2. To remove the ring, spread its ends and pull the ring up through the piston. To prevent the ring from falling into the free grooves, you can insert two or three old feeler gauges under the ring (Fig. 12.2). The rings have sharp ends - do not injure your hands or scratch the piston surface. In addition, the rings are very fragile and break easily. Keep in mind that the third (oil scraper) the ring is equipped with a spring. Remove the rings only through the piston bottom. If the rings are to be used further, store them together with the corresponding piston. Remember which side of each ring is up.
3. Scrape off any carbon deposits from the piston bottom. Any remaining carbon deposits can be removed with a wire brush or sandpaper.
4. Clean the ring grooves from carbon deposits. You can use a broken piston ring for this. Remove only carbon deposits from the grooves, not the piston metal. The piston is made of aluminum alloy and can be easily scratched by a hard and sharp ring.
5. After cleaning off the carbon deposits, wash the piston and connecting rod with kerosene or another solvent and dry them well. Don't forget to clean the oil holes in the bottom ring groove.
6. If the cylinders and pistons are not too worn and have no damage, you can skip boring the cylinders and replace the pistons. Measure the piston diameters and make sure that their diameters are within the tolerance. Check the clearances between the pistons and cylinders. If the clearances are too large, the cylinders will still have to be bored and the pistons replaced. A normally run-in piston may have a light uniform strip on one side and a small play in the upper compression ring in the groove. When assembling the engine, the piston rings are replaced with new ones in any case.
7. Carefully inspect each piston for cracks in the skirt, in the area of the piston pin holes and between the ring grooves.
8. Inspect the piston skirt for scratches and scoring, the piston crown for burnouts and scorched or overheated areas along the edges. If there are scratches and scoring on the skirt, it is possible that the engine overheated due to insufficiently efficient cooling or due to improper combustion. In this case, it is necessary to carefully check the cooling and lubrication systems. Scorched areas along the edges of the piston crown indicate a gas breakthrough into the crankcase. Burnouts on the bottom or overheated areas along the edges of the piston occur due to improper ignition (spontaneous combustion, detonation). If such defects are detected, their cause must be identified and eliminated, otherwise the defects will reoccur. The causes may be incorrect ignition timing, leaks in the intake system, or incorrect mixture composition.
9. Pitting on the piston crown means that coolant is leaking into the combustion chamber or crankcase. In this case, the cause must also be found and eliminated.
10. New pistons can be ordered from your BMW dealer.
11. Inspect each connecting rod for damage, such as cracks around the upper and lower head bores. Make sure the connecting rods are not bent or warped. Such defects are unlikely unless the engine has seized or overheated. A full inspection of the connecting rods can be performed by a service technician using special equipment.
12. Piston pins of the floating type, secured from axial displacement by spring rings. The connecting rod can be separated from the piston as follows.
13. Using a small screwdriver, remove the spring rings holding the pin (fig. 12.13,a,b). The force of the hand should be enough to remove the pin. Mark the piston and its pin for subsequent assembly. The spring rings can be thrown away - new ones are needed for assembly. The manufacturer does not recommend changing the pins separately from the pistons - the pins are selected for specific pistons according to the dimensions within the tolerance.
14. Inspect the piston pins and the upper heads of the corresponding connecting rods for wear and damage. The pin should fit into the connecting rod by hand, without noticeable play. In case of significant wear, the piston and pin should be replaced as a pair.
15. Changing connecting rods is usually not necessary, except in the case of engine seizure or other serious malfunction. Visually assess the straightness of the connecting rod and, if any suspicions arise, take it to a specialist for a more thorough inspection.
16. Check all parts and purchase the necessary replacements. Pistons are usually supplied complete with pins and spring rings.
17. When connecting the connecting rod to the piston, turn them relative to each other so that after installation in the engine, the numbers on the connecting rod and the cover of its lower head are facing the exhaust manifold, and the arrow on the bottom of the piston is facing the front side of the engine (towards the chain) (fig. 12.17,a,b).
18. Lubricate the piston pin with engine oil and insert it into the piston and into the upper head of the connecting rod. Check that the piston rotates easily relative to the connecting rod. Secure the pin with two spring rings. Make sure that the rings are positioned in the grooves intended for them.