AMC small block V8 was first introduced in 1966. All
six engine sizes (290, 304, 343, 360, 390, 401) share the
same basic block design. The different displacements
are achieved by different bore and stroke combinations.
All blocks share the same external measurements and thus can
be swapped easily. Contrary to a popular myth the AMC
V8's were not made by Ford or anyone else. They did
make use of some Motorola electrical parts (Alternator, Starter
etc) and carburators, but the engine itself is all AMC.
(290, 343, 390)
AMC small block was first introduced as a 290 in 1966.
The 343 came out in 1967 Ambassadors and Marlins. The
390 arrived in 1968 and provided major kick for the new AMX.
These engine blocks were unchanged until the end of 69.
In addition to the largest bore and stroke, the 390 motor
also got heavier main bearing support webbing and a forged
steel crankshaft and connecting rods. The head used
during this time, are the rectangle port heads, named after
their exhaust port shape. The 290 heads use smaller
valves (1.787 intake, 1.406 exhaust) in order to prevent problems
with the small bore. The 343 and 390 used the same,
larger valve head (2.025 intake, 1.625 exhaust). You
can swap the large valve heads onto a 290 but will need to
notch the top of the cylinder bore to prevent the valves hitting
and will probably need different pistons as well. It
probably isn't worth it though, the 290 works better with
the smaller valves. The large valves are so close to
the cylinder wall (on a 290) that they are shrouded by it
and will actually flow less than the smaller valves.
and Later (304, 360, 390, 401)
1970 all three blocks grew in deck height and the strokes
were increased on the 290 and 343. The 290 became the
304, and the 343 became the 360. For some reason, however,
the 390 remained a 390 in 1970. It wasn't until 71
that the 390 was stroked to the 401. Like the 390, the
401's crankshaft and connecting rods are forged steel.
The other change in 70 was the switch to the dog leg heads
(again named after the exhaust port shape). These heads
are reported to flow 50% better on the exhaust side than the
rectangle port heads and are thus the best for performance.
There are two reasons for the flow increase. Firstly
the area of the port is larger, due to the dog leg.
Secondly the shape of the port floor was changed from a concave
to a convex curve. The concave floor tended to bend
the exhaust flow upwards which caused turbulence when the
flow was forced to go down into the exhaust manifolds.
By switching to a concave floor the curvature of the flow
starts in the head and proceeds much more smoothly into the
exhaust manifold resulting in less turbulence and better flow.
As before there were two versions, a small valve one for the
304 and a large valve one for the 360/390/401. Again,
like the 290, unless you have an extremely radical 304, you're
probably best to stick with the small valve head on the 304.
The late model 304 head is a good option for the early 290's
but there are a few other things to consider, which are discussed
later. The 304 also went into Jeep CJ5 and CJ7s. The
360 2bbl was the common V8 for the Jeep SJs (Wagoneer, Cherokee,
J series trucks) from 1971 through 1991. The 360 4bbl
and 401 4bbl were also options from 1974 through 1979.
the lives of the heads there have been some changes made,
mostly exact valve sizes, and chamber volumes. In general
the 70 and earlier heads have about 51cc chambers and run
around 10.2:1 compression. Some of the early (pre 68)
heads had 53-54 cc chambers for 9:1, the later heads (71 and
up) had about 58cc, which gave 8.0-8.5:1 ratio's when combined
with the dished pistons used at the time. Using these
late heads with the earlier flat top pistons will result in
a compression ratio of about 9.5:1. In addition to chamber
and compression ratio's some small changes in valve size were
made. For example the exhaust went from 1.625 to 1.68
in the early 70's. These changes in valve size are pretty
subtle and most drivers would never notice them. In
any event you can get your heads machined to take the larger
valves if you want, there's plenty of room to do it.
You can even put the 343/360/390/401 valves in the 290/304
heads, although this is a bit risky as the castings are a
bit thinner on the small valve heads and you run the risk
of cracking a head if you open it up too much.
minor change was made to the rocker arm assembly in 1973.
Earlier heads used a round rocker arm pivot. This allowed
the rocker some freedom to rotate. The rotation was
limited by the width of the push rod slot. Later rocker
arms used a cylindrical pivot. This pivot has a bridge
which joins the intake and exhaust together maintaining proper
can swap heads around pretty easily between these different
engines. The 343 heads swap with the 68/69 390 heads,
the 70-390, 360, and 401 heads swap. If you want to
swap across the 1970 deck height change, you have to watch
two things. The early block used 7/16 head bolts where
the later used 1/2, and the center intake manifold bolts were
re-located. To swap the late heads to a early engine
you need a step dowel to ensure the heads are centered properly
and you need to file the center manifold bolt holes to re-align
the holes. Note that you can't escape the filing by
swapping the late manifold onto the early engine because of
the deck height difference. To swap a late model intake
onto an early block you need to have the gasket face of the
manifold machined down to compensate for the different deck