There were two engine types in the Ford engine family known as
the 335 series,
the 351 Cleveland (351C) and the “M-block.” There were several
minor variants of
the 351C engine (including the basic 2V and several
high-performance 4V versions),
but there were only two M-block variants — the 351M and the
400. From 1971 to
1982, Ford manufactured millions of M-block engines for use in
mid-sized cars,
full-sized cars, luxury cars, and light-duty trucks.
The 400 featured a unique engine block design with the
big-block (429/460) bell
housing bolt pattern, and the same bore diameter (4.00") and
bore spacing (4.38")
as the 351C and the later Windsor-class small blocks (i.e.,
289, 302, and 351W).
Compared to the 351C block, the 400’s deck height is over 1"
taller (10.297"
vs. 9.206"), the 400 has larger crankshaft main bearing
journals (3.00"
vs. 2.75"), and the 400 uses longer connecting rods
(6.58" vs. 5.78").
The 400 achieves its additional 49 cubic inches of advertised
displacement with
a 4.00" stroke of the crankshaft (compared to the 3.50" stroke
of the 351C). In
fact, the 400 has the longest stroke of any Ford pushrod V8
engine!
Both engine types in the 335 series (351C and M-block) share
many design
characteristics, but the only major component that is
interchangeable between 351C
and M-block engines is the cylinder head. Several internal
components and accessories
interchange, including camshafts, timing sets, water pumps,
fuel pumps, oil pans,
distributors, and thermostat housings (see the
interchange
table
below for more details). All 335-series engines are externally
balanced with a 28 oz-in
imbalance on the crankshaft (the same as 302 and 351W engines
up to 1982).
Development of the 400
When it was introduced in the fall of 1970 (MY 1971), one year
after the introduction
of the 351C, the 400 was available in Ford’s Custom, Galaxie,
and LTD car lines, and
in the Mercury Monterey, Marquis, and Brougham car lines. Billed as
the 351C’s big
brother, the 400 was designed to provide brisk acceleration for
big, heavy,
full-sized cars.
Big torque at low engine speeds was a major priority of the
400’s design, and the
400’s long stroke helped to assure that performance in a
package that was both smaller
and lighter than its predecessor, the 385 series (429/460) big
block.
Unfortunately, the 400’s design had one major flaw that was not
resolved before
production began. When the 351C 2V cylinder heads were
installed on an engine with
15% more stroke (15% more displacement), the compression ratio
produced by flat-top
pistons similar to the 351C 2V pistons was 10.2:1! To get the
compression down to a
more reasonable level in the 400, Ford engineers developed a
piston with reduced
compression height and a small dish (8 cc), and a new
400-specific cylinder head
with a slightly larger combustion chamber (77.8 cc). With the
relatively high-octane
leaded fuel available in the early 1970s, this was an
acceptable compromise, and it
produced a compression ratio of 9.2:1. However, when unleaded
fuels were mandated a
few years later, the increased deck clearance of the 400 piston
would cause problems.
Beginning in MY 1973, Ford retarded camshaft timing by 6
degrees on all 335-series
engines (except the 351C 4V) to meet emission control
regulations. Camshaft timing was
retarded by changing the keyway offset from the alignment mark
on the crankshaft
timing sprocket.
Production of the 351C ended at the end of MY 1974. To keep up
with the market demand
for engines in the 350 cubic inch class (which apparently Ford
couldn’t do with its
production of the 351 Windsor alone), the 351M was developed.
The 351M was a simple
de-stroked adaptation of the 400, and it allowed Ford to offer
both 351 cid and 400 cid
engines with fewer different parts and more common packaging
than the 351C allowed.
Since the 351M and 400 use the same block, the engine mounts
and bell housing bolt
patterns are common to the two engines. The 351M was introduced
in passenger cars in
MY 1975.
The 351M uses the same block and heads as the 400, with a
shorter stroke crankshaft
(3.50"), the same connecting rods, and a unique piston with a
taller compression height
to compensate for shorter stroke with the same rods in the same
block. In fact, the
only functional difference between the 351M and 400 is the
crankshaft and pistons. Ford
did use slightly different components in the 351M and 400, such
as harmonic balancers,
camshafts, and valve springs, but functionally, the engines
were identical except for
the crankshaft and pistons. The 351M’s unique piston was a
dished type that produced
a compression ratio of 8.6:1 when it was introduced.
The 351 Cleveland built such a strong performance reputation
and market appeal that
Ford referred to the 351M as a “351 Cleveland” in their own
marketing literature for
at least the first couple years after the 351M was introduced.
Ultimately, this led
to some confusion, and even a mistaken belief among some pickup
owners that their
trucks had 351 Cleveland engines. In fact, the only US market
trucks ever equipped
with a 351 Cleveland engine by the factory were '70-'74
Rancheros, which shared the
Torino/Montego platform and powertrain options.
In MY 1975, the USEPA mandated catalytic converters and
unleaded fuel for all
passenger cars, and the 400’s main design flaw came back to
haunt it. From the beginning,
the 400 had an unusually large deck clearance (0.067"). With
the low octane of unleaded
gasoline, and no unleaded premium available for a few years,
Ford had to make drastic
changes to the 400. To avoid detonation problems, Ford
introduced a lower-compression
piston with a 15 cc dish, and specified static ignition timing
at just 4 degrees BTDC.
Advertised horsepower dropped from 170 in 1974 to 158 in 1975.
Even with retarded ignition timing and lower compression, the
400’s excessive deck
clearance was simply incompatible with low-octane unleaded
fuel, so the 400 acquired a
reputation in the mid-to-late-'70s for persistent pinging and
detonation problems. Ford
continued to tweak the 400 specs for cars, switching to even
lower compression pistons
with larger and larger dishes (up to 32 cc), until by 1978, the
400’s compression ratio
was down to only 7.33:1 in car applications.
Though not as bad as the 400, the 351M had its own detonation
problems, even with a
better deck clearance (0.020"). Attempting to solve the 351M
problems, Ford changed 351M
pistons every year from MY 1977 to MY 1979. In 1977, they
dropped the compression ratio
from the original 8.6:1 to 8.3:1, then in 1978, they dropped
the compression to 7.74:1.
In 1979, Ford went back to 8.3:1 pistons in the car 351Ms.
As Ford planned a MY 1977 makeover for their light-duty truck
line, they decided to
replace the aging FE 360-390 engines with the M-block 351M and
400 engines. Before MY
1977, the M-block had only been available in cars, and only
with an automatic
transmission, so Ford redesigned the M-block for its truck
debut.
Main bearing support webs were revised, particularly the #3
main (thrust) bearing support
web, which was strengthened to withstand the force imparted by a
clutch. This redesign
for truck applications was the only major change in the
M-block’s engine block design
throughout its production life.
Several unique parts were developed for truck M-blocks,
including flywheels for manual
transmissions, harmonic balancers, and truck-specific intake
and exhaust manifolds. The
original “non-retarded” crankshaft timing gear was also
re-introduced on truck engines.
Other than the strengthened D7TE truck block, the truck 351M
was basically the same as
the car engine, with truck-only 8.0:1 pistons and a slightly
different camshaft. The truck
400 also used a truck-only piston that produced 8.4:1
compression ratio, and a slightly
different camshaft with more lift at both intake and exhaust
valves. In MY 1978, car
M-blocks received the D7TE engine block enhancements in their
own D8AE engine block.
In the late 1970s, American manufacturers began using metric
designations for engine
displacement. The 351 cubic inch engines (both 351 Windsor and
351M) were referred to as
5.8 liters, and the 400 cubic inch engine was referred to as
6.6 liters. The W (Windsor)
and M suffixes were attached to the 351s’ metric designations
to distinguish the two types
(i.e., 5.8W and 5.8M).
Interchangeable components in the 335
series
Even though the M-block was relegated to smog oblivion by the
factory, and even though
the M-block has been largely ignored by the aftermarket
performance industry, the 351
Cleveland is reasonably well supported and the M-block has just
enough in common with
the 351C that it can share several key performance enhancing
components. Even though
you might have heard otherwise, there are several
high-performance options for Ford’s
M-block engines.
When looking for M-block performance components or (more
likely) performance components
that can be adapted to the M-block, you need to keep in mind
which of the 335-series
(351C/351M/400) engine components are interchangeable.
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