How To Rebuild Your Small-Block Ford.pdf - Index of

A leak-down tester is the most-accurate device for measuring a cylinder's sealing capability because
it eliminates factors which can affect compression-gauge readings. Because leak-down testing is done
without cranking the engine, cranking speed, valve timing and cam condition do not affect the readings.
Photo courtesy Sun Corporation.
testing is done by most tune-up shops.
A leak-down tester is an integral part of
Sun Electric's electronic analyzer. Sun
recommends that an engine which leaks
20% of its test pressure needs attention,
whereas a good cylinder will have 5 to
10% leakage. So, if leak-down testing
your engine finds a problem cylinder,
you'll know the problem will be rings,
valves or a head gasket and possibly a
piston or a crack in the cylinder head or
the cylinder wall.
Head Gasket or Valves-Two candidates
may be causing the problem at this pointa
leaky head gasket or a valve. If two cylinders
whlch are down on pressure and
they are adjacent to one another, chances
are good the problem is a blown head
gasket between the two cylinders. It could
also be a head gasket when only one cylinder
is affected. If it is a gasket, there will
have been an unusual amount of,coolant
loss from your radiator if cylinder pressure
is leaking into your cooling system. This
situation is easy to diagnose. Remove your
radiator cap and look at the coolant
surface when your engine is running and
warmed up. If cylinder pressure is escaping
into it, you'll see bubbles. Before
making this check, make sure your coolant
level is up to the mark. If you see bubbles,
smell the coolant. If they are caused by a
blown gasket, you should be able to detect
gasoline or exhaust fumes as the bubbles
surface and burst. A sure way to test for
this is to take your car to a professional
who has a device which "sniffs" the coolant.
It indicates whether or not the bubbles
are caused by escaping cylinder pressures
and are not just recirculating air
bubbles. If a bad gasket is found, you'll
have to remove a head to replace it. Use
the procedures outlined in the engine teardown
and assembly chapters for this job.
Check the head and block surfaces for
flatness. Fix any problems or you may
have to repeat the job.
Now for the valves. If you didn't find
your compression-loss problem with the
piston rings or a head gasket, the last
probable customer will be a valvels. There
are numerous reasons for valves leaking,
but the cause is always improper seating.
A valve may not be fully closing or it may
be burned. Both result in an unsealed
combustion chamber. If a fully closed
valve leaks it's probably burned, so check
for full closure fi;st. You'll need vernier
calipers or a dial indicatior.
Pull off the valve covers and locate the
cylinder you want to check. It should be
on TDC (top dead center) of its power
stroke-not between its exhaust and intake
stroke. This ensures both valves
should be fully closed. To do this, trace
the spark-plug wires of the cylinders you
are going to check to the distributor. Put
a mark/s on the distributor body in line
with the lead on the distributor cap. Remove
the cap.
When you crank your engine over and
line up the distributor rotor with the mark
on the distributor you'll know the cylinder
will be reasonably near TDC on the
power stroke.
If both valves are fully closed, you
should be able to rotate the pushrods
with your fingers. It's very unlikely the
cam is holding a valve open unless the
valves have been recently misadjusted. I
know of no instances where the valve adjustment
of a small-block Ford has decreased.
If adjustment changes, it gets
looser. If the pushrod rotates, the rocker
arm has unloaded the valve so it is free to
close. If it won't rotate, back off that
valve's adjustment according to the procedure
in the engine assembly chapter.
One word of caution, if your valve
train is the same as what was installed at
the factory it will have hydraulic lifters,
unless you have a HP289 or Boss 302.
Hydraulic lifters slightly load the pushrods,
making them a little hard, but not
impossible to turn. Therefore, don't let
this fool you into thinking the valve in
question is open. If you have the HP289,
Boss 302, or your cam has been changed
to a mechanical type, the pushrods will
be loose. That's why mechanical cams are
noisy. They are loose because the required
clearance to ensure closing is about 0.020
inch when hot, as opposed to the zero
lash of hydraulic lifters.
CHECKING VALVE LIFT
If all the valves appear to be closing as
just described, check to see if the pushrods
feel too loose. If one is, it could mean
that a valve is sticking in its guide, preventing
it from fully closing. To confirm
this, you'll have to use your dial indicator
to get an accurate reading on actual valve
lift. The depth-gauge end of a vernier
caliper will work if you don't have a dial
indicator, but it's not as easy to use. If
you don't have either one of these you
can use a 6-inch rule with a slide for measuring
depth. It's not accurate, nor is it
very easy to use but you've got to use
what you have.
Bump the engine over until the valve
you're checking is fully opened, compressing
the valve spring. Make sure the
lifters are fully primed by cranking the
engine so you're getting full valve lift. If
you have a dial indicator, set the indicator
plunger against the top of the spring retainer
and in line with the valve stem so
you get a true reading, and zero the indicator.
If you're using a vernier caliper or a
scale, measure from the spring-pad surface-where
the spring sits on the cylinder
head-to the top of the spring retainer