Sunbeam ALPINE I SERIES: APLINE II SERIES, Workshop Manual

Page: 77 / 413

WSM.114
Section B (Engine)
highest and lowest cylinder bore limits, in which
there are four grades is ·0016" (·040 mm.). By
means of the grading system the correct piston tit is
obtained when pistons are titted to new cylinder
bores having similar grade letters as the pistons.
The diameter difference between similar grade
letters for the cylinder block and piston is the
required clearance for the piston.
Example
(1494 c.c.
engine dimensions used)
Cylinder bore Piston
Piston
grade A grade A Clearance
3·1106" 3·1084" ·0022*
(79·009 mm.) - (78·953 mm.) = (·056 mm.)
3·1102*
- 3·1088* ·OOW
(78·999 mm.) - (78·963 mm.) = (·036 mm.)
Mean piston clearance is therefore ·0018* ( ·046 mm.)
Series IV Alpine and onwards, are fitted with pistons having
a bottom skirt clearance of ·0006
1
-
·0014. (·0IS mm - ·035mm)
which gives a mean clearance of ·001
#
(·025 mm). It is there­
fore necessary to fit these pistons by measurement of the
cylinder bore and piston as previously described. Should
this be impossible the feeler pull method can be used as given
in paragraph 4 provided it Is realised that this will give the
piston its maximum specified clearance of ·0014
1
(·0JS mm)
rather than the desired mean clearance of ·001 • (·02.S mm).
The cylinder grade letters are stamped on two
machined bosses at each end of the cylinder block
below the level of the cylinder head on the exhaust
manifold side, and visible with the cylinder head in
place. The piston grade Is stamped on the top face
of each piston.
FITTING PISTONS TO LOW MILEAGE
ENGINES or
THE APPLICATION OF PISTON
GRADING IN SERVICE
After a new engine has been run for a few hundred
miles the cylinder grade letter size no longer applies
because the running In process will result in a slight
initial increase in piston clearance.
If a piston, or pistons have to be changed under ser­
vice conditions the following procedure must be
followed.
I. Check the cylinder bore diameter with a
Mercer, or other dial type bore measuring
gauge, after setting its zero reading to .a suitable
size ring gauge. (see Section S)
3rd re-issue
Pa1e 39
2. From the size obtained subtract the correct
piston clearance previously given and from this
size choose a suitable grade of piston.
Example
(/494 c.c.
engine dimensions used)
Bore size given by clock gauge 3·1116*.
3·1116* - ·0018* = 3·1098'
Nearest piston size to 3 · 1098" ls D grade. Therefore
tit a D piston.
3. If the cylinder bore is slightly damaged by selzu re
or is slightly tapered it should be honed out to
take a larger grade, or ·005" (· 127 mm.) oversize
piston
4. In the absence of suitable cylinder bore measur­
ing equipment pistons may be titted by checking
their clearance with a feeler gauge and spring
balance as shown in Fig. 46.
To do this invert the piston and insert in the
bore with a ·0015* (·038 mm.) x ½" (12·7 mm.)
wide feeler in line with the thrust face of the
piston skirt as shown In Fig. 46.
A pull of 3-4 lbs. (1 ·36--1 ·81 kgs.) is needed to
withdraw the feeler with a correctly fitting'
piston, when the cylinder bore has been
wiped clean from an oiled condition. 1.f the
cylinder hasbeen reboredor honed,it should be
well oiled and wiped clean with clean lintless
cloth.
TO REBORE CYLINDER BLOCK AND
FIT OVERSIZE PISTONS
Oversize pistons are supplied for rebored cylinders,
and conform to grade B diameter plus the requisite
oversize.
When reborlng cylinders to suit oversize pistons It
is imperative that each bore is machined to the
actual diameter of the piston to be. titted, plus the
specified clearance in the bore.
For service use with rebored cylinders, oversize
pistons will not be supplied to any specific grade,
since grading Is Incidental to manufacture.
The recommended bore finish ls a "cross hatched"
hone finish of 20-40 micro inches. This is equivalent
to that obtained by thoroughly rubbing a used
cylinder bore with partly worn number one grade
emery cloth.
The cutting tool of the boringmachlne should be set