There were two engine types in the Ford engine family
known as the 335 series, the 351 Cleveland (351C) and the ďM-block.Ē
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 ó 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 all later Windsor-class
small blocks (289, 302, and 351W).
Compared to the 351C block, the 400ís 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
Fordís Custom, Galaxie, and LTD lines, and in the Mercury Monterey,
Marquis, and Brougham lines. Billed as the 351Cís big brother, the 400
was designed to provide brisk acceleration for big, heavy, full-sized
Big torque at low engine speeds was a major priority of
the 400ís design, and the 400ís 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 400ís 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
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 couldnít 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 351Mís 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 ď351
ClevelandĒ 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
In MY 1975, the USEPA mandated catalytic converters and
unleaded fuel for all passenger cars, and the 400ís 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 400ís 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 400ís compression ratio was down to only 7.33:1 in car
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
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-blockís 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 ďnon-retardedĒ
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 351sí 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 Fordís 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.