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

get into expensive trouble. For instance,
crankshaft throw, connecting-rod centerto-center
length, piston compression
height and deck-height clearance must be
compatible with the block's deck height.
For a given stroke crankshaft, a longer
rod will cause a given piston to operate
higher in its bore. Installing a shorter connecting
rod causes the same piston to
operate lower in the same bore with the
same crankshaft. This also applies to piston
compression height. As compression
height is increased-the wrist-pin bore is
moved down in relation to the piston topthe
top of the piston will operate higher
in the cylinder bore. Reducing compression
height moves it down. The last item,
deck-height clearance, is simply how far
short a piston at TDC is from coming
even with a block's deck or gasket surface.
This clearance is necessary to prevent a
piston from hitting the cylinder head.
The question now becomes, "How does
all this relate to swapping parts"
The sum of the crankshaft throw,
connecting-rod length, piston compression
height and deck-height clearance must
not exceed a block's deck height, otherwise
serious engine damage will result.
This is particularly true if it is exceeded
by more than the specified deck-height
clearance. In this case the piston will
come higher than the deck surface and
impact against the cylinder head. On the
other hand, if this sum is less, you'll have
a very underpowered engine because its
compression ratio will be low.
There are also other limitations to consider
when swapping bottom-end components.
A 302 crankshaft cannot be
installed in a 221 even though the numbers
add up right: the bottom of the 221
pistons will hit the connecting rods. 221
pistons are the only ones that can possibly
work because of the 221s bore size.
Oddly enough a similar situation exists
with the 289 even though the 302 and
289 bores and piston compression heights
are the same. 289 piston skirts will hit a
302s crankshaft counterbalances because
289 piston skirts are longer than a 302s.
Consequently, 302 rods and pistons must
be used when installing a 302 crank in a
289 block. This interchange of parts
makes a 302 out of a 289. Since 1978,
Ford service pistons for the 289-2V and
'68-'72 302s are interchangeable.
One final and vital factor you must
consider when changing bottom-end
components is balance. Pistons, connecting
rods, crankshaft and crankshaft
damper and flywheel are matched for
proper engine balance. Consequently, to
ensure you'll end up with a balanced
engine, particularly if the crankshaft and
rods are changed, the complete assembly
should be checked for balance. Many
engine machine shops are equipped to do
this.
CYLINDER HEADS
Cylinder-head interchangeability gets
very confusing, but not because the heads
are so different-except for the Boss 302
heads which are radically different from
all the others. Detail changes made to the
heads over the years prevent interchanging
heads in many instances. Another factor
contributing to the cylinder-head-complexity
problem is government emission
standards and the design changes which
have been made to meet these standards.
All small-block heads will bolt on all smallblock
engines, but this does not mean
they'll all work. Some will and some won't.
Cylinder-head Basics-To interchange
cylinder heads you'll have to understand
some cylinder-head basics. Let's start
with compression ratio and how it is figured.
As a cylinder's piston travels from
BDC to TDC it sweeps through or displaces
a volume called swept volume
(S.V.) With the piston at TDC, the volume
above it is called clearance volunze (C.V.).
Clearance volume includes the combustion-chamber
volume in the cylinder head,
volume created by the head gasket spacing
the head above the block deck surface,
additional volume created by piston deckheight
clearance and the shape of the piston
top. A concave or dished piston adds
to the clearance volume. A convex or popup
piston will reduce it. Of these four
factors, cylinder-head combustion-charnber
volume is the biggest contributor to
an engine's clearance volume.
A cylinder's compression ratio is directly
proportional to its clearance volume
and its swept volume or displacement:
Compression Ratio
- Swept Volume +
Clearance Volume
CRANKSHAF
Compression ratio is determined by its swept
volume and clearance volume. Swept volume is
the volume an engine's piston displaces in its
travel from TDC to BDC. It is also an engine's
displacement divided by its number of pistons.
Volume above a piston at TDC is clearanoe
volume.
The compression ratio formula says
that as an engine's displacement is increased,
so must its clearance volume increase
to maintain the same compression
ratio. This is a very important consideration
when interchanging cylinder heads,
particularly these days when gasoline
octane ratings have fallen like the value of
the dollar. If you end up with a compression
much over 9: 1 you're going to have a
difficult time finding fuel with a high
enough octane rating to avoid detonation.
If detonation is severe, engine damage will
surely result. To see exactly what I mean,
let's take a look at a couple of examples