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Pistons
As noted in the cylinder head discussion, thc piston de-
sign has a great effect on the combustion process. PBS pis-
tons for the racing X1/9 engines are machined from Cos-
worth forged aluminum blanks. These blanks are of the
highest quality material available and are heat treated for
optimum hardness and stability. They can be fitted sig-
nificantly tighter in the cylinder bore than most forged
pistons without problems of galling or seizing. About .004
inch skirt clearance is adequate with these forgings com-
pared with roughly twice that much for most forged pis-
tons in the X1/9 size. The tighter clearance keeps the
piston straighter in the bore and significantly improves
ring sealing and hence engine performance. The stock
diameter piston pins are retained by snap rings. The pis-
ton crown design is discussed on page 27.
Connecting Rods
The 1500 X1/9 connecting rods are suitable for racing if
they are lightened, balanced and shotpeened. The 1300
stock rods are marginal for racing. Alternate forged steel
rods are recommended for the G-production engine. Do
not attempt t o lighten and polish the stock 1300 rods as
they will warp out of shape due t o relaxation of residual
stresses when the outer skin is broken.
Beurings
The stock Fiat main bearings and rod bearings are suitable
for racing use as long as the oil pressure is raised to about
1 0 psi per 1000 rpm and adequate oil control is provided
by a dry sump lubrication system.
ENGINE ASSEMBLY
Basic engine assembly is the same as with the stock engine
and the reader is referred to the Fiat shop manual for
general information on engine assembly. This section will
cover details which are different from the stock engine
and which are particularly critical for the racing engine.
The primary areas of concern involve clearances between
the pistons, the head and valves, and cam timing.
Checking Piston, Head and Valve Clearance
A trial assembly is recornmended t o checkpiston to head
clearance and valve to piston clearance.
Bore and hone the cylinders to get the desired piston
skirt clearance (.004 inch if Cosworth forged piston blanks
are used).
Assemble the rods. pins and pistons being certain that
the flat side of the piston pin retaining circlips face
outward.
Check piston ring end gaps in the bore before instal-
ling them on the pistons. End gap must be .0 12 inch mini-
mum. The rings should be off the pistons for the trial
assembly.
Assemble the pistons, rods, crank and bearings into
the block.
Verify that side clearance exists between the connect-
ing rod small ends and the piston pin bosses in the assem-
bled condition.
Install the head assembly using a used (compressed)
head gasket. Put modeling clay on the piston crowns.
Two narrow strips across the piston crown at right angles
will be sufficient t o monitor clearances. Torque the head
down.
Time the cam as described below. Rotate the crank
through top-dead-center on all cylinders t o compress the
clay between the head and the piston crowns.
Check the exhaust valve to piston clearance at about
1 0 '(crank) before top-dead-center (TDC) on valve over-
lap. This can be accomplished by monitoring tappet mo-
tion with a dial indicator while the valve is pried open with
a screwdriver until it touches the piston. This will be eas-
ier t o do if light springs are used on the valves during this
trial assembly phase. The exhaust valve t o piston clearance
should be a minimum of . I 0 0 inch when measured this
way.
Check the intake valve to piston clearance at about
10' (crank) after TDC on valve overlap. The clearance
should be .070 inch minimum.
Remove the cylinder head and check the thickness
of the compressed rriodelirig clay. The rriinimurri head-to-
piston clearance at the piston crown periphery should be
between .025 and .035 inch.
Checking Cam Timing
The cam timing must be checked and adjusted t o the op-
timum position as defined by the cam supplier or dyno
testing. If not otherwise specified. a symmetrical timing
pattern (split overlap) will generally give good overall
performance for racing. The cam can be advanced slightly
(up to 4O crank) t o enhance mid-range torque with some
loss in top end power. Or it can be retarded slightly to
give a small increase in top end power with a significant
loss in mid-range torque.
The easiest way t o check for split overlap on the cam
timing is to check the lift at TDC on both intake and ex-
haust lobes during valve overlap. First, set the valve lash to
the same arbitrary value on both the intake and exhaust
lobes on one cylinder such as number one. ,010 inch
clearance would be acceptable, for example.
Install a dial indicator so that it monitors the lift
of one of the tappets, for example the intake tappet.
The piston should be near TDC and the two valves on
overlap. Note that an accurate indicator for TDC will be
required. This should be carefully checked for accuracy
before the cylinder head is installed by monitoring pis-
ton travel with a dial indicator and verifying that the TDC
mark does accurately determine the point where top-
dead-center occurs.
To check intake lift at TDC, the crank must be ro-
tated CCW, from the front, until the indicator stops
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