Enthusiasts' Directory: No. 3 Oil Filters




Only of comparatively recent times has efficient engine-oil filtration been understood by both designers and users. In the bad old days the sole attempt at filtering the lubricant consisted of a suction filter to protect the pump and a gauze-tray below the crankcase through which oil ran back into the sump. As the position of the suction filter inside the engine rendered it prone to neglect and as its main function was to safeguard the pump against drawing in large solids, it was often of 16 mesh or greater, to avoid impeded flow as it became clogged, and so served no real purpose as an oil cleaner. Some designers, Georges Roesch for example, realised the value of properly filtered oil in the ‘twenties, providing additional filters, accessible for cleaning or otherwise according to their lights. But the majority, although often lavish with gauzes in the water, fuel and oil fillers, neglected to filter the lubricant in circulation.

In the late nineteen-thirties it became generally known that oil could be kept clean over big mileages, thus saving engine-wear while obviating the need for frequent sump draining, if a proper filter was incorporated in the lubrication system. Today most manufacturers fit such filters as standard and they are available as accessories for the older cars.

Space precludes a description of the filtering problems to be solved in the light of what impurities in liquid and solid states (such as metal, carbon, dust and gum) exist in engine lubrication systems, but much information on this subject will be found in a paper presented by T. C. Worth, of British Filters, Ltd., before the then I.A.E. and published in the Automobile Engineer dated November, 1940. Microphotography has shown that abrasive particles in the oil rarely exceed 0.001 in. in diameter, larger particles being pulverised by the piston rings. Filters should stop particles of two microns (one micron = 0.00004 in.).

Two general systems of oil filtration are in use, the Full-Flow and the By-Pass methods. In the former all the oil is taken through the filter in its passage from pump to bearings. On paper the Full-Flow system is ideal but in practice difficulties arise. To work effectively the filter should have a volume equal to the full flow for one minute and as a modern car engine frequently passes three gallons of oil in that time, obviously the filter would require to hold three gallons, which renders it impossible to install in the available space. If a smaller filter is used, or clogging takes place, the oil flow to vital engine parts will be impeded.

For these reasons the By-Pass system is normally employed, in which only a percentage of the oil enters the filter, flow to the bearings being unimpeded. Filtered oil returns direct to the sump. Usually about 5 to 10 per cent, of the flow is filtered, so that for a 25-h.p. engine the filter can be about the size of a quart pot. As such filtration represents replacement of dirty with clean sump-oil at a rate of approximately a pint every minute, all the oil is maintained in good condition and examination of the dip-stick will show the oil to look almost like fresh oil while the filter element is in good order. If the element is neglected and clogs completely, flow is unimpeded. In 1940 60 per cent. of American cars had By-Pass filtration as standard and they were regarded as essential when hydraulic tappets were used, as these are particularly susceptible to dirty oil. The By-Pass filter has since gained ground in Britain and instances are known of such filtration keeping the oil in excellent condition up to a mileage of 33,000. A combination of full and partial filtration is sometimes used.

Another filter development is the use of magnetic filters or drain plugs, which collect metallic impurities and prevent them from entering pumps and bearings. Until a recent discovery in Sweden no steel was available which would retain its magnetism in short bar-magnet form under all conditions. This obstacle has been overcome and magnetic filtration is an additional safeguard. “Chip” Magnet-plugs sell at the rate of 250,000 per annum in Sweden and 70,000 have been fitted in nine months in Denmark. Parion Products, Ltd., Parion House, Witham, Essex, now supply them in this country and Philips Electrical Ltd., Century House, Shaftesbury Avenue, W.C.2, list a wide range of magnetic filters and drain plugs, about which Mr. E. P. Roche of their Industrial Group will be pleased to supply details. Notes on some well-known proprietary oil filters follow. Their elements usually remain sound for about 10,000 miles but should be changed when the colour of the sump-oil shows it to be getting dirty.


A.C. oil filters have replacement elements, positive end-seals, a machined seat with oil-resisting gasket and their strong metal cases have drain-plugs. Filters with renewable elements are made in a variety of sizes, from the SA model, measuring 31/16 in. in dia. by 39/16 in., to the ZR1-C and AR1 filters which are 8 in. high, with a dia. of 3½ in. Prices range from 13s. 6d. to 20s. each.

A.C. replacement elements cost from 6s. 6d. to 12s. 6d. each according to size. These filters cope with badly fouled oil in Full Flow systems, the ZR1 taking up to 11 gallons an hour at 25 lb./sq. in., the larger ZS1 filter up to 15 gallons an hour at the same pressure. The detachable-lid, renewable-element filters are intended for By-Pass systems and will continue to filter by absorption even when fully choked. Bracket and flange mountings are available.

Manufacturers: A.C. Sphinx Sparking Plug Co. Ltd., Dunstable, Beds. (Tel.: Dunstable 360).


The Amal Type-166 oil-filter is made in two sizes, each having a bowl dia. of 3¼ in. but the long-model being 10 in. deep, the short-model 5¾ in. deep. These filters take  -in. o.d. piping and -in. B.S.P. unions and have a capacity of 150 pints an hour under one ft. head, when clean at normal temperature. The direction of flow is left to right. A drain-plug is provided. If required, two filters may be coupled together via a two-way tap, so that either one or other is in action. Attachment is by two -in. pins, 1? in, between centres, in the filter head. Spares are available and the price is £4 14s. for the long-model, £3 15s. for the short-model, the respective weights being 4¼ and 3 lb.

Various replacement elements, such as 320 by 20 or 24 by 110 monel cloth or 80, 60 or 30 monel gauze, are available, priced at 25s. for the long-model, 13s. 6d. for the short-model. The filter bowls are low-pressure aluminium die-castings.

Manufacturers: Amal Ltd., Holford Works, Perry Barr, Birmingham 20. (Tel.: Birmingham 4571).


Auto-Klean Strainers Ltd. are of the opinion that the main oil supply should be filtered by gauze of 0.0015 in., supplemented by a fine fabric or paper filter acting as a by-pass. To prevent the main-supply filter from clogging they introduced a self-cleaning mechanism, operated by a rod from clutch or brake-pedal to a ratchet on the filter spindle. This is rather costly, so not normally used by private-car manufacturers, but for racing cars a ratchet-type self-cleaning filter gives a high degree of protection to the main oil supply. Parry Thomas was a user, so was Sir Henry Birkin, Bt., for his Bentleys, and the Hon. Mrs. Victor Bruce used the same Auto-Klean on five different competition cars.

The PK model is recommended for racing cars, but the TK is a useful filter, with handle for operating with cleaning element. It passes up to 1 gallon an hour and has 0.010 in. thick metal plate-threaded on a spindle with 0.003 in. thick washers. The plates are of two sizes, alternating, and the large plates are 0.016 in. apart, the final access being through 0.003 in. wide slots. Final access slots of 0.001 in. wide are available if required. It is claimed that the Auto-Klean filter passes the same oil flow at the same pressure loss at 0.003 in. mesh as the equivalent-size element of standard construction at 0.005 in. mesh. The cleaner blades are 0.015 in. thick pivoted on a ground-steel support rod attached to the top cover and a smaller rod passing through and attached to the cleaners ensures their correct alignment and free movement between designed limits. When the cleaner element is rotated by handle or ratchet the cleaning blades remove solids deposited in the slots.

Manufacturers: Auto-Klean Strainers Ltd., Lascar House, Hounslow, Middlesex. (Tel.: Hounslow 6441).


There are over 15 million users of Fram oil filters and they are standard equipment on A.C., Fiat, Ford, Kaiser-Frazer, Morris, Renault, Simca, Studebaker and Wolseley cars.

Models are made to suit practically every known engine and garages can fit the Fram within an hour. A Fram cartridge has collected 26½ oz. of impurities after only 5,000 miles. The filter element is absorbent cotton specially treated by a patented process. It does not remove detergents from detergent oils. Normally the element should be renewed when the dip-stick shows the oil to be dirty but as detergent oils look dirty almost as soon as the engine is run, when such lubricant is used the element must be renewed on a mileage basis.

The popular Fram models are:–

F3     Engines up to 12 h.p.          Price 66/6     C3 Replacement cartridge     Price of cartridge 12/6

F4     Engines up to 30 h.p.          Price 79/-      C4 Replacement cartridge     Price of cartidge 17/-

F30   Largest size engines           Price 95/-      C30 Replacement cartridge    Price of cartridge 22/6

F40   Diesels, buses etc              Price 162/6    C40 Replacement cartridge   Price of cartridge 34/6

These prices are complete with cartridge and fitting kit. The fitting kits alone cost 22s. 6d. for the F3 and F4, 27s. 6d. for the F30 and F40. Other replacement cartridges are available, priced from 11s. 6d. for the CH804 to 53s. 6d. for the C174, for special applications. Fram “throw-away” oil cleaners are also made, the PS5 costing 15s., the PB½ costing 25s. Most Fram installations are of by-pass type. The fitting kits include flexible hoselines where necessary, adaptors, brackets and bolts, etc., and contain detailed instructions.

Manufacturers: Stenor Ltd„ Kew Foot Road, Richmond. Surrey.

Purolator Micronic

These filters are made by the Automotive Products Co. Ltd. The filter element consists of a plastic impregnated paper, claimed to be heat-resistant, waterproof and warp-proof. An unusually large surface is provided for any given filter unit, the popular car-sizes giving approximately 500 sq. in. in a 3⅜-in. dia., 4¼-in. deep unit. The original sheet of filter paper is, indeed, ten feet long. It filters down to one micron at high rates of flow.

The casing is a deep-drawn steel casting with dirt-resisting grey metallic finish, which lasts as long as the engine to which it is fitted. The filter paper has a protective outer oil-board casing and leak-proof unions obviate gaskets.

The makers claim that impregnation does not impair the porosity of the paper and that it retains foreign matter on its outer surface which would penetrate below the surface of felt, cotton waste and other materials. Moreover, no injurious particles or fluff are shed by a paper filter. A “Micronic” element remained unharmed after complete immersion in water for 300 hours. Prices are available on application.

The Full-Flow pad-mounted models with replaceable element are as follows :

Type          Nominal flow         Max pressure     Relief valve pressure     Weight          Dia. in.     Length in.   

MF 2600    120 gall. per hr.     100 lb./sq. in.     15-20 lb./sq. in.              3 lb.             3.50         9.5

MF 2603    120 gall. per hr.     100 lb./sq. in.     15-20 lb./sq. in.              3 lb. 7 oz.     3.59        10.5 

MF 2604    120 gall. per hr.     100 lb./sq. in.     15-20 lb./sq. in.              3 lb. 1 oz.     3.59         9.5   

MF 3601      90 gall. per hr.     100 lb./sq. in.     15-20 lb./sq. in.              2 lb. 12 oz.   3.59         5.87

MF 3900     70 gall. per hr.      100 lb./sq. in.     15-20 lb./sq. in.              2 lb.             3.52         7.47

MF 3901     70 gall. per hr.      100 lb./ sq. in.    15-20 lb./sq. in.              2 lb. 5 oz.     3.59         7.5

MF 4300   200 gall. per hr.      100 lb./sq. in.      10-15 lb./sq. in.             6 lb. 11 oz.    4.69         12.12

MF 4301   200 gall. per hr.      100 lb./sq. in.      10-15 lb./sq. in.             6 lb. 8 oz.     4.68          9.04

In most cases inlet and outlet are tapped ½-in. B.S.P. Additional depth is necessary to allow for sump removal.

The By-Pass replaceable element filters are:–

Type          Engine suitable for Max pressure     Relief valve pressure     Weight          Dia. in.     Length in.   

MF 5201    Up to 20 h.p.         100 lb./sq. in.     57-63 lb./sq. in.              2 lb. 12 oz.    3.5          10.38

MF 2301    Up to 12 h.p.         100 lb./sq. in.     57-63 lb./sq. in.              2 lb.              3.95          5.62

MF 2901    Up to 25 h.p.         100 lb./sq. in.     57-63 lb./sq. in.              3 lb. 8 oz.      4.14          6.67

The By-Pass Cartridge-type filters are:–

Type          Engine suitable for     Outlet and Inlet                                  Weight          Dia. in.     Length in.   

MF 5201    Up to 12 h.p.             ⅛-in. Am st. pipe                                15 oz.           3.5           4.03

MF 2501    Up to 25 h.p.             ⅛-in. Briggs                                        1lb. 3 oz.      4.28         5.9

MF 6100    Up to 10 h.p.             0.4375 in. X 20th U.N.F.                       14 oz.          3.12         4.03

Manufacturers: Automotive Products Co. Ltd., Tachbrook Road, Leamington Spa. (Tel.: Leamington Spa 2700).


Stream-line Filters Ltd., make a self-cleaning filter with a number of paper packs as the filter element. This filter is intended for commercial-vehicle application.

Manufacturers: Stream-Line Filters Ltd., Ingate Place, London. S.W.8. (Tel.: Macaulay 1011-3).


Vokes oil filters are used on Alfa-Romeo, Bentley, Rolls-Royce, M.G., Frazer-Nash, Bristol, Jowett and other high-performance cars, and in racing by the Ecurie Richmond and Mike Hawthorn on their Cooper-Bristols. It is claimed by Vokes, Ltd., that their patented filter element, of radial V-section felt and gauge, provides the greatest area in the smallest possible space and removes at least 97 per cent. of impurities causing wear. On the other hand, sub-micron-size carbon particles are allowed to pass, as they do no harm, and a filter so arranged does not remove valuable additives from detergent oils, or become blocked by such additives. Vokes filters have inside to outside flow, so that impurities remain in situ within the element assembly until this is removed. Filters for individual engines cannot be supplied at present and spares for existing assemblies should be ordered from the car manufacturer or agent.

Manufacturers: Yokes, Ltd., Henley Park, Guildford, Surrey (Tel. : Guildford 62861).


The patented Wipac By-Pass oil cleaner consists of only two parts, the adaptor base which becomes an integral part of the engine and the “Instant Fit” factory-sealed cleaning element. The used element can be unscrewed and a new one fitted in a minute. The filtering element is free from chemical treatment and intended for use with detergent oils. Clear fitting and servicing instructions are issued, initial installation occupying a little over one hour. Only ½-in. clearance is required for removing the replaceable element and the filter can he mounted vertically or horizontally, on engine, wing valance, bulkhead, etc. For engines up to 12 h.p. the E.1 cleaning element is used, for engines of 12-20 h.p. the E.2 element is required, for 20-25-h.p. engines two E.1 elements on a twin adaptor base. The complete installations, less fitting charges, cost from £2 15s. to £4 3s. The cost of replacement elements is 13s. 9d. each in the case of the E.1, 17s. each for the E.2. The Wipac oil filter, first produced in 1947, is now fitted as standard to Rover 75, Marauder and Alvis-Healey cars. There are over 2,000 fitting agents throughout this country.

Manufacturers : Wico-Pacy Sales Corporation Ltd., (Filter Division), Bletchley, (Tel. : Bletchley 320).