BMW Coupes and Sedans 1970-1988 Repair Guide

Pistons and Connecting Rods


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Exploded view of piston and connecting rod assembly


Keep all of the following points in mind when rebuilding a BMW engine:

The pistons and connecting rods may be removed from the engine after the cylinder head, oil pan and oil pump are removed. It may be necessary to first remove a ridge worn into the cylinder above the top ring. See the engine rebuilding section. The connecting rods and caps are marked for each cylinder with No. 1 cylinder at the sprocket end of the engine. Codes pairing the connecting rods with the matching cap are located on the exhaust side of the engine. However, it is a good idea to mark the exact relationship between each rod and the crankshaft to ensure replacement in the exact same position, in case the bearings can be re-used.

On the 524td engine, oil nozzles which are critically aimed must be protected from damage by studs (11 2 050) which screw into the connecting rod cap bolt holes before the rod and piston are shoved upward and out the top of the block. Make sure each crankpin is precisely at BDC prior to removal.

To disassemble rods and pistons, remove the circlip and press out the piston pin. Note that pistons and piston pins come as a matched set. Do not mix them up.

A piston pin must always slide through the connecting rod under light pressure.

If replacing pistons, make sure all are of the same make and weight class written on the crown.

Piston installed clearance must meet specifications. On the 318i, check installed clearance at a point measured up from the lower skirt edge, depending on the piston manufacturer: Mahle-14mm; KS-31mm; Alcan-15.5mm.

On the engines used in 325e and 528e models, check piston diameter according to total height and manufacturer. On pistons 68.7mm high manufactured by Mahle, check the diameter at a point 8mm above the low point on the skirt; on those of this height manufactured by KS, check diameter 14mm above the low point of the skirt. If the total height is 77.7mm, check the diameter of both Mahle and KS pistons 23mm above the low point of the skirt. On the engines used in 325i, check the diameter 9mm above the low point of the skirt.

On the 3.3 liter engine, measure Mahle pistons 26mm up from the lower skirt edge, and KS pistons 34mm up from the skirt edge. On the 3.5 liter engine, measure 14mm up from the skirt edge.

On the 3.5 liter, engine, measure the Mahle pistons at a point 14mm up from the bottom of the skirt.

On the 524td engine, the dimensions are: Alcan-15mm; KS -18mm; Mahle/Konig-12mm. Measure 14mm up from the skirt edge.

On the engines used in the M3, M5, and M6, measure the piston at a point 6mm below the deepest part of the skirt.

Lubricate the piston and rings with engine oil prior to installation. Offset ring gaps 120° apart. Install circlips facing downward.
The side of rings marked TOP must face upward. 2.7, 2.5, 3.3 and 3.5 liter engines and the M3 engine use a plain compression ring at top, tapered or beveled second compression ring, and an oil control ring at the bottom. The 524td uses a keystone ring at the top, a taper face lower compression ring, and a beveled oil control ring with a rubber-lined expander at the bottom.


See Figures 2,3,4,5,6 and 7

While the pistons and connecting rods can be removed from the engine with the engine installed in the vehicle most of the time, it is better to remove the engine to do this procedure. With the engine removed, it is easier to inspect the engine and to manipulate the tool and parts.

Click image to see an enlarged view

Fig. Fig. 2: Remove the ridge from the cylinder bore using a ridge cutter

Click image to see an enlarged view

Fig. Fig. 3: Matchmarking the connecting rod and cap

Click image to see an enlarged view

Fig. Fig. 4: With the engine installed in the vehicle, use a breaker bar and socket to loosen the bearing cap.

Click image to see an enlarged view

Fig. Fig. 5: Bearing cap removed

Click image to see an enlarged view

Fig. Fig. 6: With the engine inverted on a workstand, the pistons can be more easily removed by tapping them out with a piece of wood and a hammer

Click image to see an enlarged view

Fig. Fig. 7: If the pistons you are removing have studs attached to the connecting rods, place rubber hoses over the ends prior to removal

Before removing the pistons and connecting rods, mark the pistons, rod bearing caps and the rods with the cylinder number and orientation. This will ensure replacement of the pistons and bearing into the correct positions.

Before removing the pistons, the top of the cylinder bore must be examined for a ridge. A ridge at the top of the bore is the result of normal cylinder wear; caused by the piston rings only traveling so far up the bore in the cause of the piston stroke.

BMW uses a very high quality material to cast the cylinder blocks and cylinder ridges tend to be slight. If the ridge can be felt by hand, it must be removed before the pistons are removed. This is to prevent the rings from breaking while the piston is pushed past the ridge.

A ridge reamer is necessary for this operation. Place the piston at the bottom of its stroke, and cover it with a rag. Cut the ridge away with the ridge reamer, using extreme care to avoid cutting too deeply. Remove the rag, and remove the cuttings that remain on the piston with a magnet and a rag soaked in clean oil. Make sure the piston top and cylinder bore are absolutely clean before moving the piston.

  1. Remove the cylinder head.
  3. Remove the oil pan.
  5. Remove the oil pump or oil pump pickup.
  7. Matchmark the connecting rod cap to the connecting rod with a scribe; each cap must be reinstalled on its proper rod in the proper direction. Remove the connecting rod bearing cap and the rod bearing. It may be necessary to rock the cap back and forth to free it. Number the top of each piston with silver paint or a felt-tip pen for later assembly.
  9. Cut lengths of 3 / 8 inch diameter rubber hose to use as rod bolt guides. Install the hose over the threads of the rod bolts, to prevent the bolt threads from damaging the crankshaft journals and cylinder walls when the piston is removed.
  11. Squirt some clean engine oil onto the cylinder wall from above until the wall is coated. Carefully push the piston and rod assembly up and out of the cylinder by tapping on the bottom of the connecting rod with a wooden hammer handle.
  13. Place the rod bearing and cap back on the connecting rod, and install the nuts temporarily. Using a number stamp or punch, stamp the cylinder number on the side of the connecting rod and cap this will help keep the proper piston and rod assembly on the proper cylinder. Do not stamp in the web area of the rod.

On all BMW engines, the cylinders are numbered 1-4 or 1-6 from front to back.

  1. Remove the remaining pistons in a similar manner.
  3. When ready for reassembly, please not the following:
    1. Connecting rods/caps must be reinstalled in the same cylinder and are so marked. Make sure markings on rod and cap are on the same side when reassembling.
    3. The piston pins are matched to the pistons and are not interchangeable.
    5. The arrow on top of the piston must face forward (toward the timing chain). Pistons are also marked as to manufacturer and weight class. All pistons must be of the same manufacturer and weight class.
    7. Offset each ring gap 120 degrees from each other. Do not align any of the gaps with the piston pin bore. This reduces blowby of combustion gases.



See Figures 8,9 and 10

Before any kind of inspection is done to the piston, cylinder or piston pin, it is imperative that they be clean. All traces of varnish, carbon or build-up be removed. Measurements will be inaccurate and signs of wear hidden on a dirty part.

Clean the piston with solvents and chip off any carbon build-up with a hardwood chisel. Do not hot tank aluminum pistons. Clean the cylinder bore with solvent, then wipe down with clean engine oil.

Click image to see an enlarged view

Fig. Fig. 8: Use a flexible honer to remove any cylinder glazing

Click image to see an enlarged view

Fig. Fig. 9: Use a telescopic gauge to measure the cylinder bore

Click image to see an enlarged view

Fig. Fig. 10: Measure the cylinder bore in three direction to determine if it is out of round

Inspect the piston for signs of scoring, scuffing, cracks, pitting or burning. Check the piston for mechanical damage such as valve strikes or denting due to ingested objects. Measure the piston diameter with a micrometer. Measure at points 90 degrees to the piston pin bore. Measure down from the edge of the piston skirt the proper distance from the piston skirt edge.

Measure the inside diameter of the cylinder at 3 points; top, middle and bottom. Measure at points which are contacted by the piston rings. If the cylinder walls are found to be tapered, scored or otherwise damaged, the cylinder will need to be honed or bored oversized.

Subtract the diameter of the piston from the diameter of the cylinder to find the piston clearance. Check that the clearance does not exceed the maximum permitted. If the clearance is too great, the cylinders will need to be bored oversized for the next available sized piston


There are 2 basic machining operations that are done to a cylinder wall, boring and honing. Both procedures remove metal from the cylinder walls. The major difference is that boring removes a larger amount of metal than honing does. Boring is used to enlarge the cylinder so a larger diameter piston can be used. Honing is a finer process that gives the cylinder wall the correct machined finish for proper operation and sealing of the piston rings.

If the cylinders are out of round or tapered beyond allowable limits, the cylinders will need to be bored to the next standard oversize. Boring is done by a qualified machine shop. Provide the machine shop with the new oversized pistons. The machine shop will match the bore of the cylinder to diameter of the new piston, taking into account the piston clearance required. The machine shop will return the block and piston marked as to which piston goes to which cylinder.

Honing should be done by a machine shop to guarantee the proper cylinder wall finish. Honing can be accomplished with a hand held honing tool available from most automotive shops, but a machine shop can provide the best result. Honing is done to prepare the cylinder wall with the proper machined finish. The finish is a crosshatch pattern that catches and holds oil for the piston rings to ride on. Honing can be used without boring the cylinder when only new rings are being installed, or after the cylinder has been bored for a new piston and the proper finish is required.


See Figures 11 and 12

The piston pins are matched to the pistons and must not be interchanged. The piston pins run in bushings pressed into the small end of the connecting rod. The pins are retained by snaprings.

Remove the snaprings with a snapring removal tool. The piston pin should slide out of the bore with some finger pressure. If the pin resists being pushed out, check that varnish or carbon hasn't collected at the end of the pin bore. Clean the bore and the pin should slide freely out.

Click image to see an enlarged view

Fig. Fig. 11: The piston pin is secured with a snapring

Click image to see an enlarged view

Fig. Fig. 12: The piston pin should slide through the connecting rod with minimal force

Clean the pin and connecting rod bushing. Lightly lubricate the pin and slide it into the connecting rod. The pin should slide in with only hand pressure. There should be no discernible play with the pin installed in the rod. Check the pin bushing for scoring or damage. A sign of worn bushing is a knocking sound during engine acceleration.

Have new bushing pressed into the rod if the old bushing are found to be worn or damaged. The new bushing will be drilled for the oil lubrication hole at the top of the rod.

The piston pin is installed in the piston and connecting rod, making sure the orientation of the rod and piston is correct. Check the marks previously made on the rod during removal and the arrow on the piston top. The arrow should face the timing chain or belt. Replace the snapring if the old ring was damaged, though it is a good idea to always use new snaprings. Place the gap in the snapring opposite the groove cut into the piston used for snapring removal.


See Figures 13,14,15,16,17 and 18

Piston rings provide the seal between the combustion gases and the crankcase. The rings do a tremendous job of separating the extreme pressure of the combustion chamber and the semi-vacuum of the crankcase. If the piston rings can not do the job properly, there will be leakage of oil into the combustion chamber and the resulting blue oil smoke from the tailpipe. There will be a reduction in compression resulting in a loss of performance.

Click image to see an enlarged view

Fig. Fig. 13: Use a ring expander to remove the piston rings

Click image to see an enlarged view

Fig. Fig. 14: Clean the piston grooves with a ring groove cleaner

Click image to see an enlarged view

Fig. Fig. 15: Also part of an old piston ring can be used to clean the piston grooves

Click image to see an enlarged view

Fig. Fig. 16: Mosr new rings are marked as to which side faces upward

Click image to see an enlarged view

Fig. Fig. 17: Check the ring-to-ring groove (ring side) clearance

Click image to see an enlarged view

Fig. Fig. 18: Piston ring placement in piston body

The piston rings are located in grooves, called lands, machined into the pistons. As the piston moves up and down, the rings slowly rotate in the lands. This allows the rings to seal against the cylinder wall and the piston. If the ring lands get filled with carbon or varnish, the rings will stick and not be allowed to move. This will reduce the ability of the rings to seal. If the rings stick, the rings need to be removed, the lands cleaned and the rings replaced.

The piston rings should be removed with a ring remover tool. The tool will grasp the ends of the ring at the gap and spread the ring enough to remove it from the piston. Do not use screwdrivers, pliers or any other tool not designed to do the job. The piston rings can break into sharp edged pieces of stretched too far.

With the piston rings removed, inspect the lands for carbon and varnish buildup. Clean the lands with solvent if varnish is found. Clean the lands with a mechanical cleaning tool if carbon is found. A piece of old ring can be used to remove light deposits. Check the bottom of the lands for wear. The pistons will need to be replaced if the lands are damaged or worn.

Piston ring end gap should be checked while the rings are removed from the pistons. Incorrect end gap indicates that the wrong size rings are being used; ring breakage could occur. Compress the piston rings to be used in a cylinder, one at a time, into that cylinder. Squirt clean oil into the cylinder, so that the rings and the top half of cylinder wall are coated. Using an inverted piston, press the rings approximately 1 inch below the deck of the block. Measure the ring end gap with a feeler gauge, and compare to the specifications chart in this section. Carefully pull the ring out of the cylinder and file the ends squarely with a fine file to obtain the proper clearance.

Install the rings on the piston, lowest ring first, using a piston ring expander. There is a high risk of breaking or distorting the rings, or scratching the piston, if the rings are installed by hand or other means. When installing new rings, refer to the installation diagram furnished with new parts.

Check the pistons to see that the ring grooves and oil return holes have been properly cleaned. Slide a piston ring into its groove, and check the side clearance with a feeler gauge. Make sure that you insert the gauge between the ring and its lower land (lower edge of the groove), because any wear that occurs, forms a step at the inner portion of the lower land. If the piston grooves have worn to the extent that relatively high steps exist on the lower land, the piston should be replaced, because these will interfere with the operation of the new rings and ring clearances will be excessive. Piston rings are not furnished in oversize widths to compensate for ring groove wear.


See Figures 19 and 20

Connecting rod bearings for the engines covered in this guide consist of 2 halves or shells which are interchangeable in the rod and cap. When the shells are placed in position, the ends extend slightly beyond the rod and cap surfaces so that when the rod bolts are tightened, the shells will be clamped tightly in place to insure positive seating and to prevent turning. A tang holds the shells in place. Place one end of the bearing into the journal and press the other end down. Squeeze the bearing shell a bit while pressing down. The bearing shell will snap into place.

The ends of the bearing shells must never be filed flush with the mating surface of the rod and cap.

If a rod bearing becomes noisy or is worn so that its clearance on the crank journal is sloppy, a new bearing of the correct size must be selected and installed since there is no provision for adjustment.

Click image to see an enlarged view

Fig. Fig. 19: Place a strip of gauging material to the bearing journal, then install and tighten down

Click image to see an enlarged view

Fig. Fig. 20: After the bearing cap has been removed, use the gauge supplied with the gauging material to check the bearing clearance

Under no circumstances should the rod end or cap be filed to adjust the bearing clearance, nor should shims of any kind be used.

Inspect the rod bearings while the rod assemblies are out to the engine. If the shells are scored or show flaking, they should be replaced. If they are in good shape check for proper clearance on the crank journal. Any scoring or ridges on the crank journal means the crankshaft must be replaced.

The crankshaft is surface hardened at the factory and it must not be reground without the proper surface treatment being done to the reground crankshaft. It is recommended to use a factory reground or new crankshaft if the crankshaft must be replaced. Use the proper sized undersized bearings if a reground crankshaft is used.

Connecting rod-to-crankshaft bearing clearance is checked using Plastigage® having a range of 0.0005-0.0030 inches.

  1. Remove the rod cap with the bearing shell. Completely clean the bearing shell and crank journal, and blow any oil from the oil hole in the crankshaft; Plastigage® is soluble in oil.
  3. Place a piece of Plastigage® lengthwise along the bottom center of the lower bearing shell, then install the cap with shell and tighten to specification. DO NOT turn the crankshaft with the Plastigage® in the bearing.
  5. Remove the bearing cap with the shell. The flattened Plastigage will be found sticking to either the bearing shell or crank journal. Do not remove it yet.
  7. Use the scale printed on the Plastigage® envelope to measure the flattened material at its widest point. The number within the scale which most closely corresponds to the width of the Plastigage® indicates bearing clearance in thousandths of a inch.
  9. Check the specifications chart in this section for the desired clearance.

With the proper bearing clearance and the nuts tightened, it should be possible to move the connecting rod back and forth freely on the crank journal slightly. If the rod cannot be moved, either the rod bearing is too far undersize or the rod is misaligned.


See Figures 21, 22 and 23

Before installing the pistons and rods, but after checking bearing clearances, it is recommended to replace the connecting rod bolts. BMW uses forged steel rods which are very strong and will last an extremely long time. The rod bolts should be replaced after they have been tightened and released once. It is acceptable to used the old bolts during the bearing clearance checking procedure, but they should be replaced before the final assembly and installation is completed. The bolts are pressed into the rods. A machine shop can do this for a nominal fee. The bolts have to pressed in while supporting the rod to prevent any damage to either the rod or the bolt.

Click image to see an enlarged view

Fig. Fig. 21: Check to make sure the marks on the connecting rod and rod cap match

Click image to see an enlarged view

Fig. Fig. 22: Most pistons are marked on the top to indicate their relative position

Click image to see an enlarged view

Fig. Fig. 23: Use a ring compressor and a piece of wood to carefully install the piston into the cylinder bore

Install the connecting rod to the piston, making sure piston installation notches and any marks on the rod are in proper relation to one another. Lubricate the wrist pin with clean engine oil, and install the pin into the rod and piston assembly. Install snaprings and rotate them in their grooves to make sure they are seated. Position the snaprings with the ends opposite of the removal groove. To install the piston and connecting rod assembly:

  1. Make sure that the connecting rod big-ends bearings (including end cap) are of the correct size and properly installed.
  3. Fit rubber hoses over the connecting rod bolts to protect the crankshaft journals, as in the Piston Removal procedure. Coat the rod bearings with clean oil.
  5. Using the proper ring compressor, insert the piston assembly into the cylinder so that the word TOP faces the front of the engine (this assumes that the dimple(s) or other markings on the connecting rods are in the correct relationship.
  7. From beneath the engine, coat each crank journal with clean oil. Pull the connecting rod, with the bearing shell in place, into position against the crank journal.
  9. Remove the rubber hoses. Install the bearing cap and cap nuts and tighten to the proper specifications.
  11. If more than one rod and piston assembly is being installed, the connecting rod cap attaching nuts should only be tightened enough to keep each rod in position until all have been installed. This will ease the installation of the remaining piston assemblies as the crankshaft is rotated.
  13. Replace the oil pump if removed and the oil pan.
  15. Install the cylinder head.