Note: If the engine has been severely overheated, check for warped cylinder head.
Cleaning
2. Remove all traces of the old gasket from the cylinder head.
3. Scrape carbon deposits from the combustion chamber and holes. Then thoroughly wash the cylinder head with kerosene or other suitable solvent.
4. Scrape off thick deposits of carbon from the valve plates with a scraper, and finally clean the plates and valve stems from carbon deposits with a wire brush attached to an electric drill.
Status check
Note: Perform all the checks listed below before deciding whether to contact a service station. Make a list of items that need attention.
Cylinder head
5. Carefully inspect the head for cracks, visible coolant leaks, or other damage. If cracks are found, the head must be replaced.
6. Using a ruler and feeler gauge, check for warping of the surface adjacent to the cylinder block (see fig. 9.6). If warping is found, you can try to eliminate it by machining. It is important not to exceed the minimum head height. We also note that on diesel engines it is necessary to control the protrusion of the combustion chamber if the surface of the cylinder head is ground - see p.p. 11-13.
![](https://www.mercedesman.ru/img/c-class/2431.jpg)
Pic. 9.6. Checking the warping of the cylinder head surface with a metal ruler and feeler gauges
7. Check the valve seats in each combustion chamber. If they have significant pitting, cracks, or burnouts, they should be replaced or re-sharpened by a specialist. If seats and valves have only slight pitting, they can be lapped in with a special paste as described below. If the valve seats are reground, check that the valve recess, measured between the cylinder head sealing plane and the center of the valve head, is within the required limits.
8. Check for wear on the valve guides. Lower the valve into the appropriate hole and rock it from side to side. A slight play is allowed. If there is a significant gap, check the diameter of the valve stem (see below) and if it is worn, replace it. If the valve is not worn, the wear may be in the guide bushing. The guide sleeve should be countersunk or replaced and a repair valve installed. The replacement of the guide bush is best done at a service station, where all the necessary tools are available
9. If the valve seats are to be regrinded, this can only be done after replacing the guide bushings.
10. Check for wear or damage to the camshaft bearing surfaces in the cylinder head and bearing caps. If the bearings are significantly worn, it is possible to bore out the hole in the cylinder head/cover to install the camshaft with overhaul journals. Consult a Mercedes-Benz dealer or engine rebuilder.
Combustion chambers - diesel engines
11. When checking the cylinder head, the protrusion of the combustion chamber should be checked - this is especially important if the head has been ground. If the protrusion is too large, the pistons can hit the chamber while the engine is running, causing serious damage.
12. Measure the protrusion of the combustion chamber from the sealing plane of the cylinder head (see fig. 9.12). If the protrusion exceeds the maximum allowable protrusion, the protrusion can be changed by removing the combustion chamber and installing gaskets of various thicknesses to achieve the required protrusion. If the head has been ground, inserts of increased thickness should be installed, corresponding to the thickness of the metal removed from the head.
![](https://www.mercedesman.ru/img/c-class/2432.jpg)
Pic. 9.12. Combustion chamber protrusion measurement (With) - diesel engines
13. Removal and installation of combustion chambers and installation of appropriate inserts should be entrusted to a Mercedes-Benz dealer or an engine repair specialist.
Valve
Warning: To improve heat transfer, the exhaust valves on 4-cylinder petrol and diesel engines are filled with sodium. Sodium filled valves can be installed on other engines. Sodium is a highly reactive metal that, when in contact with water [including water vapor in air) ignites and explodes. These valves cannot be disposed of in the usual way. Consult your dealer for advice.
14. Inspect each valve head for pitting, cracks, burnout, and general wear. Check valve stems for scoring and wear. Rotate the valve and make sure it is not bent. Look for dents and heavy wear on the ends of the valve stems. Replace valves that show such damage.
15. If the valve looks satisfactory at this stage of the check, use a micrometer to measure the diameter of the valve stem at several places along the length (see fig. 9.15). Any difference in readings indicates rod wear. If wear is out of tolerance, the valve must be replaced.
![](https://www.mercedesman.ru/img/c-class/2433.jpg)
Pic. 9.15. Valve Stem Diameter Measurement
16. If the valves are in satisfactory condition, check their fit to the seat to ensure good sealing of the gases. If the saddle has only slight pitting, or if it has been reground, only fine paste should be used for finishing. Do not use coarse lapping paste unless the saddle is deeply pitted or burned. If this is the case, have the head and valves shown to a specialist who will decide if the seat can be reground or if the valve or seat insert needs to be replaced.
17. Lapping of valves is performed as follows. Place the head upside down on the stand with a block of wood on each side.
18. Apply a small amount of lapping paste to the seat face and press the head of the valve lapping tool on top of the valve disc (see fig. 9.18). Use a back and forth motion to lap the valve against the seat, lifting the valve from time to time to change the paste. To facilitate this operation, it is recommended to insert a weak spring under the valve disc.
![](https://www.mercedesman.ru/img/c-class/2434.jpg)
Pic. 9.18. Lapping valves with tool
19. If using a coarse grinding paste, lap until all pitting is gone and the lapping surfaces of the seat and valve disc have a dull matt appearance. After that, remove the remaining paste and proceed to work with a thin paste. Lapping should be continued until a light continuous ring band appears on both surfaces. Do not lap the valves more than necessary or the seat will drop below head level prematurely.
20. After lapping all valves, thoroughly wash off all traces of lapping paste with kerosene or other solvent before reassembling the head.
Valve train parts
21. Inspect the valve springs for damage and discoloration, and measure their free length by comparing existing springs with new ones.
22. Place each spring on a flat surface and check it for shape. If at least one of the springs is damaged, changed shape or has lost its elasticity, the entire set of springs should be replaced. It is considered normal to replace all springs when overhauling the engine.
23. Replace all valve stem seals, regardless of condition.
Cam followers gear / valve lifters
24. Check contact surfaces. If significant wear or damage is found, the parts should be replaced. Refer to parts A, B and IN of this chapter.