Cleaning
2. Scrape off the remnants of the old gasket from the cylinder head.
3. Scrape carbon deposits from combustion chamber surfaces and valve passages. Wash the head in kerosene or other suitable solvent.
4. Scrape off thick deposits of carbon from the valve plates, then clean the valve plates and valve stems with a wire brush clamped in an electric drill.
Examination
Note. Before deciding on the need to machine parts and contacting a service station for help, perform all of the following checks. Make a list of items you need professional help with.
Cylinder head
5. Carefully inspect the head for cracks, obvious leaks, or other damage. If cracks are found, the head must be replaced.
6. Check the warping of the machined head surface. To do this, attach the edge of a steel ruler to the surface to be checked and, using flat probes, measure the gap between the ruler and the surface in several places (pic. 7.6). If the surface is warped, it may be possible to restore it by machining, if after processing its height does not become less than acceptable. Note. If the head is ground by 0.3 mm, then when assembling the engine, a gasket 0.3 mm thicker than usual should be installed under it. This is necessary to maintain the compression ratio to prevent the valves from sticking into the pistons, to maintain the correct distances between the valve plates and guides.
7. Inspect the valve seats in each combustion chamber. If the saddles have cracks, burnouts, or severe pitting. Seats need to be replaced or reground (this is a job for a specialist). If the seat surfaces are only slightly pitted. this defect can be eliminated by lapping the valves to the seats, as indicated below.
8. Check valve guides for wear. To do this, insert the valve into the appropriate sleeve. Rock the valve from side to side. A small gap is acceptable. If the play seems excessive, replace the valve Separate guide bushings are not available for spare parts, while valves are available with stems of various repair sizes.
9. On a 6-cylinder engine, unscrew the bypass valve from the bottom plane of the cylinder head. Make sure the valve is blowing from bottom to top and not from top to bottom. Flush the valve, replace the o-ring on its body and screw it into place, tightening it securely (pic. 7.9).
10. Inspect the camshaft bearing bores and their covers for significant wear or damage.
11. On engines with separate camshaft bearings, check the mating surfaces of the bearings and cylinder heads for warpage using a steel ruler placed on edge and flat feelers. If warpage exceeds the acceptable level, the bearings and head will have to be replaced.
Valves
Attention! The exhaust valves of some engines are filled with sodium to increase their thermal conductivity. Sodium is a highly reactive metal that can cause a fire or explosion, especially when in contact with water or steam. Such valves cannot be disposed of as ordinary junk. For disposal of these valves, consult your local authorities or BMW dealer.
12. Inspect the valve heads for cracks, burnouts, wear, and pitting. Check valve stems for nicks and significant wear. Rotate the valve in the guide and see if the valve stem is bent. Inspect the ends of the rods for significant wear and wear. If defects of this kind are found, replace the valve.
13. If the inspection showed that the valve is in good condition, do not rush to rejoice. Measure the diameter of the valve stem in several places with a micrometer (pic. 7.13). A significant difference in diameter indicates unacceptable wear of the rods. In this case, the valve should also be replaced.
14. If the valves are in good condition, they should be lapped against the seats to create a tight, gas-tight joint. If the saddle is not badly pitted, or if it has been re-sharpened, lapping can be limited to a fine abrasive paste. Coarse paste should only be used if the saddle is burned or deeply pitted. In this case, it is recommended to show the cylinder head to a specialist who will decide whether it is possible to restore the seat by lapping, or whether it needs to be regrinded, or even replaced.
15. Lapping of valves is performed as follows. Lay the cylinder head on a workbench with the combustion chambers up.
16. Coat the seat surface with a suitable grit abrasive paste. Insert the valve and press it against the seat with the suction cup of the lapping tool. Hold the lapping rod between your palms and rotate the valve back and forth (pic. 7.16). Raise the valve from time to time and redistribute the paste evenly over the seat. The operation is greatly facilitated if a weak spring is placed under the valve disc.
17. When using coarse lapping paste, stop lapping when a matte band without breaks forms on the seat and on the valve disc. After that, wipe off the coarse paste from the surfaces and continue lapping with the fine paste. Continue lapping until both surfaces are dull grey. Do not lap the valves more than necessary.
18. After lapping all valves, wash off any remaining paste with kerosene or other suitable solvent.
Valve train parts
19. Inspect the valve springs for damage and overheating. The manufacturer does not specify the length of the springs in the free state, so they can only be compared with a new spring.
20. Place each spring on a smooth surface and check that it is perpendicular to that surface. If at least one of the springs is defective, if it is bent or has lost elasticity, replace the entire set of valve springs. It is recommended that all valve springs be replaced, regardless of condition, if the engine is being overhauled.
21. Replace all valve stem seals, regardless of their condition.
Pushers and hydraulic supports
22. Inspect the contact surfaces of these parts for significant wear and tear. If significant wear is evident, replace parts.