Manual - Chapter 1 Engine

21. Examination and renovation: pistons and piston rings

  1. Remove all traces of carbon from the piston crowns, using a blunt ended scraper to avoid scratching the surface. Finish off by polishing the crowns of each piston with metal polish. so that carbon will not adhere so rapidly in the future. Never use emery cloth on the soft aluminium.
  2. Piston wear usually occurs at the skirt or lower end of the piston and takes the form of vertical streaks or score marks on the thrust side of the piston. Damage of this nature will necessitate renewal and is checked by measuring the piston outside diameters at a point 12.0 mm (0.47 in) above the base of the skirt on KS pistons; on Mahle pistons the measuring points are 7.6 mm (0.30 in) on 75 models and 8.6 mm (0.34 in) on 100 models. In all cases the measurement is made at right angles to the gudgeon pin axis and the reading obtained is compared with that specified. Stamped marks in the piston crown will show the manufacturer and the size group code letter.
  3. Actual piston wear limits are not given, therefore unless any piston is found to be worn to significantly less than the specified tolerances, the only way of calculating wear is to subtract the piston diameter from its respective bore's measurement (see Section 20) to calculate the piston/cylinder clearance. If this exceeds the specified service limit either the pistons or the cylinder block. or both must be renewed; only careful measurement, preferably by an authorised BMW dealer or similar expert, will indicate which.
  4. Note that if any of the pistons is renewed at any time it must be of the same make and size code as the original. and must also be of the same weight group as the other pistons, ie all must carry the same weight group marking (+ or - stamped in the piston crown).
  5. Check the fit of the gudgeon pin in the piston bosses; it should be a tight sliding fit with no sign of free play when installed and no trace of scoring or wear on any bearing surface. If the necessary equipment is available, the components can be checked by direct measurement. The piston and gudgeon pin should be treated as a matched pair and never interchanged.
  6. Discard any gudgeon pin retaining circlips that were disturbed on dismantling and obtain new ones for refitting; these circlips should never be re-used.
  7. After the engine has covered a high mileage, it is possible that the ring grooves may have become enlarged. To check this. measure the clearance between the ring and groove with a feeler gauge. A clearance in excess of the limits given will mean that the piston or rings must be renewed.
  8. To measure the end gap. insert each piston ring into its cylinder bore, using the crown of the bare piston to locate it about 1 inch from the cylinder head end of the bore. Make sure it is square in the bore and insert a feeler gauge in the end gap of the ring. If the end gap exceeds the limits given, the ring must be renewed.
  9. When fitting new piston rings, it is also necessary to check the end gap. If there is insufficient clearance, the rings will break up in the bore whilst the engine is running and cause extensive damage. The ring gap may be increased by filing the ends of the rings with a fine file.
  10. The ring should be supported on the end as much as possible to avoid breakage when filing, and should be filed square with the end. Remove only a small amount of metal at a time and keep rechecking the clearance in the bore.

21.2 Measuring piston outside diameter - see text for measuring points

21.8 Measuring piston ring installed end gap