MG Midget J1 1933 Instruction Manual page 28

and removed by passing it through t h e valve cover saddles. Removal of th,e
camshaft enables all t h e valv,e rock'er-arms to b e swung clear of the valves.
A
small wood block slightly thickter than t h e depth of the combustion
spaces and an easy fit within them should now b e prepared. Slip this block
into the combustion space so that the valve heads are resting upon it, in
order that the valve springs can be compressed with a suitable tool such as a
screwdriver, without forcing the valves open. Depression of th,e springs will
expose two small conical cotters engaging in a groove in t h e pencil-like e n d
of the valve stem. Removal of the cotters
w i l l
release the valve springs cap
from t h e valv'e stem, permitting their removal and releasing the valve springs.
W h e n t h e springs of both valves have been remov'ed, the head may be raised
from the bench and the wood block withdrawn, thus allowing the valvres to
b e drawn from their guides. Repeat this operation o n th'e remaining valves
until they are all removed.
Grinding-in the Valves.
Examination of the valves
w i l l
show that the
edges of their mushroom-like heads are bevelled off at an angle to correspond
with t h e similar bevelled edges of the valve ports in the cylinder head and
thus provide a gastight joint when they a r e in contact. Obviously, gastight-
ness is not attained
if
these bevelled surfaces a r e dirty o r
"
pitted," and in
o r d e r to clean them u p so that they make perfect contact over the whole of
their surfaces it is necessary to grind them in. W h e n grinding-in the valves
t h e utmost care should be taken to see that they are inserted into the correct
port. Each valve is clearly numbered o n its head, and o n the lower faoe of
the cylinder head adjacent to the valve port will b e found its corresponding
number.
T h e grinding-in process consists in coating th'e bevelled face of the valve
with a small quantity of valve-grinding paste-applied o n the end of a match-
stick-reinserting the valve in its guide and partially rotating it backwards
and forwards o n its seating by means of a ~ c r e ~ d r i v e r . H e r e w8e come ro
the secret of good valve grinding. T h e valve should b e raised from its
seating every few reciprocations and given a half turn in o r d e r that the
grinding compound may spread itself evenly over t h e whole surface. If this
is not d o n e there is the possibility that minute circular grooves will be cut
into the face of both the valve and its seating, which will a b s ~ l u t ~ e l y prevent
o n e from obtaining a good gastight fit. Probably t h e most convenient way
of carrying o u t this periodical lifting is to obtain a light coil spring (similar
to the valve spring but much lighter), and insert it into the valve port beneath
the valve head. W h e n pressure is released o n the screwdriver the valve
will p o p up, wh'en it can easily b e rotated into a fresh position.
I t
is not n'ecessary to continue grinding the valves once the faces of both
valve and seating have assumed a clean, even, matt-surfaced appearance.
A polished surface must not be expect'ed and is quite unnecessary. If the
engine has bemen run f o r a long period without being decarbonised, th.e valve
may be badly
"
pitted "-that is to say, it will have a numbmer of small black
spots o r depressions o n its fac,e. Should these depressions b e at all excessive
o r d'eep, it is best to hav,e th'e valve face trued u p o n
3
special machine at a
garage. This will prev.ent needless grinding away of th,e valve seating in the
cylinder head-a matter of importance, as it cannot b'e r,enewed. Any valv'es
which are distorted should immediat,ely b e replaced by new ones. T o
attempt to grind thmem in will only produce ext,ensiv8e damage to the seating.
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