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PeterI
PeterI

Posted: Mon Nov 14, 2016 6:44pm

Post Subject: Fuel pipe sizes

Hi, I have a Dawncraft 22 with bmc 1500 diesel, and am replacing the fuel system. I've noticed that the existing fuel supply from tank to lift pump is 8mm, but the return pipe is 6mm and 1/4". The spill rail at the injectors is 3.5mm approx OD. The return fitting into the tank has a very small internal diameter - much less than the pipe. Is there science behind creating back pressure in the return line, or can I ignore this and just use convenient pipe sizes (6 or 8mm) and fittings? I will also attempt to solder in replacement parts of the spill rail, which is broken...again would a bigger diameter be OK? Thanks Pete

Tony-B
Tony-B

Posted: Wed Nov 16, 2016 7:35am

Post Subject: Fuel pipe sizes

The way the BMC 1.5 & 1.8 fuel system is designed the lift pump will simply stop pumping if there is too much fuel/pressure in the system. There shoul;d be all but 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 on the bolts so 0.5 mm dis should be fine for that pipe. It is this hole that creates any "back pressure" not any of the pipes so there is no reason not to use a larger bore pipe on the return.

Tony-B
Tony-B

Posted: Wed Nov 16, 2016 9:33am

Post Subject: Fuel pipe sizes

I have just noticed your comment about soldering the spill rail. If you use soft solder it will fail the next BSS inspection. You will need to silver solder or braze it. Otherwise buy a new one from Calcutt boats or ASAP supplies.

PeterI
PeterI

Posted: Wed Nov 16, 2016 6:22pm

Post Subject: Fuel pipe sizes

Tony, Thank you. Pete

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