Diesel Fuel: Filter, Polish, Transfer, Distribute

Dirty fuel is the number one cause of marine diesel engine troubles/failures. Engine troubles often happen at the worst time, when weather is rough, the boat is bouncing and you NEED that engine to work for quick maneuvering. A bouncing boat will stir up dirt and sediment in the fuel tanks. If that fuel is not filtered properly, the dirt can find its way into the diesel engine injectors and injector pumps causing engine troubles that cannot be easily fixed till you get back to the dock (and the fix can be quite expensive).

This assembly is the last piece of the fuel system before installing fuel consuming items like heaters and engines. I originally wrote on this blog thinking I would install one of the FilterBOSS devices, but in the end I decided against. I determined I could build my own at half the cost. It does not have some of the nifty features of the FilterBOSS, but it adds a couple more. Call that compromise I guess.

Features of the assembly are:

  • Select-able primary filters. Choose one, the other, or both filters with a single valve lever.
  • Vacuum gauge for primary filters (high vacuum indicates a clogged filter)
  • Fuel distribution for up to three diesel consuming devices (engine, heater and TBD)
  • Fuel polishing through a dedicated 10 micron filter
  • Fuel transfer between fuel tanks via the tank selector valves (already installed)


The assembly being “bench tested” checking for leaks.

The components of the assembly are:

  • Parker/Racor 500MA Series dual turbine filter/water separator 
  • Parker/Racor 120 Series “spin on” filter/water separator
  • Walbro fuel transfer pump (24 volt)
  • aluminum manifold blocks for fuel supply and return distribution
  • ball valves
  • flare fittings
  • custom length fuel hoses

The assembly is designed for easy installation and removal.  The components are mounted on a piece of plywood (painted white) that will bolt onto mounting rails in the engine room. One only need connect/disconnect the fuel hoses to/from the tanks, the diesel devices and disconnect the 24volt power plug for the pump. All hose connections use flare fittings. There are NO hose barb/clamp connections.

As for the primary functions of this assembly…


All fuel that is drawn from the fuel tanks first flows through the 500MA Series filters fitted with cartridges that will remove particles as small as 30 microns. From there fuel is distributed to the diesel consuming devices via the distribution manifolds. Each diesel consuming device (engine, heater, etc) will have its own dedicated 10 micron filter installed near the device.


“Fuel polishing” is a term used to describe fuel drawn from the tanks, filtering/cleaning particles from from it, then sending it back into the tank. Polished fuel flows through the same 500MA Series 30 micron filter, then through the 120 Series 10 micron filter (dedicated to polishing), then back to the tank.


Fuel can be transferred from one tank to another, by simply selecting the appropriate fuel supply and fuel return valves (already installed) to control tank distribution. The fuel is polished during the transfer.


Filtered fuel is supplied through a manifold off of which comes three connections. One for the engine, one for the heater, and a spare (for some future device). Each connection has its own valve for easy shutoff. There is a similar manifold for fuel return.

Next, the assembly gets bolted to the wall in the engine room and the fuel tank supply/return fittings connected. At which point leak testing of the previously installed tank plumbing can begin.

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