BMW 1989-1995 525i Service Manual page 118

116-20
CYLINDER HEAD AND VALVETRA IN
O'JO t 1111
Fig . 41. Before cutting valve seats on M50 engines, measure valve
stem protrusion (A). Illen cut seats and make measurement
again. Install valves with thicker heads if difference exceeds
0.20 mrn (0.008 in.).
Fig. 40. Valve stem all seal being removed from valve gu ide using
special removal tool.
NOTE-
• On M50 engines. BMW recommends that valves with
thicker heads be installed if the vatve seats are ma-
chined more than 0.20
mm
(0.008 in.). If thicker
vatves are not installed. engine compression will be
adversely affected.
• To determine the amount ot material machined from
the seat. first insert
a
new valve into the valve guide
and measure the amount of valve stem extending be-
yond the top of the valve guide. See Fig. 4 1. Repeat
this measureme nt after cutting the seats. If the ditter-
ence in the measurements exceeds 0.20
mm
(0.008
in.}. valves with thicker heads should be installed.
Thicker valves are available from BMW in two sizes:
0.20 mm and 0.40 mm.
Replacing the valve seats requires heating the cylinder
head and chilling the valve seats to a specific tem perature as
listed in Table i. When installing new valve seats. they should
be installed flush to the cylinder head surface.
NOTE -
International Organization for Standardization (tSO)
tolerance classes are given in T able i for machine shop
reference. ISO allowances are based on nomina l shaft
and bore tolerances and should be used to determine
proper fits. Most qualified machine shops should have
this information available.
CYLINDER HEAD AND VA LVETRAIN RECONDITIONING
Table h lists valve seat dimensions. Tab le i lists valve seat
replacement specifications. including correct installation tem-
peratures.
Valve Springs
Valve springs should be checked for squareness and fa-
tigue. To quickly check the springs. line them up in a row.
Place a straight edge across the top of the springs. Any spring
that is significantly shorter than the others is significantly worn
and should be replaced . Use a high quality square to check
spring out of squareness. Any spring that is out of square
should be replaced .
NOTE-
Vatve spring specifications and wear limits are not
available from BMW.