Part 1: History of the Aluminum Slant Six

At the dawn of the musclecar era, Chrysler engineers were pulling their hair out, replacing hefty iron parts with the svelte aluminum alternatives we've all come to know and love. From alternator housings to automatic transmission cases and fenders to Slant Six blocks, Chrysler led the industry in its quest for increased efficiency through reduced mass. Woah! Did we say aluminum Slant-Six blocks? Few people today realize that nearly 50,000 aluminum-block Slant Sixes were installed in the '61 and '62 model year passenger cars. Today so few survive that most Mopar enthusiasts have never seen one. But that hasn't stopped the rumors from flying, and today they're shrouded in a veil of mystery, misinformation, and confusion.

In this first installment of the Buzzing Half Dozen, we'll clear the smoke and relate the story of how these lightweight aluminum wonders came to be. In our next installment, we'll hammer one together that makes better than one horsepower per cubic inch on 91-octane pump gas. Then we'll stick it in a 3,000-pound A-Body and show you how easy it is to run low fourteens at over 90 mph with any Slant Six, cast iron, or aluminum.

In early 1957, Chrysler launched a program to develop its first compact car to compete with inexpensive foreign invaders like the Volkswagen Beetle and low-line domestic products from Rambler and Studebaker. Word was out that General Motors and Ford were also developing their own compact cars, and the '60 model year was the target release date for what would become the Corvair and Falcon. This gave Chrysler three years to come up with a winner. To have any advantage in the marketplace, Chrysler needed its new small car--code named the A-901--known today as the Valiant, to seat six, carry plenty of luggage, offer exceptional fuel economy, and deliver acceptable performance without becoming too expensive for its penny-pinching target customers. To get these results, a group of approximately 100 designers, draftsmen, engineers, and mechanics were completely segregated from the regular design and engineering staff and put to work in a separate building. Chrysler was banking heavily on its new contender and didn't want its details to leak out prematurely. Under the leadership of Robert M. Sinclair, the Valiant project was started in a brick building located at 403 Midland Avenue in Detroit. The security at the Midland Avenue annex was so tight that many outsiders assumed it housed top-secret military contract work. Many called it the "Mystery of Midland Avenue."

While the A-Body platform was evolving in great secrecy, the engine-development department at Central Engineering faced the reality that Chrysler's 230-cubic-inch flathead six was a heavy, antiquated dinosaur. Designed in 1932, it couldn't deliver the fuel efficiency and power needed in an advanced engineering statement like the Valiant. A deadline of May 1, 1958, was given to the engineers to select a final design configuration for the new Valiant powerplant. Records show they toyed with a 150-cubic-inch inline four-cylinder, a 150-cubic-inch cast iron inline six-cylinder, and even an aluminum V6. All were equipped with overhead valves, but it was a 170-cubic-inch inline six that got the nod. Unlike the upright inline designs under consideration, its novel 30-degree slant allowed for a lower hood line and reduced overall length thanks to its off-center water pump location. These details complemented tight vehicle packaging and were major factors in it being selected for the job of powering the new Valiant.

Aluminum Block Bonanza!

We found three aluminum Slant Sixes in the last few years just by keeping our eyes open. The first was found at the Ontario, California, Pick-A-Part self-serve wrecking yard in April 1998 in the battered '62 Valiant shown here. I bought the entire motor for $135. The second was pulled from a '62 Lancer GT in the San Pedro, California, Pick-A-Part about a year later. This time I disassembled the motor and took only the block, main caps, and main and head bolts for under $100. Both cars must have sat inoperable for years before being sold to the scrap yards, which crushed them after they'd been picked clean of useful parts (typically after 30 days). The first two discoveries are so badly corroded that they are beyond reasonable repair. They still make excellent conversation pieces, though. The third was found at a local machine shop and is a cherry. No corrosion, no cracks, and it still retains a standard bore.

Dare to Compare

Here's an exclusive look (check the pictures at right) at the visual differences that distinguish the aluminum block from its cast iron brother. For easy comparison, these blocks have been stripped of paint, so the shiny die-cast aluminum surface contrasts with the dull cast iron.

Soon after, a request was made for ways to increase displacement to allow enough power for installation in future full-size Dodge and Plymouth C-Bodies and trucks so that the outdated 230-cubic-inch flathead six could at last be retired. The result was a one-inch deck height increase and a two-engine family of 170 and 225 inline six-cylinder engines with a common bore of 3.40 and strokes of 3.125 and 4.125, respectively. On November 26, 1958, less than seven months after the May '58 go-ahead, the first prototype Valiant engine--an all-iron 170--ran under its own power, and the Slant Six was born. Within two weeks, an all-aluminum 170 also sprang to life, and by March 1959, iron and aluminum tall-deck 225s were also among the living.

So what's the deal with the aluminum motors? From the start, Chrysler's intention was to use aluminum for the cylinder block in order to excise dead weight and maximize vehicle efficiency. An aluminum cylinder head was also under consideration, but the cost-to-benefit tally of the light head wasn't nearly as significant as the efficiency bonus offered by a low-mass engine block. After all, the single most bulky item in any car is the engine block, not the head. Although the engineering department did produce some aluminum heads for testing and evaluation (a few of which are still floating around among hardcore Slant Six collectors), economics prevailed, and the aluminum head was never approved for mass production.