Manual - Chapter 1 Engine

19. Examination and renovation: valves, valve seats and valve guides

  1. Obtain eight suitable containers and clearly mark each one with the type and cylinder number of the valve components that will be stored in it, eg Number 1 cylinder intake. Always keep components separate and in their marked containers so that all can be refitted in their original locations. As far as possible work on one valve assembly at a time so that there is no risk of swapping components.
  2. Use a rubber sucker or a strong magnet to remove the shim and cam follower over each valve, then compress the valve spring, using a specially-modified valve spring compressor (see accompanying photograph) which will fit securely over the spring top retaining collar and yet reach down inside the bores in the cylinder head without marking their walls. Withdraw the two split collets using a slim magnetised rod (draw a permanent magnet several times down the shank of a slim screwdriver). release the compressor gradually and withdraw the top collar, the spring (noting carefully which way round it was fitted), the spring seat, the valve and the valve guide oil seal. The magnet will also be required to remove the spring seats. Store all components from each valve assembly together in their marked container.
  3. Note: If any valve is difficult to remove through its guide check for any burrs or raised edges at the collet locating groove in the stem; use fine emery paper, if necessary, to polish away any that are found until the valve stem will slide easily through the guide. Be careful to wash away all particles of abrasive material.
  4. Inspect the valves for wear, overheating or burning, and replace them as necessary. Normally, the exhaust valves will need renewal more often than the inlet valves, as the latter run at relatively low temperatures. If any of the valve seating faces are badly pitted, do not attempt to cure this by grinding them, as this will invariably cause the valve seats to become pocketed. It is permissible to have the valve(s) refaced by a motorcycle specialist or small engineering works. The valve must be renewed if the head thickness (the area between the edge of the seating surface and the top of the head) is reduced to the service limit specified, if its overall length is shorter than that specified, or if its stem is bent by more than the maximum specified runout, measured at the valve head.
  5. Check the valve stems and guides for wear either by direct measurement or by inserting a valve and rocking it to and fro both along the direction of cam lobe thrust and at right angles to it; any wear will be most noticeable when the valve is at the maximum lift position. Compare the original valve with a new component to check if wear is excessive. If a small bore gauge and micrometer are available, the two components can be measured at three or four points along their bearing surfaces, both in the direction of cam lobe thrust and at right angles to it. Subtract the smallest stem diameter measurement obtained from the largest guide bore diameter; if the stem/guide clearance figure thus calculated exceeds the specified service limit, one or both components must be renewed. 
  6. Valve guide renewal is not easy, and will require that the valve seats be recut after the guide has been fitted and reamed. It is also remarkably easy to damage the cylinder head unless great care is taken during these operations. It may, therefore, be considered better to entrust these jobs to a competent engineering company or to an authorised BMW dealer. For the more skilled and better equipped owner, the procedure is as follows:
  7. Heat the cylinder head slowly and evenly, in an oven to prevent warpage, to 220 - 240 °C (428 - 464 °F). Using a stepped drift, tap the guide(s) lightly out of the head from the combustion chamber side, taking care not to burn yourself on the hot casting.
  8. With the guide(s) removed, allow the head to cool and measure the guide bore in the cylinder head. If worn beyond the specified limits, the bore must be reamed out to the next oversize and the correspondingly oversized guide fitted.
  9. To fit a valve guide, heat the cylinder head as described above and use the stepped drift to drive the guide into place from the camshaft side until it seats on the locating circlip.
  10. After a new guide has been fitted it must be reamed to 7.0 mm (0.28 in) using a BMW reamer, and the seat must be recut to centre it on the new guide.
  11. If a valve guide has been renewed, or if a valve seat face is worn or badly pitted, it must be recut to ensure efficient sealing. This process requires the use of the necessary cutters, with their adaptors, pilots and other equipment. It also requires some skill and experience if the cylinder head is not to be severely damaged. Accordingly, owners are strongly recommended to take the cylinder head to an authorised BMW dealer or similar expert for this sort of work to be done.
  12. For those who have access to the necessary equipment and the experience, proceed as follows. Fit the appropriate cutter to the pilot bar and insert the pilot bar into the guide until the cutter makes contact with the valve seat. Using firm hand pressure, rotate the cutter through one or two full turns to clean the seat then withdraw the cutter and examine the seat. If the seat is continuous and free from pitting, proceed to the next step, but if pitting is still evident, refit the cutter and repeat the procedure until all pitting has been removed.
  13. Be very careful to remove only the bare minimum of material necessary to achieve a clean surface. If valve seats become pocketed (sunk into the head) through excessive re-cutting, new valve seats must be fitted. While BMW supply separate replacement valve seats in standard size and one oversize, the removal and refitting of these seats is quite definitely a task for the expert alone who has the necessary skill, experience and equipment.
  14. With the seat face cleaned up check that it is 1.5 mm (0.06 in) wide at the contact area with the valve face. If it is too wide it must be narrowed, using the appropriate cutter to remove material either from the combustion chamber side or from the port side. Again, remove only the barest minimum of material.
  15. The valves should be ground in, using ordinary oil-bound grinding paste, to remove any light pitting or to finish off a newly cut seat. Note that it is not normally essential to resort to using the coarse grade of paste which is supplied in dual-grade containers. Valve grinding is a simple task. Commence by smearing a trace of fine valve grinding compound (carborundum paste) on the valve seat and apply a suction tool to the head of the valve. Oil the valve stem and insert the valve in the guide so that the two surfaces to be ground in make contact with one another. With a semi-rotary motion, grind in the valve head to the seat, using a backward and forward motion. Lift the valve occasionally so that the grinding compound is distributed evenly. Repeat the application until an unbroken ring of light grey matt finish is obtained on both valve and seat. This denotes the grinding operation is now complete. Before passing to the next valve, make sure that all traces of the valve grinding compound have been removed from both the valve and its seat and that none has entered the valve guide. If this precaution is not observed, rapid wear will take place due to the highly abrasive nature of the carborundum paste.
  16. If the necessary equipment is available, measure the diameter of each cam follower at several points, also the inside diameter of each respective cylinder head well. If necessary, renew any component that is found to be damaged, scored or excessively worn.
  17. Examine the spring retaining collars and split collets, renewing any that are marked, worn, or damaged in any way. Measure the free length of each valve spring. If any spring has settled to a length significantly less than the nominal length specified, or if anyone varies in length (check particularly the exhaust valve springs against the intakes) by a significant amount, it should be renewed. Note that while it is possible to buy the springs individually, it is considered good practice to renew them all as a set, and that many mechanics renew the springs as a matter of course to ensure good engine performance.
  18. Place the spring seats over the guides and press new oil seals into place on each guide upper end. Liberally oil the guide bore and valve stem before refitting the valves. The springs are fitted next; while new springs, being linear-wound, have no particular direction of installation, the original springs (if re-used) must be fitted as they were found. On the machine featured in the accompanying photographs the spring lower end was marked by a dab of blue paint; similar markings may be found on other machines.
  19. Refit the retaining collars, ensuring that the springs are correctly seated, compress the springs and refit the split collets. Give the end of each valve stem a light tap with a hammer to ensure that the collets have located correctly.
  20. Refit the shims to their follower recesses, ensuring that the marked surface is downwards and using a smear of grease to retain each shim as the cam follower is oiled and refitted. Be careful to keep the followers absolutely square in their housings; the slightest tilt will jam them and make removal very difficult.
  21. Refit the dowels (if used) to retain the followers.

19.2a Use rubber sucker or strong magnet to extract shim and cam follower - store carefully and do not interchange

19.2b Valve spring compressor must be modefied as shown to reach inside cam follower bores

19.18a Valve guide oil seals must be fitted to a suitable tool ...

19.18b ... so that they can be pressed correctly on to guide ends

19.18c Oil valve stems and guides liberally on reassembly

19.18d Do not forget to refit spring seat as shown ...

19.18e ... before insert spring. Re-used springs must be refitted as found

19.19a Fit spring retaining collar and compress spring ...

19.19b ... so that collets can be refitted to secure assembly

19.20 Be very careful to ensure that followers are square in the bores on refitting