Dear Boat Doctor, 

What size filter element should I use in my Racor fuel filter? I’ve been told both a two-micron and a 10-micron filter. 

Ned Bittner 

Tacoma, Washington 

Dear Ned, 

There’s a reason you’re hearing two different things; believe it or not, this is a contentious topic. 

I use a 10-micron filter in the primary filter. My logic is based on the principle of multistage filtration. The first filter pulls out everything 10 microns and above and the second gets the stuff that is two microns and up. Filters are best at filtering contaminants close in size to their rating, so a two-micron filter will not work well if called upon to filter larger contaminants. Ideally you’ll catch the big stuff in the primary filter and the small stuff in the secondary on-engine filter. 

Arguments in support of using a very fine primary filter usually involve on-engine filters that are expensive or hard to change, but not changing the primary filter is a false economy. Secondary fuel filters

Bob Pingel, a Coast Guard-licensed captain, runs Custom Line Splicing LLC. Send your questions to boatdoctor@sailingmagazine.net.