Cleaning
1. Before checking, the pistons and connecting rods should be washed, and the old rings removed from the pistons.
2. Carefully spread the ring over the edges of the gap and remove it from the piston upwards. Two or three old feeler gauges can be useful in this case, so that the removed rings do not fall into already free grooves (see fig. 14.2). Do not scratch the pistons with the ends of the rings. The rings are made of a brittle material and can break if spread too wide. In addition, they have sharp edges - take care of your hands and fingers. Always remove the rings towards the top of the piston. Note that the third ring has an expander. If rings are to be used, store the rings with the piston. For proper installation, write down how each ring is installed.
Pic. 14.2. Removing piston rings with feeler gauges
3. Scrape off any carbon deposits from the piston crown. Residues of soot can be removed with a manual carriage or sandpaper.
4. Remove carbon deposits from the grooves using the old ring. To do this, break the ring in half (be careful - the ring has sharp edges). Be careful to remove only carbon deposits, not metal, and do not scratch the walls of the grooves.
5. After removing deposits, wash pistons and connecting rods with kerosene or other suitable solvent and dry thoroughly. Make sure the oil drain holes in the oil scraper grooves are clean.
Examination
6. If the pistons and cylinder bores are not damaged or too worn, and if the cylinder block does not require boring, old pistons can be installed during assembly. Measure the piston diameters and check that they are within the respective group. If the clearances between the pistons and cylinders are increased, the cylinder block must be bored out and new repair pistons and rings installed. A normally worn piston has vertical, even wear marks on the cylinder contact surface and a slightly worn top ring groove. In any case, when assembling the engine, install new piston rings. Recall that the dimensions of the pistons and cylinders are knocked out on the bottom of the pistons and on the mating surface of the cylinder block.
7. Carefully inspect the pistons for cracks in the skirt, around the piston pin holes, and in between the grooves.
8. Look for a lot of wear and scuffing on the skirt, burnouts in the piston bottom and burns around the perimeter of the bottom. If there are scratches or scuffs on the skirt, this may mean that the engine is overheating during operation or the combustion process is abnormal at high operating temperatures. In this case, it is necessary to carefully check the cooling and lubrication systems. Signs of scoring on the side surfaces of the piston mean that there was a breakthrough of gases between the piston and the cylinder wall. Burnout and burns on the piston crown could be due to violations of the combustion process (incorrectly adjusted ignition timing, detonation). If there are any of the listed problems, it is necessary to find out and eliminate the causes, otherwise the problems will arise again. Causes may be incorrectly set ignition or intake timing or faulty injectors (diesel engines).
9. Pitting of the piston in the form of pitting means that the coolant enters the combustion chamber or the crankcase. In this case, it is also necessary to find out and eliminate the cause, otherwise these problems will occur again in the repaired engine.
10. New pistons are available from your Mercedes-Benz dealer.
11. Carefully inspect each connecting rod for damage such as top and bottom head cracks, warping, or warping. Such damage is unlikely unless the engine has been severely overheated or seized. A detailed inspection of the connecting rods can be performed by an engine rebuilder with special equipment.
12. The piston pins of the floating type are secured against longitudinal mixing by two spring rings. If necessary, their disassembly is carried out in the following order.
13. Using a small screwdriver, remove one of the spring tracks holding the pin. Push the pin out of the piston and connecting rod (see fig. 14.13, a, b). To remove the finger, the force of the hand is sufficient. Label the connecting rod and piston to assemble them correctly. At installation it is necessary to put new spring rings.
Pic. 14.13, a. Remove snap ring (16 valve engines)
Pic. 14.13.6....and push out the piston pin
14. Check up, whether there is no wear or damage of piston pins and plugs of the top heads of rods. Wear can only be repaired by replacing the pin and bushing. Replacing the connecting rod head bushing is a complex operation that requires pressing equipment.
15. Connecting rods do not require replacement unless jamming or other mechanical problems have occurred. Check the straightness of the connecting rod and if in doubt seek expert advice.
16. Inspect all parts and, if required, purchase new ones. If you buy new pistons, they must be fitted with new circlip pins. Spring rings can also be purchased separately.
17. Position the piston in relation to the connecting rod located as follows (see fig 14.17).
Pic. 14.17. Small grease hole (shown by arrow) must be located on the drive chain side - petrol engines
- A) On gasoline engines, the small grease hole at the top of the connecting rod and the arrow on the bottom of the piston must point towards the engine camshaft drive chain.
- b) On diesel engines, the bushing slots in the connecting rod and bearing caps must be located on the high pressure pump side and the arrow on the piston crown must point towards the drive chain.
18. Lubricate the piston pin. Install the connecting rod in the piston and insert the pin. Make sure the piston turns freely on the connecting rod. Secure your finger with a snap ring. Position the ring so that its lock is facing down.