The smallest plug in the range at 1/8 inch, which is the standard size for gauge and sender ports across most engines and fittings. It is exactly what you need to close a spare sensor port on a fuel rail, a gauge adapter, a thermostat housing or a manifold once you have decided not to run that particular sender. As a tapered pipe thread it seals on the thread, so a wrap of PTFE tape or a smear of thread sealant is required, kept back from the leading thread. Small plugs are easy to overtighten into aluminium, so nip it up snugly rather than leaning on the spanner.