Piston And Rings; Big-End Bearing; Small-End Bearing; Valves, Springs And Guides - Royal Enfield CO Workshop Maintenance Manual

ROYAL ENFIELD 350cc O.H.V. WORKSHOP MANUAL

5. Piston and Rings.

If the piston rings are in good condition they can
be replaced, taking care to fit them in their original
grooves and the same way up. If the rings show brown
or black patches on their working faces or if their gaps
when in position in the barrel are more than 1/16 in.
new rings should be fitted. The correct gap for new
rings is .011 - .015 in. The gap should be measured in
the least worn part of the cylinder which will be found
to be at the extreme top or bottom of the bore.
The original size of the cylinder bore is 2.751 in.
If the wear at any point in the bore exceeds .008 -
.010 in. the cylinder should be rebored and an
oversize piston fitted. Pistons are available in various
oversizes. The original diameter of the piston skirt,
measured fore and aft, is 2.748 in. at the bottom and
2.745 in. at the top. Diameters parallel to the gudgeon
pin are .008 in. less. The original side clearance
between the piston rings and the grooves is .003 in. If
the piston skirt or the grooves show .005 in. wear the
piston should be replaced.
6. Big End Bearing.
While the piston is removed examine the
condition of the bigend. This should have about .010 -
.020 in. side play and it will be possible to rock the
connecting rod slightly. The big-end is a plain bearing
and has an original clearance of approximately .003
in. which is rather more than is usual with a roller
bearing. If, however, definite up and down play can
be felt the engine should be stripped further to have
the big-end renewed.
7. Small End Bearing.
The gudgeon pin should be a push fit in the piston
(when cold) and a free working fit (.001 in. clearance)
in the small end bush. The small end bush can be
INSERTING NEW BUSH
TOOL FOR REMOVING OLD AND INSERTING NEW
BUSH IN SMALL END OF CONNECTING ROD
Page 8
Fig. 4
renewed if worn, using a draw bolt as shown in Fig. 4,
both for withdrawing the old bush and fitting the new
one. After fitting, the bush must be reamered, the size
to suit a new gudgeon pin being .7507 - .7505 in.

8. Valves, Springs and Guides.

Wear on the valve stems can be seen on
examination and if a definite step has formed the
valves should be renewed. Test the valve guides for
wear by trying the fit of a new valve in them. Both
valves should be quite free, but the exhaust valve has
.002 in. more clearance than the inlet valve. The
guides are removed by knocking or pressing them out
from inside the head using a drift 9/16 in. maximum
diameter with one end reduced to 21/64 in. diameter,
supporting the head on a tube 13/16 in. inside
diameter 2 ins. long slipped over the collar on the
guide. The same drift can be used for fitting the new
guide. Check the length of the valve springs which are
originally 1.3/4 ins. for both outer and inner springs.
If these have closed more than 1/8 in. they should be
renewed.
Before replacing the valves they must be ground
on to their seats. If good faces are not formed with a
reasonable amount of grinding the seats must be cut
with a cutter (included angle 90°) and the valve
refaced in a Universal Grinder, or if this is not
available by spinning in a chuck and holding a strip of
emery cloth on the back of a file at 45° to the valve
stem. Do not attempt to form good seats by an
excessive amount of grinding. This will cause
pocketing which restricts the flow of the gases (see
Fig. 5). If a pocket has already been formed this must
be removed by cutting with a valve seat cutter larger
in diameter than the valve head. Do not interchange
CORRECT AND INCORRECT
CUTTING OF VALVE SEATS
Fig. 5