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Odd pipe sizes; which end goes where?; should I move my stove?; prop rope-cutters
QI am replacing the fuel system on my BMC 1500 diesel and have noticed that the existing fuel supply from tank to lift pump is 8mm, but the return pipe is 6mm and 1/4in. The spill rail at the injectors is 3.5mm and the return fitting into the tank has a very small internal diameter, much less than the pipe. Can I ignore this and just use convenient pipe sizes and fittings? I will also attempt to solder in replacement parts of the broken spill rail: again, would a bigger diameter be okay?
PETERI, via the CB website
ATONY REPLIES… On no account soft solder the spill rail. The Boat Safety Scheme requires such fuel system joints to be silver soldered or compression joints. It would be easier to buy a new rail.
The way the BMC fuel system is designed, the lift pump will simply stop pumping if there is too much fuel/pressure in the system. There should be almost no flow through the small injector leak off pipes so its size does not matter.
The main return should be fed from the banjo bolt in the top of the fuel filter via a 0.5mm hole in the bolt so 0.5mm diameter or larger should be fine for that pipe. It is this hole that creates any ‘back pressure’, so there is no reason not to use a larger bore pipe on the return.
Back pressure is only required to ensure sufficient fuel gets to the injector pump rather than flow back into the tank.