My fuel pump failed about 9 months ago and in a hurry I got an aftermarket one for about £20. It did the job fine and appreared to fit well but the engine gradually developed an oil leak. I put this down to the Perkins rear crank seal as these have a reputation for leaking and it was dripping from the sump, but recently its got too bad to ignore - 6" pool in a few minutes. Cleaned everything and waited, it was coming from the fuel pump. Pulled it off and found it was 1/8" thinner than the AC OEM one, just enough for the nuts to feel like they were doing up but actually binding on the stud and not pinching up the gasket. A few thick washers cured it for now, but very annoying and worth looking out for.
Thing is its obvious once you put the two side by side and look at the flanges, but of course I looked very carefully at the bolt spacing and inlet and outlet positions and threads but missed the key difference that's been responsible for a lot (and I mean pints) of oil loss and mess.
