Oil filters

by Richard Atkinson

In selecting a replacement oil filter, there are a couple of points one should bear in mind. Oil filters made for automotive use are 3/4" x 16 inside thread, with a 2 1/2" diameter O-ring (Ford, Mercury, Lincoln, Plymouth, Dodge, Chrysler, and most imports, including Rover), 13/16" x 16 with 2 1/2" or 3 1/8" O-ring (Chevy small and big blocks, small block AMC, Buick, Cadillac, Olds and Pontiac V8s), 18mm x 1.5 with 2 1/2" O-ring (Buick, GM, Chevy V6s), and 20mm x 1.5 with either 2 1/2" or 3 1/8" O-ring (many Honda, Mazda, Isuzu, Mitsubishi, Opel). With a given thread and O-ring size, the major differences between filters and manufacturers are 1) the filtration medium, 2) overall dimensions, 3) antidrainback valve, and 4) overpressure valve.

The first point is up to the filter manufacturers. The 'allowed' particle Sizes and flow per unit area are usually found in comparison reports, but, to be acceptable to consumers and not to void manufacturers' warranties, I'd say there are very few 'bad' ones out there.

Specifying the size of the filter is really up to the engine designers. A Large capacity filter acts as an extra reservoir, as well as an additional cooler, but adds a couple of pounds cantilever weight to its attachment point. A smaller filter will clog faster for a given total flow so a compromise must be made here. A point to remember in an elderly(!) design like the Rover V8, is that the lubrication system relies on volume, rather than pressure. In fact, raising the pressure results in more 'co-lateral damage' to the pump drive parts, the timing chain, cogs, and cam and distributor drive gears specifically. The minimum allowable pressure for the V8 is usually taken to be 10psi per 1000 RPM. From this, you will appreciate that a larger filter, the size of the old FL1A/PH8A, should be better. However, remember that you don't want a huge cantilever on the pump mountings when you're hitting the big bumps while out 'wheelin'! So, there's your first compromise...

The third point is VERY important to the Rover V8. Because the pump is mounted far remote from, and above the level of, the oil in the pan, air will 'break the vacuum' in a loose engine, and allow the oil to drain back through the pump. Most filters in use today have an antidrainback valve (it's the black, or reddish color, neoprene or silicone flap under the ring of filter inlet holes - they're just inside the sealing gasket), since modern engines often use the front of the crankshaft to drive a crescent gear pump, and therefore, again, are above the mean oil level. To check if a filter has such a valve, avoiding the startled looks of sales clerks and other customers, purse your lips and blow into the filter outlet (the threaded hole). If your breath rustles through your moustache out the ring of inlet holes, the valve does not exist or is inadequate for our (Rover) use. If your ears pop and eyeballs bulge, it has the required antidrainback valve.

The fourth characteristic is the overpressure valve. This can usually be found by looking down inside the threaded outlet hole and seeing if there is a coil spring, usually about an inch diameter and an inch long, near the bottom. This spring holds a steel disk over an aperture, and if the pressure differential across the filter element exceeds a preset amount (usually 8 to 12psi), such as may occur when starting an engine long overdue for an oil change on a very cold winter's day, the valve disk lifts up against the spring and allows (dirty, unfiltered!) oil into the outlet, thereby saving the oil pump and the engine from immediate (not long-term!) damage. Later filters have a phenolic plastic overpressure valve, viewed by looking into the threaded hole to the bottom, and noting the approximately one inch diameter head of the valve. However, in the Rover V8 application, this feature of the filter is less important, since on many of the engine derivatives, there is already such a valve built into the pump. The small thimble-shaped screen covering the pressure relief valve is usually directly visible. (In applications which have the oil cooler adapter is fitted, this will have to be removed to see the overpressure valve.)

In summary, then, filter size has nothing to do with pressure (per se), and if you change your oil and filter at the recommended intervals, the PH16-type equivalents) are totally adequate.

From the OVLR Newsletter, March 1999