|The MGA With An Attitude
The Nature of BRONZE BUSHINGS -- UT-103
Bronze: a) an alloy of copper and tin and sometimes other elements. b) any of various copper-base alloys with little or no tin. There are many different alloys of bronze, but here I will only cover those (some of those) which are used for various bushings in the MG.
Bronze bushings come in a few basic types. Let me chat about the more rare variety first. This would be a high alloy and high strength part for relatively heavy loading, and hopefully long life. The most obvious application of this in the MG would be for upgraded valve guides in the engine cylinder head to replace the original iron valve guides. The material here would be either silicone-bronze (slightly cheaper) or manganese-bronze (somewhat more robust). The Mn-Bz alloy is primarily copper-zinc with only a small amount of manganese in the alloy. The Si-Bz alloy may be considered the poorer cousin used for similar function. The important point here is that this alloy is moderately hard and very tough (both metallurgical terms). It is also "self lubricating" when running against a smooth steel surface, so these bushings may need little or no oil to provide low friction and to survive with very little wear. Bronze in general also has a higher rate of thermal expansion than steel or iron, so as a side note, the bronze valve guides need a little more working clearance that iron guides, which is covered in more detail under Engine Tech.
The more common type of bronze bushing is a softer material. Type 66 bronze is a soft and inexpensive alloy, primarily copper-tin-lead with a small amount of zinc. This is commonly used as a thin layer of bronze with a steel backing shell, as in engine bearings and kingpin bushings (MGB), and the sliding tail bearing in the MGA 1500 gearbox. Type 660 bronze is also soft, but has a few more minor alloying elements for slightly higher strength and substantially higher toughness. This type can be deformed more without breaking. It is commonly used in solid cast bearing parts such as thrust washers and the bushings on the mainshaft in the gearbox carrying rotating gears (2nd and 3rd gears in the MG, and also the reverse idler gear). I have used this material successfully to replace the original tail bushing in the MGA 1500 gearbox. All of these applications using solid cast bronze bushings will normally require oil lubrication for running at speed.
The last type of bronze bushing I will mention is the sintered bronze bushing. This is made by compacting powdered materials, followed by heating to bond the material together into a solid part without melting (sintering). This can produce a bushing to finished size without machining, and may even produce some moderately complex shapes such as having a thrust washer type flange on one end of the bushing. But there is another trick of interest to us. Some of the original powder particles may be volatile material that will burn off when the bushing is sintered. This gives a finished part with a porous structure, which under a microscope looks much like a sponge full of small holes. This part may then be impregnated with oil by placing it in a vacuum to withdraw the air, then returning atmospheric pressure to force oil into the bushing. A bushing made in this manner is commonly called an "Oilite" bearing after the original trade name from Chrysler Corporation. They contain about 18% by volume of oil which is "metered" as required to the surface of the bearing by capillary action when heat or pressure is applied. There are several different alloys used for Oilite bearings, including mostly copper (bronze parts), mostly iron, or mostly aluminum.
The bronze Oilite bearings are quite common in the MG, being used for electric motors and small gearboxes such as the windscreen wiper drive and heater motor. They are also used for heavier parts such as front and rear bearings in the starter motor, rear bearing in the generator, and the spigot bushing in the tail of the crankshaft which carries the nose end of the gearbox input shaft. One very significant advantage of the Oilite bearing is that the original oil content may provide lifetime lubrication. In fact in the MG these bearings are intended to do exactly that. The one in the tail of the generator which runs continuously at high speed likes to have a few drops of oil added occasionally, but the rest of them have no required maintenance schedule. Since I restored my MGA, the wiper drive, heater motor and starter motor have been working flawlessly for 184,000 miles (Sept 03). The generator has been repaired or replaced a few times, but not because of failure of the rear bearing. The spigot bushing in the crankshaft commonly lasts as long as the clutch and is replaced any time the clutch is serviced.