Most fittings and adapters used in the Motorsport industry are based on a 37º sealing surface. A 37º fitting and a 37º adapter mate together for a tight, leak-proof assembly. However, there are other similar fittings and adapters that use a 45º sealing surface. These 45º fittings and adapters can be found in other automotive applications. They may also look like a 37º fitting or adapter, but are not interchangeable. In most sizes, they may thread together but will not seal, due to the different sealing surface angles. Make sure you know the sealing angle of the parts you are assembling.