21. Examination and renovation: pistons and piston rings
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.
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.
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.
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).
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.
Discard any gudgeon pin retaining circlips that
were disturbed on dismantling and obtain new ones for refitting; these
circlips should never be re-used.
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.
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.
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.
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.