Bypass Filters

When Ford did introduce an oil filter it was the bypass style, which meant only a portion of the oil was filtered. Oil was delivered to the filter from a boss at the back of the block where the oil pressure gauge sender was located. From the filter oil was returned to the pan. A simple arrangement, it’s always been said that 10 percent of the engine’s oil passes through this style filter, which Joe Abbin’s recent testing confirmed (see his sidebar on page 83 for more information).

Partial Filtration

A popular oil filter conversion is the partial flow style. Basically the oil is rerouted at the back of the block. On the pad where the oil pressure sender is found there are two holes with 1/4-inch pipe threads (one is horizontal and normally plugged, the other is vertical and is for the oil pressure sender). To add a filter the vertical passage from the pump to the oil pressure sender is drilled to 9/16-inch for increased flow and both outlets are drilled and tapped for a 3/8-inch NPT fittings. Oil will be taken out of the engine at this point and routed to a remote spin-on filter bracket (do not use a Ford canister or beehive filter with this conversion).

To return oil to the engine a hole is drilled into the block to intersect the oil galley. Some blocks have a raised, round protrusion from the casting core that marks the spot. In others a wooden dowel slipped into the horizontal passage can be used to determine the depth of the passage and the spot to drill.

The final step is to block the connection between the two internal oil passages in the block. Just past the vertical passage from the pump, the horizontal passageway leading to the oil distribution tube is plugged by drilling a tapping for a 7/16-inch set screw. Oil will now leave the block, pass through the filter, and return to the distribution tube. However, the rear main and rods, numbers 4 and 8, will still receive unfiltered oil directly from the pump.

It should go without saying that these modifications should be done with the engine disassembled to allow for through removal of all the debris from drilling and tapping.

Full Flow Filters

A system to filter 100 percent of the oil flowing through a Flathead is available from Phil Goller. Originally developed by Mark Kirby (formerly from Motor City Flatheads, now Motor City Speed Equipment), a collaboration of the two Flathead gurus has resulted in the complete filter kit that is available from Goller’s Hot Rods.

This system uses a modified pump with a line that exits the side of the pan, delivers the oil to a remote filter, after which it returns to the engine through a fitting in the back of the block. Oil is no longer delivered directly from the pump and back rods. Filtered oil is now fed “backward” to the rear main down through the passage that runs from the pump to the oil distribution tube.

Installing the full-flow system couldn’t be easier. Other than the installation of the pump, the only required modifications to the engine are a bulkhead fitting in the pan and the installation of the 1/4-inch pipe return fitting in the oil pressure sender boss. Which is Best?

There are pros and cons to all the choices we’ve talked about. The bypass style filters the least amount of oil but is the cheapest and easiest to install, the full-flow version filters all the oil all the time, is easy to install, but is the most expensive, retailing for around $350, and the partial flow is in the middle; it filters most of the oil, is the most labor intensive to install, but the parts are cheap.

Which you use is up to you, however we suggest using some sort of filter, changing the oil regularly, and your Flathead will lead a long, healthy life.