Yamaha TDM850 User Manual page 61

2»22 Engine, clutch and transmission
U --- "A"—*J
illustration). If it is thinner than specified, renew
the valve. The margin is the portion of the valve
head which is below the valve seat.
17 Check the end of each valve spring for wear
and pitting. Measure the spring free length and
compare it to that listed in the specifications (see
illustration). If any spring is shorter than
specified it has sagged and must be renewed.
Also place the spring upright on a flat surface
and check it for bend by placing a ruler against it
(see illustration). If the bend in any spring is
excessive, it must be renewed.
18 Check the spring retainers and collets for
obvious wear and cracks. Any questionable parts
should not be reused, as extensive damage will
occur in the event of failure during engine
operation.
19 If the inspection indicates that no overhaul
work is required, the valve components can be
reinstalled in the head.
12.17a Measure the free length of the
valve springs
12.22a Rotate the valve grinding tool back
and forth between the palms of your hands
B
^ '
12.16 Valve head measurement points
Reassembly
20 Unless a valve service has been performed,
before installing the valves in the head they
should be ground in (lapped) to ensure a positive
seal between the valves and seats. This
procedure requires coarse and fine valve
grinding compound and a valve grinding tool. If a
grinding tool is not available, a piece of rubber or
plastic hose can be slipped over the valve stem
(after the valve has been installed in the guide)
and used to turn the valve.
21 Apply a small amount of coarse grinding
compound to the valve face, then slip the valve
into the guide (see illustration). Note: Make
sure each valve is installed in its correct guide
and be careful not to get any grinding compound
on the valve stem.
22 Attach the grinding tool (or hose) to the valve
and rotate the tool between the palms of
12.17b Check the valve springs for
squareness
12.22b The valve face and seat should
show a uniform unbroken ring ...
A Head diameter
B Face width
C Seat width
D Margin thickness
your hands. Use a back-and-forth motion (as
though rubbing your hands together) rather than
a circular motion (ie so that the valve rotates
alternately clockwise and anticlockwise rather
than in one direction only) (see illustration). Lift
the valve off the seat and turn it at regular
intervals to distribute the grinding compound
properly. Continue the grinding procedure until
the valve face and seat contact area is of uniform
width and unbroken around
circumference of the valve face and seat (see
illustrations).
23 Carefully remove the valve from the guide
and wipe off all traces of grinding compound.
Use solvent to clean the valve and wipe the seat
area thoroughly with a solvent soaked cloth.
24 Repeat the procedure with fine valve
grinding compound, then repeat the entire
procedure for the remaining valves.
25 Lay the spring seat for each valve in place
12.21 Apply the lapping compound very
sparingly, in small dabs, to the valve face
only
12.22c ... and the seat (arrowed) should be
the specified width all the way round
Scaned by Stalker
the entire