Iain_S

Very often a male BSP thread on a pipe will be parallel, as it's easier to cut. Depending on wall thickness, it may be difficult to get a taper in any case. Best practice is both male and female threads either parallel or taper. Male taper into female parallel works, but male parallel into female taper is very iffy, and is a recipe for stripping threads.

BSP threads come in both parallel and taper forms. Both will seal in an exhaust system, but threads on pipe are normally parallel, as are the threads on unions and sockets, so best to use parallel, which also allows for a slight length adjustment.

Not really : a car exhaust is usually rigidly fixed to the engine, and vibrates with it. On a boat, the exhaust is normally fixed to the boat, with a flexible section between system and the manifold to prevent, or, at least, greatly reduce, the engine vibration being passed to the exhaust.