BMW 1989-1995 525i Service Manual page 116

116-18
CYLINDER H EAD AND VALVETRAIN
Bl 1108
B338.ENGB
Fig. 38. Rocker arm being checked for wear on rocker arm shaft.
Valve Guides
BMW does not supply valve guides as replacement parts .
If
a valve guide is excessively worn. the valve guides should be
reamed to accept oversized valve stems. Valve guide specifi-
cations are listed in Table f.
Valve guides can be checked for wea r using a new valve as
shown in Fig. 39. Be sure to inspect the valve seats to ensure
that the cylinder head can be reconditioned before rework ing
the valve guides.
NOTE -
• International Organization for Standardization (fSO)
toterence classes are listed in Table f. ISO al/owances
are based on nominal sizes and should be used to de-
termine proper fits. Most machine shops should have
this information readily available.
• BMW supplies oversize valves. See Valves for addi-
tional information.
• Replacement valve guides may be available through
aftermarket suppliers. Valve guide replacement re-
quires special tools and a press. It is also necessary
to heat the cylinder head and chill the valve gu ides
when replacing the guides.
Valves
Valve specifications are listed below in Table g . Remove
carbon deposits from the valves using a wire brush or wire
wheel.
CYLINDER HEAD AND VALVETRAIN RECONDITIONING
Fig. 39. Valve guide wear being checked with dial indica tor. insert new
valve until stem end is flush with end of guide
(white
arrow).
See Table f for wear limits.
Testing Valves for Leakage
To test the valves for leakage. the camshaft and rocker arm
assembl ies (where applicable) should first be removed. Install
the valve assemblies and the spark plugs in each cylinder.
Place the cylinder head on a workbench with the combustion
chamber facing upward. Fill each combustion cham ber with a
thin non-flammable liquid. such as a parts cleaning fluid. After
fifteen minutes . check the level of the fluid. If the fluid level in
any cylinder drops. that cylinder is not sealing properly .
Valve Stem Oil Seals
The purpo se of the valve stem oil seal is to prevent excess
oil from entering the combustion chamber. The sign of faulty
valve stem oil seals is excessive oil consumption and smoke
from the exha ust immediately after starting and during decel-
eration
NOTE -
Valve stem oil seals should not be reused. If the valves
are removed. new valve stem oil seals should be in-
stal/ed.
Valve stem oil seal replacement requires that the cylinder
head be fully disasse mbled as described above under Cy lin-
der Head Disassembly and Assembly.