Many myths surround America's fourth-largest automaker. The most persistent is that AMC simply bought parts from other manufacturers and built only the bodies themselves. This isn't entirely true. While AMC did source some parts externally, these represented less than 15% of any given vehicle in any year.
AMC mastered the art of using resources wisely. When purchasing a part from an outside contractor proved more cost-efficient than developing their own—and didn't compromise the unique character of their products—that's exactly what they did. In this approach, they were actually ahead of other manufacturers! Today, even the largest automakers like GM and Ford subcontract many parts and assemblies to other companies.
Like other manufacturers, AMC owned outright or held interests in many smaller companies that produced parts for them and other automakers. AMC purchased the following major components from other automotive companies:
Other parts such as belts, hoses, tires, and batteries have been purchased from outside sources by all manufacturers for many years. AMC always built their own engines with only a few exceptions, and always manufactured their own bodies.
AMC’s first V8 wasn’t an in-house design. For 1955 and some 1956 models, AMC used the Packard 320, available only with Packard’s Ultramatic automatic transmission. In the late 1940s, AMC and Packard had discussed a possible merger, and AMC leaders hoped a parts-sharing arrangement would deepen cooperation between the companies.
Packard president James Nance saw it differently. He viewed engine sales as a one-way favor and had little interest in buying AMC parts in return. Packard solicited bids largely for appearance, then rejected them as too expensive. The snub infuriated AMC president George Mason, who immediately directed his engineers to develop an AMC-built V8 as quickly as possible.
To meet this challenge, AMC hired former Kaiser Engineer Dave Potter, who had already worked on V8 designs at Kaiser. Remarkably, he delivered an engine ready for installation in less than 18 months.
Due to the tight timeline, the engine couldn't incorporate cutting-edge technology. Instead, it used proven design and build techniques. While on par with other V8s of the era, it couldn't match the then-new Chevrolet small block (also introduced in 1955). The engine was relatively bulky and heavy for its displacement, but exceptionally strong. Without time to test cast components, AMC used forged rods and crankshafts, creating a robust foundation that today's turbo racers have discovered can handle high boost applications with only custom forged pistons.
The main limitation was the cylinder heads. They used conventional vertical overhead valves, which restricted valve size due to space constraints and shrouding effects. While the block could support over 400 cubic inches, the heads would require redesign to accommodate larger valves. Modern turbo racers overcome this limitation with boost pressures up to 23 psi while keeping displacement near stock.
This early AMC V8 more closely resembled Chevrolet's 396 big block (noting that Chevy's small block 327 didn't arrive until 1962) and represents the closest thing to a "big block" AMC ever produced.
AMC engineers recognized that the heavy V8 couldn't compete with modern small blocks from the competition (Chevy in 1955, Chrysler in 1955, Ford in 1963), nor did it align with AMC's economy image—despite contributing significantly to Rambler's reputation for reliability and smoothness.
Development of a new V8 began shortly after the first entered full production. The redesigned engine, introduced in mid-1966, combined features of modern small blocks with elements common to big blocks of the era. It featured wide bore centers comparable to a big block, making it slightly longer but providing ample room for future growth and increased stability. Some publications call it a "mid block" for this reason. It incorporated a Buick-style oil pump integrated into the timing cover, with the distributor driven by a gear bolted to the front of the camshaft.
A 390 cubic inch performance model debuted for the 1968 AMX, using the same basic dimensions as the 290 and 343 but with thicker main bearing webs for added stiffness. AMC never produced a factory four-bolt main bearing engine, confident that two-bolt caps were adequate, but they cast the webs thick enough to accommodate aftermarket four-bolt main caps for racing.
To maintain high reliability with the longer stroke, all 390 and larger AMC engines used forged crankshafts and rods. According to AMC engineers, forgings were initially used due to inadequate testing time for cast parts, but AMC chose to retain them—either due to low production numbers or to preserve high reliability. As a result, an AMC 390/401 is substantially stronger than comparable small block 400 engines, requiring no aftermarket cranks or rods for racing—just careful preparation of stock parts.
In 1970, displacements increased by lengthening the block's deck height by 0.16" to accommodate a longer stroke. The 390's strong performance reputation prompted AMC to design a new rod to maintain a 390 option for 1970. The stroke changed only 0.11" for the largest engine, bringing displacement to 401 cubic inches for 1971. AMC felt more than 400 inches would be excessive. The block required notching at the bottom of the bores to clear crankshaft counterweights, and the higher deck height necessitated a slightly wider intake manifold.
The cylinder heads were also redesigned in 1970. Earlier 1966-1969 heads featured rectangular exhaust ports, while 1970 and later heads introduced a "dog leg" or "pork chop" shaped exhaust port. This larger port increased exhaust flow by approximately 50%, making AMC heads the best-flowing production heads available. The Chrysler "Magnum" V8 head was later based on this AMC design. The new ports also required new exhaust manifolds.
AMC V8 engines are classified as GEN-1, GEN-2, and GEN-3:
Although GEN-2 and GEN-3 share essentially the same block except for the 0.16" deck height increase, changes to heads, intake manifolds, and exhaust manifolds justify the separate designation. GEN-3 engines use 1/2" head bolts while GEN-2 uses 7/16" head bolts. Heads interchange between the two with proper attention to bolt size—step dowels allow GEN-3 heads on GEN-2 blocks, though for racing it's preferable to drill and tap the older block for 1/2" head bolts.
Technically, AMC didn't build separate small block and big block engines—they produced one V8 design (excluding the brief 1966 overlap). However, when compared to the competition, the GEN-1 can be considered a "big block" due to its external dimensions and weight, while the GEN-2 and GEN-3 qualify as small blocks. Some publications mistakenly call the 390 and 401 "big blocks" based solely on displacement.
Externally, all GEN-2 and GEN-3 engines are the same size except for height and width variations. All bolt patterns and external parts (except intake and exhaust manifolds) are identical. All internal parts interchange, though crankshaft and rod swaps may require custom pistons. GEN-1 parts are unique to that engine family.
All 1970 and later AMC heads use the same port design. All 360, 390, and 401 heads are identical, featuring 2.025" intake and 1.680" exhaust valves (early 1970 used 1.625" exhaust valves) with high-flow "dog leg" exhaust ports.
When building an AMC race engine, simply order pistons for the desired compression ratio rather than sourcing smaller chamber heads—it won't cost more (and may cost less) if pistons need replacing anyway.
The 1970-early 1971 304 heads use a different casting (3199517) but are essentially the same as 360/390/401 heads. Ports may be slightly smaller and castings a bit thinner, but according to AMC technical data, 2.02"/1.62" valves can be installed. The 304 head has a 52.20cc combustion chamber volume, producing a 9.0:1 compression ratio with stock pistons. Later 304 heads have a 58.92cc chamber and produce 8.4:1 compression.
The bore size of GEN-1 engines is cast into the right rear of the block just behind the cylinder head, in the space between the head and bell housing flange. This area is difficult to see with the engine installed due to the heater housing's proximity, but may be viewable with a small inspection mirror and might require cleaning with a wire brush.
The cubic inch displacement of all GEN-2 and GEN-3 engines is cast into each side of the block just behind the engine mount plates in the center of the engine. Exceptions include the 1970 thick-cast 360 (used in Donohue Javelins to homologate the thick-wall 360 block for Trans-Am racing) and some service replacement engines. Since the Engine Day Build Code or serial number is located on a removable tag, the casting is the only reliable method to identify engine size. This does not apply to the Packard V8 engine.
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Copyright © 2025, Alexander Ortiz. All Rights Reserved. | AMCHornet.com was established 2002 | Updated March 27, 2026