FIP on an m51?
Depends which seals and what’s leaking. Generally you should manage. There’s enough room if you take things off to get in there, you need to scribe everything up and will have fun readjusting modulation if you move the pump top. You need proper tool to hold the sprocket in place if you remove the FIP drop that and your stripping engine right down
Good read of RAVE beforehand so you know what not to touch
 
I take by back you mean pressure head. Very easy to change the o'ring if you know what you are doing and have the correct tooling. And follow the correct procedure. You do not remove it you back it off just enough to get at o'ring. Pump MUST be set on full cam or spacer washer in pump behind piston will fall off. And you really do not want that to happen.
 

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