If the engine has experienced severe overheating, then the cylinder head is probably bent (see point 4).
Cleaning
1. Remove with a scraper from the condensed surfaces of a head of cylinders, the inlet pipeline and a final collector all remnants of an old laying and a sealing compound. Be especially careful not to damage the cylinder head. Special solvents for removing gaskets are available from auto parts stores.
2. Remove all deposits from the cooling channels.
3. Use a stiff brush to clean the various openings to remove deposits that may have accumulated in them.
4. To remove corrosion and thread sealant that may be present in the threaded holes, clean them with taps of the appropriate size. If compressed air is available, use compressed air to clear the holes of debris after this operation.
Wear safety goggles when using compressed air.
5. Clean the cylinder head with solvent and dry thoroughly. Compressed air will speed up the drying process and keep holes and recesses clean.
There are chemical decarbonizers available and these can be very helpful in cleaning cylinder heads and valve assembly components. However, these agents are very aggressive and should be used with caution. Follow the instructions on their packages.
6. Solvent-clean the rocker/rocker/pushrod shafts, springs, valve springs, retainers and crackers, then dry thoroughly. To avoid mixing up valve components, clean the valves one by one.
Do not clean the hydraulic pushers of the M40 engine; leave them completely immersed in oil.
7. Remove all deposits from the valves with a scraper, and then use an electric brush to remove carbon deposits from the heads and valve stems. Again, make sure that the valve components are not mixed up.
Status check
Before deciding whether to work in a machine shop, ensure that all of the indicated condition checks have been completed. Make a list of checks.
Cylinder head
1. Carefully inspect the head for cracks, coolant leaks, and other damage. If cracks are found, consult a mechanical workshop. If repair is not possible, a new cylinder head should be purchased.
2. Using a flatness gauge and feeler gauges, check for curvature of the gasketed head surface. If the curvature exceeds the limit given in Table of sizes and adjustment data at the end of the Manual, it can be ground in a mechanical workshop, provided that the thickness of the head does not turn out to be less than allowed.
3. Inspect the valve seats of each combustion chamber. If they have significant dents, cracks or burnouts, then the head will require service beyond the capabilities of a home workshop.
4. Check the clearance between the valve stem and guide by measuring the lateral deflection of the stem with a dial indicator. The valve must be in the guide and about 2 mm above the seat. The total valve stem deflection measured by the indicator should be divided by two to obtain the actual clearance value. If there is any doubt about the condition of the valve guides after this, they should be checked by a mechanical workshop (the price of this service is minimal).
Valves
1. Carefully inspect the surface of each valve for uneven wear, deformation, cracks, dents, and burn areas. Check the valve stem for wear and check the stem neck for cracks. Rotate the valve and check for any noticeable distortion. Check for dents and excessive wear at the end of the rod. The presence of any of these defects indicates the need for valve service as described in Section Valve maintenance.
1 - Valve end
2 - Groove for crackers
3 - Rod (most worn part)
4 - Rod (most worn part)
5 - Valve chamfer
6 - Rise
2. Measure the elevation of each valve. Valve with less than required elevation must be replaced
Valve components
1. Check for wear at the ends of all springs. Before deciding to use springs in a rebuilt engine, it is necessary to check their stiffness using a special tool (for this check, send the springs to a mechanical workshop).
2. Place each spring on a level surface and check that they are vertical. If any of the springs are bent or warped, or possibly have a broken coil, replace the springs.
3. Check up holders of springs and crackers on presence of obvious traces of deterioration and cracks. Any suspicious parts should be replaced, otherwise, if they fail while the engine is running, significant damage will occur.
Rocker arms (engines M10, M20 and M30)
Exhaust rocker arms are the most susceptible to wear and should be checked with particular care.
1. Check the rocker arms for excessive wear on the ends that contact the valve stem and camshaft.
2. Check radial clearance (see Section Disassembly of the cylinder head). If it is too large, then either the rocker arm bushing or the shaft (or both) worn out. To determine which element is more worn, move the rocker a certain distance along the shaft and measure the radial clearance again. If the clearance is now correct, then the shaft is probably the most worn component. If the gap does not meet the requirements, then the rocker arm bushings must be replaced.
Rocker shafts (engines M10, M20 and M30)
Check the shafts for scoring, excessive wear and other damage. The contact surfaces of the rocker arms with the shafts must be smooth. If there is a ledge at the contact boundary of the rocker arm with the shaft, then the shaft is probably worn out.
Hydraulic pushers (M40 engine)
1. Check the tappets for wear, nicks, and dents where they come into contact with the ends of the valve stems and pins. If there is significant wear on the tappets and camshaft, then a new camshaft must be purchased along with the tappets.
2. Similarly check up hydropushers where they contact wells in a head of cylinders, on presence of wear, scuffs and dents. Sometimes hydraulic pushers create noise during operation and require replacement, and this becomes noticeable when the engine is running. It is quite difficult to detect internal damage or wear to the hydraulic tappet when it is removed; in case of doubt, it is necessary to install a set of new hydraulic pushers.
Camshaft
1. Check the camshaft journals (round sliding surfaces) and cams for nicks, dents, chips, and excessive wear. Measure the height of each intake and exhaust cam with a micrometer. Compare the height of all intake and exhaust cams. If all intake or exhaust cams differ by more than 0.08 mm, or if any signs of wear are found on the camshaft, replace the camshaft.
2. Check up a condition of surfaces of sliding bearings in a head of cylinders for presence of scuffs and other damages. If the bearing surfaces are scored or damaged, then the cylinder head is usually replaced, since the bearings are the machined surfaces in the cylinder head.
mechanical workshop (especially specialized in BMW) or the dealership may offer an alternative to replacing the cylinder head if the only problem is minor scratches on the camshaft bearing surfaces.
3. Measure with a micrometer of a neck of a camshaft and write down results of measurements.
4. Using a telescopic gauge or micrometer - inside gauge, measure the diameters of the camshaft bearings in the cylinder head (on the M40 engine, pre-install the bearing caps). To obtain the lubrication gap from the size of the camshaft journal, subtract the inner diameter of the corresponding bearing. Compare the lubrication gap with the value given in Table of sizes and adjustment data at the end of the Guide. If it does not match, a new cylinder head and/or a new camshaft will be required.
Before installing a new cylinder head, check the old one in a mechanical workshop (especially in the BMW-specialized). Maybe they can restore it.