<P> Because the rims of bicycles drilled for Presta valves cannot accommodate the wider Schrader valves, it is often the case that rims need to be drilled for such replacements which can structurally weaken the rim . Conversely, when a Presta valve is fitted into the larger Schrader rim hole, grommets or reducers are sometimes used to take up the extra space . </P> <P> The standard Presta valve has an external thread . An adapter can be fitted onto this external thread to permit the Presta valve to be connected to a pump with a Schrader chuck . The same adapter, because of a coincidence of thread sizes, may be able to convert a Schrader pump into one that can connect to flexible adapters of either kind . </P> <P> On many race wheels, they have such a high, thick, rim for aerodynamics, that a standard Presta valve is too short . Valve extenders have been developed to adapt shorter valves to deeper, thicker, rims . There are two types depending on whether the Presta core is removable . </P> <P> The valve threads for Presta valves follow the ISO 4570 standard . The external threads at the tip of both "threaded" and "unthreaded" Presta valves are 5V2, which measures out to 5.2 × 1.058 mm . The external threads on the main body of "threaded" Presta valves are 6V1, which measures 6 × 0.80 . </P>

What are the two types of bike valves