Royal Enfield 500 TWIN 1949 Workshop Maintenance Manual page 17

ROYAL ENFIELD WORKSHOP MANUAL
This should be done with the pistons at bottom
dead centre.
When replacing the cylinder, clean off the
joint faces and fit new paper joints, two to each
cylinder.
In the case of the "500 Twin" engine, see that
there are the correct holes in the paper joint for
the internal oil passages.
16. Removal of Pistons
Remove the cylinder heads and cylinders.
With the tang of a file remove one of the wire
circlips retaining the gudgeon pins. If necessary
rotate the engine slightly until the pistons are in
such a position that the gudgeon pins will clear
the long cylinder studs when being withdrawn.
Use Special Tool No. E5477 to extract the
gudgeon pin or using a rod about 1/4 in. in
diameter, insert this right through one gudgeon
pin and drive the other pin out of its piston,
supporting the connecting rod substantially
meanwhile to prevent distortion.
Having lifted the first piston away, the other
one may be readily removed in the same
manner. Mark the pistons and gudgeon pins so
that they go back into the same pistons the same
way round and so that the pistons go back into
the same barrels the same way round.
Take care not to drop the gudgeon pin circlip
into the crankcase. A clean cloth should be put
over the mouth of the crankcase to prevent this.
17. Decarbonising
Having removed the cylinder heads as
described in Subsection 10, scrape away all
carbon, bearing in mind that you are dealing
with aluminium which is easily damaged.
Scrape gently and avoid scoring the combustion
chamber or the valve seats which are of
austenitic iron shrunk into the head. Be careful
while performing this work not to injure the
joint face which beds down on to the head
gaskets.
Do not, in any circumstances, use caustic
soda or potash for the removal of carbon from
aluminium alloy.
Scrape away all carbon from the valve heads
and beneath the heads, being very careful not to
cause any damage to the valve faces.
If the piston rings are removed the grooves
should be cleaned out and new rings fitted. For
cleaning the grooves, a piece of discarded ring
thrust into a wooden handle and filed to a chisel
point is a useful tool.
If the piston ring gaps exceed 1/16 in. when the
rings are in position in the barrel, 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 the extreme top or bottom of the
bore.
Section C1 Page 6
While the cylinders and pistons are not in
position on the engine, cover the crankcase with
a clean cloth to prevent the ingress of dust and
dirt of all kinds. Do not, of course, attempt to
scrape the carbon from the pistons when the
mouths of the crankcase are open.
18. Grinding-in Valves
Wipe the valve faces clean and examine them
carefully. If they are at all pitted, have the faces
re-cut. Pay similar attention to the valve seats in
the head; excessive grinding will form a pocket
and the gas flow will be restricted. The angle of
the valve face should be 45 degrees to the axis of
the valve stem.
To grind a valve, smear the seating with a
little grinding in compound, place a light, short
coil spring over the valve stem and beneath the
head, insert the valve into its appropriate guide,
press it on to the seat using a tool with a suction
cup and with a backwards and forwards rotary
motion, grind it on to its seat. Frequently lift the
valve and move it round so that an even and true
seating is obtained. Continue grinding until a
bright ring is visible on both valve and seating.
19. Re-assembly after Decarbonising
Before building up the engine, see that all
parts are scrupulously clean and place them
conveniently to hand on a clean sheet of brown
paper.
It is advisable to fit new gaskets to the
cylinder base and cylinder head. Two paper
gaskets are fitted to the base of each cylinder; in
the case of the 500c.c.engine, make sure that
each has two holes which register with the oil
passages.
Smear clean oil over the pistons and space
the ring gaps, having replaced the rings if these
have been removed, lower the piston over the
connecting rod and insert the gudgeon pin from
the outer side. Fit the circlip and then fit the
second piston in a similar manner.
Oil the cylinder bores and lower the barrels
over the pistons and seat them gently on their
gaskets.
Drop the push rods down their tunnels on to
the tappet heads, shallow cups upwards. (Or
wide cups upwards in the case of steel rods.)
Fit the copper cylinder head gaskets and
replace the cylinder heads as described in
Subsection 10.
After the engine has been assembled, run it
for a brief period with the ignition fully
advanced.
When the engine has been run for some time
and has become thoroughly hot, go over all the
cylinder head and other nuts to ensure that they
are tight.