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Extracted from notes on early BMW's but equally applicable to our old Urals Even though your machine may show only 20,000 to 30,000 miles on the odometer, in most cases, you have no idea how long the previous owner neglected the machine. You don't know if it stood for many years with dirty oil and condensation. Considering the failure rate of speedometers, you don't even know if the speedometer is the original one. The service bulletins of the era recommended the removal and cleaning, or replacement of the BMW oil slingers every 45,000 miles. This recommendation was for engines that were under constant service, which is a lot less demanding than extremely occasional use and neglect. To understand why cleaning the oil slingers is necessary, you must examine how these engines oil themselves. The pre-1970 BMW engine has a caged ball and caged roller bearings. By the nature of this bearing design, they do not require a pressure feed of lubricant, but only a constant mist of oil. In fact, high pressure, forced feeding of oil to a ball bearing will cause premature bearing failure. The BMW engines have a modest gear pump, which draws the oil out of the sump, via a screened pickup. The oil is pumped out of the oil gallery, onto the face of the caged ball bearing, (or in the case of the R69 and R69S rear main, a roller bearing). The oil passes through the bearing, and hits the face of the oil slinger, (which is a sheet metal disk, with a lip around its entire circumference) where the centrifugal force of the rotating crankshaft slings the oil outward, into this lip. When this lip fills with oil, it then flows out a hole in the back of the singer, which lines up with the hole in the hollow rod pin. The oil then flows down the hollow rod pin, through two small holes, to the caged rollers of the rod. The oil constantly exits the bearing on both sides of the rod, throwing oil onto the camshaft, lifters and one of the piston skirts, and then falls back into the pan, to be recycled. The problem with this arrangement is that the slingers act as a 500 - 5000 rpm centrifuge. Anything in the oil, dirt, sludge, and the ferrous metal from the wear of the rings and cylinders is hard packed into the slingers, and no amount of solvents or prayers will remove it. The sludge sticks to a magnet because it contains a lot of ferrous metal from ring and cylinder wear. The only way to clean the slingers is to remove the crankshaft from the case and scrape out the slinger grove. You have no choice. When the deposit gets deep enough, the sludge will then migrate down the rod pin, and plug the two small oil holes, cutting off the flow of oil. The end result is a bad or thrown rod bearing. Crankshaft rebuilding is expensive, and all of this expense can be avoided if the slingers are cleaned on a regular schedule. The BMW crankshafts will run 300,000 miles on the original rod bearings, if the slingers are cleaned or replaced on a regular basis. Another helpful hint is to place large bar magnets in the oil pan during reassembly. They stick to the pan, and are isolated from the crankshaft cavity of the crankcase by a baffle screen, which separates the sump from the cavity. The magnet catches the metal of the cylinders and rings, as they wear away, and slow down, but not stop, the deposits in the oil slingers. |
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