Racor Primary Fuel Filter Upgrade

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By Mike Vaccaro

The Catalina 34/36 series boats are factory equipped with a Racor spin-on type of primary fuel. While inexpensive, the primary disadvantage to this type of filter is the cost of replacement filters and the difficulty of changing filters and bleeding the fuel system. Replacement filters cost an average of $25 to $30 depending on the source, and it is necessary to prime the system each time a filter is replaced. This is complicated if emergency maintenance must be performed in a seaway in the event of a clogged filter. Although the primary defense against an unexpectedly clogged filter is preventative maintenance (i.e., cleaning the fuel tank), often a problem doesn’t manifest itself until you are caught in heavy weather with the boat rolling and pitching to the point of churning up sludge in the tank.

The Racor 500 series “turbine” filter is a direct replacement for spin-on types, but is more expensive and takes up more room. However, it is more maintenance friendly, and over the life of the system should cost less to maintain than spin-on types due to the reduced cost of the drop-in filters, which average $7 to $10 each. Additionally, the water-separator turbine is effective and easy to check and bleed, as required. It is also quite easy to equip this filter with a vacuum gauge, which is the best way to monitor the status of the filter element. The Racor gauge is color-coded; you simply replace the filter when the needle is at the top of the yellow arc. The other primary advantages of the 500-series filters are that no tools are required for changing filters, draining water or bleeding the fuel system.

The only disadvantage to this type of filter is the cost. In addition to the filter housing, it’s necessary to purchase fittings and the vacuum gauge (if desired). Although there is room under the head sink to mount this style filter, it is larger than the spin-on type as well.


Removing the old filter is simply a matter of removing disconnecting the fuel lines and unbolting the housing from the bulkhead. If your fuel lines are older than 5 years, it’s worthwhile to check their condition and replace if necessary. It might be simplest to change the filter when you perform scheduled maintenance on your fuel system (filter change, tank cleaning, etc.).

The 500-series housing uses simple lag bolt attachments via the metal flanges on the unit. It does not use the same bolt pattern as the spin-on type, so it will be necessary to drill new pilot holes. As with any project, measure twice, drill once and be damn sure you know what you’re drilling into. You can hold the housing against the bulkhead and mark the hole locations with a permanent marker for reference. It’s sufficient to drill through the liner at this point and not necessary to drill all the way through to the engine compartment.

Figure 1
Once the new housing is installed, it’s easiest to fill it with diesel, drop in a filter, and screw on the cap with the T-handle or vacuum gauge (hand-tight is sufficient to make a seal—over-tightening will damage the o-ring). Then follow standard Universal bleeding procedures. As a technique, with M-25 series engines most folks leave the bleed valve cracked open all of the time to make the system “self-bleeding.” Remember that when you bleed the engine, it should be run in gear long enough to ensure 100% of the fuel from the pick-up point to the fuel return is cycled through. A typical error when bleeding is to not run the engine long enough with varied RPM, which may leave air in the system and cause a vapor lock the next time the engine is run. Leaving the bleed valve partially open can help avoid trouble.

If your old housing is still serviceable (or can be made so with a new seal kit), you may want to consider re-installing it and plumbing the system with two y-valves. This will provide you with redundant fuel filters that can be “changed” by turning two valves (the best way to deal with a clogged filter in a seaway!). Unfortunately, there is limited room for the second filter, and it may be necessary to add a second boost pump to the system depending on the run of the line required to affect the y-valve circuit. If you do add a second circuit, keep in mind that it’s necessary to prime and bleed both systems (one at a time), and then check for uninterrupted engine operation after you shunt back and forth between the two filters.

Figure 2

Figure 1 shows the 500-series filter installed under the head sink. Note the T-handle mounted to the bulkhead adjacent to the filter. This is the handle supplied with the unit that is replaced by the vacuum gauge. The handle is maintained for emergency use if there is a problem with the gauge assembly. Also note the markings on the gauge. If the filter is not clogged, the system will pull little or no vacuum, but as the filter begins to collect particulate matter, the vacuum will increase, pulling the needle up into the yellow arc. The filter should be replaced as the need approaches the top of the yellow arc. Keep in mind that the gauge only reads accurately when the engine is operating and pulling fuel. The fuel line in the foreground runs to the engine. The fuel line not visible on the back of the housing runs up to the top of the electric pump and is the line from the tank. The filter is mounted as high as practical to allow for limited gravity feed to the engine in the event of a boost pump failure. The water separator “turbine” is visible in the fuel bowl at the bottom of the housing. Water is easily drained out of the bottom of the bowl by loosing the nylon fitting. This can be accomplished without the use of tools.

Figure 2 shows a simplified schematic of a duel filter system. In this depiction, the engine is being fed fuel from the #2 filter. Note that it’s possible to incorrectly position the Y-valves resulting in eventual fuel starvation to the engine. If you install this type of system, be sure to label the system clearly so that there is no doubt that the valves are properly positioned. Additionally, the goal of fuel system design is to ensure that fuel has an opportunity to flow “down” to the engine; this provides a redundant gravity feed in the event a fuel pump fails. If your second filter circuit is mounted at a height much lower than the primary, it may be necessary to add a second fuel pump to that circuit. If this is the case, it will only be necessary to run that pump in the event that the primary electric pump fails and the engine is being supplied by the second filter circuit (i.e., drawing fuel from the “low” system). With dual filters and pumps, you’ll have a completely redundant system that can be used with two valve and one switch actuation.

While handy, a dual filter/pump system is no substitute for a clean fuel system. If there is a considerable amount of contamination in the tank, and it is churned up in a seaway, a second filter may clog very rapidly. Under these conditions, unless use of the motor is required to avoid a lee shore or immediate hazard, it’s best to transition to sail if the primary clogs in heavy going and wait until the motor is required for docking or you reach calmer waters before switching to the secondary circuit. The best way to avoid problems on the water is with preventative maintenance that keeps the fuel tank clean.