What is header drop?

Is it measured down from the exhaust port or out. Or is from the exhaust port?

I think the normal convention is from the exhaust port, at least that's the way I've always ordered custom headers.
Of course, during installation planning, one has to learn the crankshaft to center of exhaust port dimension; many engine manufacturers provide that on drawings.

BTW - "Drop", is usually referenced to an inverted engine, with the header distance going further away from centerline. "Rise" is for upright installations.

header drop is the offset of the header pipe from exhaust port flange away from the engine crankshaft centerline. With the engine mounted inverted, this causes the header to exit lower (nearer to spark plug level) than the exhaust port, by the amount of drop specified.

Guys

Thanks for clearing this up for me. I guess I ordered the wrong header, I think I should have got a 50mm drop, rather than no drop. Now I have to make an adapter plate, if I got the header with drop I would only needed to elongate one hole 2mm live and learn.