The connecting rod bearings in BMW engines transmit 100
percent of the load from combustion to the crankshaft. As a result, the bearing
surfaces do not wear evenly. The side of the bearing facing opposite the piston
tends to get the most wear, because the rod pushes on it with a huge amount of
force when the cylinder fires. On the opposite side of the power stroke, the
same rod pulls the piston and rod assembly back toward the crankshaft, creating
wear on the opposite side. These push–pull forces on the crankshaft bearings
are exactly opposite each other and create wear patterns on opposite sides of
the bearing journals. BMW connecting rods, like the crankshaft itself, are
robust parts. Rods typically become damaged when a more basic problem, like low
oil pressure, caused by high heat or simply low oil levels, causes the rod
bearings to run dry. The rod bearing journals are some of the last components
to receive oil, and thus are often the first to run dry when the engine oil
level runs low. The result is catastrophic failure, as the rod itself crushes
and squeezes the rod bearings against the crankshaft.
If your engine has suffered from a loss of oil pressure, the first thing I recommend you do is replace your rod bearings. They are the one component most likely to be damaged by low oil pressure and the one component that, if worn, will result in complete engine failure. When most people think of bearings, they think of ball bearings like you would find in the wheel of a bicycle. The rod bearings are very different; they are basically a strip of curved metal that an oil film floats on. When the engine is running perfectly, the connecting rod bearing never touches the crankshaft, as thin oil molecules are suspended between the bearing and the crankshaft. The only time they might touch under normal circumstances is when you first start up the engine and there’s not much oil pressure built up yet.
If your engine runs low on oil pressure, the crankshaft will ride on the actual metal bearing strip with no thin film in between. This accelerates the rod bearing wear significantly, causing the bearings to overheat and disintegrate in a matter of minutes. If the engine hasn’t seized and you’ve driven the car with the low oil pressure lamp on for even a short while, you should bite the bullet and replace the rod bearings as soon as possible. A catastrophic drop in oil pressure might be caused by hitting a large rock on the freeway that punctures the bottom of your oil sump, or having the oil pump nut come loose on your E36 six-cylinder engine (more on this later).
Rod bearing replacement is generally straightforward. The toughest part of the procedure is getting to the bearings themselves. On the E30 and E36, this involves removing the front suspension, front axle support bar, steering rack (Project 59), and lower sump oil pan (Project 19). Once all of this equipment is out of the way, you have ready access to the rod bearings.
Begin by rotating the engine so you can reach the rod bolts. The rod bolts can be accessed in the following sequence: 1 and 6, 3 and 4, and 2 and 5. Turn the crankshaft over so each pair moves to its lowest position. Use a socket and driver on the front crankshaft pulley nut (see Photo 3 of Project 17). Remove the two rod bolts from each of the two rods, and gently pull off the rod end cap. If it sticks on the end of the rod, tap it lightly with a very small hammer. With the cap removed, gently push the other rod half up into the engine until the rod can clear the crankshaft journal. Place a small strip of cardboard against the surface of the crankshaft—you don’t want to scratch the delicate polished surface of the crankshaft with the edge of the rod.
Inspect the rod bearings when you remove them. They should be a dull grey, with no indications of wear or discoloration. If they are shiny, if you see brass-colored metal, or if the bearings are missing chunks or pieces, they are surely worn and need to be replaced.
Replacement rod bearings must be matched to the crank. If you use a crankshaft that has had rod journals reground, you will have to use oversized rod bearings. Measure the rod journals on the crank before you order any replacement rod bearings.
Using a clean-room wipe and isopropyl alcohol, carefully clean each rod bearing (see Photo 4). Install each bearing into the rod cap and rod half. Depending upon which engine you have, there may be upper and lower bearing shell halves (blue for the top side and red for the lower side near the rod cap). Using a clean-room wipe and isopropyl alcohol, clean all oil and residue off of the rod bolts and nuts. Then install the new rod bolts into the cap. Apply a generous, but even, layer of assembly lube to the rod bearing. Spread the assembly lube across the rod bearing with your finger while wearing powder-free latex gloves. If you don’t gloves, wash your hands first with nonpumice soft soap and make sure they are clean of any dirt or debris. Lubricate both bearing halves, and wipe a small amount around the sides of the big end of the rod to help lubricate any side contact the rod and crankshaft might see before adequate oil pressure is achieved.
Each rod is stamped with a unique number, and rods and their matching caps must be kept together. Make sure the numbers stamped in the rod butt up against each other. If you have the cap aligned correctly, with the numbers lining up, the rod bearing tangs located on the edge of each bearing will also butt up against each other. Double-check that you have properly lubed all the bearings and lined up the rods with their matching caps before you begin to torque the rod nuts to their final values.
Rod nuts and bolts are one-time-use parts designed to be tightened to their torque values only once. Always use new rod bolts and nuts when replacing your rod bearings. When tightening the rod bolts, carefully follow the torque values and use a calibrated torque wrench. Rod bolt torque values vary considerably; see www.101Projects.com for a listing of torque values and tightening procedures. Most procedures involve tightening the rod bolt to a specific torque value, and then further tightening the bolt a certain number of degrees.
With the new bearings installed, follow BMW’s engine break-in instructions. Disconnect the fuel or DME relay, and allow the car to build oil pressure by turning over the starter in 10-second increments. Repeat this step at least three times to build oil pressure in the engine. Breaking in an engine involves all of the parts in the engine beginning to wear together, finding their groove. Close-tolerance parts actually wear and machine themselves into proper alignment with their counterparts over time. For the first 1,200 miles, do not exceed engine rpm of 5,500 or road speed of 105 miles per hour.
Interestingly enough, there is a factory recall notice (SI B 11 04 04) that dictates rod bearing replacement on the E46 M3 with the S54B32 engine, produced from February 12, 2001, through May 22, 2003. The original bearings in these engines were prone to failure if the engines were driven at high rpm for extended periods of time (exactly what you’d expect to do to an M3). The design of the S54 motor is very similar to the earlier inline six-cylinders, and the rod bearing replacement procedure is the same. The factory recall involves replacing the bearings and an update to the factory engine management software (DME) to restrict rpm when the car is in lower gears.
Oil pump nut
A relatively new failure mode is occurring on some six-cylinder BMW engines, like the ones used in the E36 series. The oil pump gear attaches to the engine crank by a chain and a sprocket held on with a small reverse-thread nut. There have been many recent reports of this nut coming loose and falling off the sprocket. As a result, the sprocket becomes loose and can separate from the oil pump, leading to zero oil pressure in the engine. This can be catastrophic to the rod bearings, which are typically the first components to fail.
It hasn’t been determined what causes this loosening of the nut. Some people I’ve spoken to seem to believe the odds of it falling off may be increased by aggressive driving and go so far as to suggest it happens when the chassis of the car is spun around on the pavement (like at the race track). Whatever the cause, this very serious problem seems to be affecting many more cars as they age and are driven harder and harder. The problem doesn’t seem to correlate to mileage, as many lower-mileage cars appear to have been affected as well (cars with less than 50,000miles on the odometer).
To solve this problem, remove the lower oil pan and secure the nut in place so that it will not fall out. The one fail-safe trick to secure the nut is to tack weld it to the sprocket. Once welded, that nut is not going to come off under almost any circumstance. Other people have run safety wire through the nut, applied Loctite, and deformed the inner edge of the nut with a punch. If you don’t weld it, at least replace the nut with a new one (part number 11-41-1-735-137). Keep in mind that this nut is reverse threaded, so to remove it, you need to turn it clockwise.
If your engine has suffered from a loss of oil pressure, the first thing I recommend you do is replace your rod bearings. They are the one component most likely to be damaged by low oil pressure and the one component that, if worn, will result in complete engine failure. When most people think of bearings, they think of ball bearings like you would find in the wheel of a bicycle. The rod bearings are very different; they are basically a strip of curved metal that an oil film floats on. When the engine is running perfectly, the connecting rod bearing never touches the crankshaft, as thin oil molecules are suspended between the bearing and the crankshaft. The only time they might touch under normal circumstances is when you first start up the engine and there’s not much oil pressure built up yet.
If your engine runs low on oil pressure, the crankshaft will ride on the actual metal bearing strip with no thin film in between. This accelerates the rod bearing wear significantly, causing the bearings to overheat and disintegrate in a matter of minutes. If the engine hasn’t seized and you’ve driven the car with the low oil pressure lamp on for even a short while, you should bite the bullet and replace the rod bearings as soon as possible. A catastrophic drop in oil pressure might be caused by hitting a large rock on the freeway that punctures the bottom of your oil sump, or having the oil pump nut come loose on your E36 six-cylinder engine (more on this later).
Rod bearing replacement is generally straightforward. The toughest part of the procedure is getting to the bearings themselves. On the E30 and E36, this involves removing the front suspension, front axle support bar, steering rack (Project 59), and lower sump oil pan (Project 19). Once all of this equipment is out of the way, you have ready access to the rod bearings.
Begin by rotating the engine so you can reach the rod bolts. The rod bolts can be accessed in the following sequence: 1 and 6, 3 and 4, and 2 and 5. Turn the crankshaft over so each pair moves to its lowest position. Use a socket and driver on the front crankshaft pulley nut (see Photo 3 of Project 17). Remove the two rod bolts from each of the two rods, and gently pull off the rod end cap. If it sticks on the end of the rod, tap it lightly with a very small hammer. With the cap removed, gently push the other rod half up into the engine until the rod can clear the crankshaft journal. Place a small strip of cardboard against the surface of the crankshaft—you don’t want to scratch the delicate polished surface of the crankshaft with the edge of the rod.
Inspect the rod bearings when you remove them. They should be a dull grey, with no indications of wear or discoloration. If they are shiny, if you see brass-colored metal, or if the bearings are missing chunks or pieces, they are surely worn and need to be replaced.
Replacement rod bearings must be matched to the crank. If you use a crankshaft that has had rod journals reground, you will have to use oversized rod bearings. Measure the rod journals on the crank before you order any replacement rod bearings.
Using a clean-room wipe and isopropyl alcohol, carefully clean each rod bearing (see Photo 4). Install each bearing into the rod cap and rod half. Depending upon which engine you have, there may be upper and lower bearing shell halves (blue for the top side and red for the lower side near the rod cap). Using a clean-room wipe and isopropyl alcohol, clean all oil and residue off of the rod bolts and nuts. Then install the new rod bolts into the cap. Apply a generous, but even, layer of assembly lube to the rod bearing. Spread the assembly lube across the rod bearing with your finger while wearing powder-free latex gloves. If you don’t gloves, wash your hands first with nonpumice soft soap and make sure they are clean of any dirt or debris. Lubricate both bearing halves, and wipe a small amount around the sides of the big end of the rod to help lubricate any side contact the rod and crankshaft might see before adequate oil pressure is achieved.
Each rod is stamped with a unique number, and rods and their matching caps must be kept together. Make sure the numbers stamped in the rod butt up against each other. If you have the cap aligned correctly, with the numbers lining up, the rod bearing tangs located on the edge of each bearing will also butt up against each other. Double-check that you have properly lubed all the bearings and lined up the rods with their matching caps before you begin to torque the rod nuts to their final values.
Rod nuts and bolts are one-time-use parts designed to be tightened to their torque values only once. Always use new rod bolts and nuts when replacing your rod bearings. When tightening the rod bolts, carefully follow the torque values and use a calibrated torque wrench. Rod bolt torque values vary considerably; see www.101Projects.com for a listing of torque values and tightening procedures. Most procedures involve tightening the rod bolt to a specific torque value, and then further tightening the bolt a certain number of degrees.
With the new bearings installed, follow BMW’s engine break-in instructions. Disconnect the fuel or DME relay, and allow the car to build oil pressure by turning over the starter in 10-second increments. Repeat this step at least three times to build oil pressure in the engine. Breaking in an engine involves all of the parts in the engine beginning to wear together, finding their groove. Close-tolerance parts actually wear and machine themselves into proper alignment with their counterparts over time. For the first 1,200 miles, do not exceed engine rpm of 5,500 or road speed of 105 miles per hour.
Interestingly enough, there is a factory recall notice (SI B 11 04 04) that dictates rod bearing replacement on the E46 M3 with the S54B32 engine, produced from February 12, 2001, through May 22, 2003. The original bearings in these engines were prone to failure if the engines were driven at high rpm for extended periods of time (exactly what you’d expect to do to an M3). The design of the S54 motor is very similar to the earlier inline six-cylinders, and the rod bearing replacement procedure is the same. The factory recall involves replacing the bearings and an update to the factory engine management software (DME) to restrict rpm when the car is in lower gears.
Oil pump nut
A relatively new failure mode is occurring on some six-cylinder BMW engines, like the ones used in the E36 series. The oil pump gear attaches to the engine crank by a chain and a sprocket held on with a small reverse-thread nut. There have been many recent reports of this nut coming loose and falling off the sprocket. As a result, the sprocket becomes loose and can separate from the oil pump, leading to zero oil pressure in the engine. This can be catastrophic to the rod bearings, which are typically the first components to fail.
It hasn’t been determined what causes this loosening of the nut. Some people I’ve spoken to seem to believe the odds of it falling off may be increased by aggressive driving and go so far as to suggest it happens when the chassis of the car is spun around on the pavement (like at the race track). Whatever the cause, this very serious problem seems to be affecting many more cars as they age and are driven harder and harder. The problem doesn’t seem to correlate to mileage, as many lower-mileage cars appear to have been affected as well (cars with less than 50,000miles on the odometer).
To solve this problem, remove the lower oil pan and secure the nut in place so that it will not fall out. The one fail-safe trick to secure the nut is to tack weld it to the sprocket. Once welded, that nut is not going to come off under almost any circumstance. Other people have run safety wire through the nut, applied Loctite, and deformed the inner edge of the nut with a punch. If you don’t weld it, at least replace the nut with a new one (part number 11-41-1-735-137). Keep in mind that this nut is reverse threaded, so to remove it, you need to turn it clockwise.
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