn itself, are completely preventable.
Valves come in two sizes. The narrower Presta valve fits through a 6mm (1⁄4 inch) hole in the rim, while the wider Schraeder valve fits through an 8.5mm (3⁄8 inch) hole.
If you ride with low pressure in your tyres (which slows you down but increases grip), you will find that the tube shifts about inside the tyre. Since the valve is held in place by the valve hole, the area near the valve gets stretched and can tear. The retaining ring on the valve stem will help by reducing the amount of movement in the valve, but this is often not enough if you’re using Presta valves in Schareder-sized holes. A better solution is to buy one of the plastic or rubber rings that fit into Schraeder-size holes to reduce them to Presta size. This helps support the valve stem and stops water getting into the gap between valve and rim. They’re properly called ’Schraeder to Presta conversion grommets’, but are more usually identified by a description of what they do: ’those little black things you put in your valve holes if you run Presta tubes’.
Some rims come with a narrower valve hole that is only big enough to take a Presta valve. You can drill these out if you want to run Schraeders – use an 8.5mm (3⁄8 inch) drill bit and drill through from outside. Cover the hub and cassette with a cloth so they don’t get covered in swarf. File off sharp edges around the valve hole or they will cut into the valve stem.
Rim tape sits around the inside of the rim to stop the heads of the spoke nipples cutting holes in the tubes. Make sure the tape sits evenly across the well of the rim, but that it doesn’t run up the sidewall anywhere because this stops the tyre seating properly. I like the thick plastic ones; they don’t move around on the rim as you fit the tyre and have to be stretched over the rim to fit.
Once it’s fitted, check that the rim tape completely covers all the spoke holes – if there are any gaps at all, the tube will creep into them as you inflate it. The sharp edges of the spoke hole and rim tape will cut gradually through the tube, puncturing it. This may not happen straight away – it’s more likely that your tube will last until the furthest point of a long loop, just before it starts to snow.
When you’ve fitted rim tape, you will often find that the valve hole in the tape doesn’t line up with the hole in the rim. Stick a screwdriver under the rim tape, lift it a little and push it over to rest across both rim walls with the tape stretched over it. Roll the screwdriver around the rim until the holes line up and remove the screwdriver.
Cloth tape is heavier than plastic tape, but it is a better solution for high pressures because it’s less flexible. Thinner rim tapes will bulge into the nipple holes as you put pressure in your tyres. If the rim tape has bubbles at every spoke hole, replace it with a thicker one. Don’t double up the rim tapes, though – if you put one on top of the other, they fill up the rim well, making it tricky to remove and refit tyres.
Schraeder to Presta rim-converting grommet
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Which tyres?”