Although many modern aftermarket...
Although many modern aftermarket aluminum V8 blocks use iron and steelmain caps without trouble, Chrysler strove for the utmost in dimensionalstability. The cast-iron upper and lower main-bearing caps are designedto keep main-bearing clearance within a range of 0.0005 to 0.0015 forlong bearing life and consistent oil pressure. Specially designed1/2x13-coarse pitch main-bearing bolts thread into the aluminumbulkheads to 50 lb-ft, securely clamping the dual main caps with a7,500-pound screw load. Each upper cap has a roll pin to assist properlocation. If these parts get lost, hang it up and turn your spiffyaluminum block into a coffee table. They're irreplaceable.
The open deck of the aluminum...
The open deck of the aluminum version is the most profound differencebetween the two blocks. It is also the Achilles heel of every aluminumblock.
The cast-iron cylinder bore...
The cast-iron cylinder bore liners are 0.10 thick. The pen points to thealuminum surrounding them, which is 0.11 thick. Although the specialhead gasket is designed for a primary gas seal against the end of theiron liner, the prolonged effects of corrosion can reduce the thicknessof the surrounding aluminum and lead to chronic head-gasket failure. Thesame goes for overboring; the more material removed from the innerdiameter of the iron liner, the less area to seal the gasket.
(above & right) Chrysler employed...
(above & right) Chrysler employed state-of-the-art gasket technology to provide acombustion seal at the open end of the cylinder bores, a coolant sealaround the periphery of the water jacket, and an oil seal around theouter wall of the tappet chamber. The basic gasket (Chrysler PN 2128279,McCord PN 6530M) is 0.050 thick (0.045 compressed) and consists of asheet of perforated steel flanked with rubber-asbestos sheets and acopper sheet facing the water jacket. A 3/16 wide fire ring surroundseach bore, and, finally, the entire gasket is coated with syntheticvarnish to improve sealing against slight surface imperfections ofmating parts. Though long since discontinued, gaskets are stillavailable from www.slantsix.org for about $75.
An equally significant reason for using aluminum over cast iron was to cut costs. The August '63 issue of Car Life reported that each unmachined aluminum block cost Chrysler $30 to produce--$6 more than cast iron. But this was the only disadvantage. Chrysler was banking on the fact that softer aluminum is easier to machine and doesn't wear out tooling as quickly as cast iron. Chrysler recognized that if tooling life was significantly increased, the capital investment in machinery required to process engine blocks could be reduced. They estimated that three cylinder-block machining lines would be required for cast-iron production, and only two for aluminum. Additionally, by using a recently developed high-pressure die casting process, manufacturing cost could be further reduced. Aluminum parts made with the sand-mold process tend to be heavier for a given size because compensating for core shift and minor discrepancies requires much thicker sections.
Die-casting definitely offers worthwhile advantages. The high injection pressure provides a consistent structure so thinner sections can be used where strength is not critical. Surfaces are smoother and dimensional tolerances closer, so less machining is required to finish the casting. Die-casting is also a lot faster. With no sand cores to set up and shake out, one casting machine can turn out six or seven times as many engine blocks per hour than a permanent mold machine.
The biggest drawback was the up-front cost of the die-casting machines. Though fewer were needed, each weighed a staggering 2,000 tons. The size and weight of the machine was needed to provide the required clamping force to prevent leakage between the dies as the molten metal was injected at 8,000 psi. Still, Chrysler was banking on the increased pieces-per-hour-per-machine to offset the initial investment in manufacturing equipment.
Although the die-casting process was chosen, it was still fairly new to Chrysler. As a precaution against unforeseen interruptions in the supply of aluminum Slant Six blocks, a nearly identical cast-iron engine block was designed in tandem that would be manufactured using the traditional low-pressure, semi-permanent mold-casting technique, and it shared the same 170 low-deck and 225 tall-deck configurations. Quoting Willem Weertman, Managing Engineer of Engine Design (1955-1962): "It was understood that if the development of the aluminum engines did not encounter major problems, the development of the cast-iron versions would be terminated at a future date when the success of the aluminum engines was assured; it was projected that by the start of the '61 model year, both the 170 and 225 engines would consist solely of aluminum blocks.
Because Chrysler banked so heavily on the reliability of aluminum, the changes made to create the "temporary" cast iron version were minimized to save money and time, and to streamline the operation. The ruggedness designed into the aluminum block becomes absolute, bulletproof strength when it is rendered in more stable cast iron. The resulting rigid crankcase endows the approximately 12,500,000 iron Slant Sixes manufactured between 1960 and 1991. Its ability to withstand extreme abuse is the root of the engine's reputation for being virtually indestructible. On the other hand, on the eve of the Valiant's '60 debut, the aluminum Slant Six program was running into trouble.
The Trenton, Michigan, engine plant began producing cast-iron 170 and 225 Slant Sixes in August 1959 using dual-tooled machining lines capable of handling either iron or aluminum blocks. The Canadian Windsor Engine Plant and the Kokomo, Indiana, facility followed within a few months. While the plants began churning out tens of thousands of iron blocks, the production of aluminum blocks was hindered by problems with the cast-iron bore liners and a costly learning curve that led to high scrappage, especially at the Kokomo plant. Although a handful of prototype aluminum block 170s and 225s are reported to have been installed in '60 model year vehicles for analysis in taxi fleets and limited sale to the public (Weertman claims to have leased an aluminum 170-powered Valiant from Chrysler), the production-line kinks weren't ironed out sufficiently to ensure the steady stream required for mass production.
Nonetheless, Chrysler must have been confident the problems had been addressed in August 1960, when it issued PTR (Production Test Request) No. 3331. It authorized the building of 15,000 aluminum-block 225 engines for installation in '61 model year vehicles, and records indicate that an actual total of 10,767 aluminum block Dodge and Plymouth cars were built. (Oddly, the 170 was not included in this production run, and all '61-'69 170s used iron blocks.) The '61 PTR was encouraging enough for Chrysler to give the aluminum-block 225 a regular production release for the '62 model year, and approximately 36,000 more were installed as optional equipment in Valiants and Lancers. We have not been able to confirm whether any regular-production full-size C-Bodies or trucks were ever built with aluminum blocks. Despite the assembly and sale of nearly 50,000 aluminum-block Slant Six vehicles in 1961 and 1962, the die-casting process failed to achieve the problem-free non stop processing required for consistent high-volume manufacture. And while the reduced weight of the aluminum block was highly praised by magazine road testers from Hot Rod, Motor Trend, and others, the benefit was lost on the buying public. At the end of the '62 model year, production of aluminum blocks was discontinued.
So, what happened to the roughly 50,000 aluminum-block 225s released into circulation? The vast majority suffered from corrosion. Because the aluminum block was an extra-cost option (see sidebar), it is safe to assume that most original owners were diligent about following Chrysler's directions to flush the coolant twice a year and use a rust inhibitor. But as the cars were traded in and entered into the used-car market, their second, third, and fourth owners were often oblivious to the presence of the special aluminum block and its tendency to corrode from the inside out when neglected. As a result, galvanic activity quickly took its toll around the tops of the free-standing cylinder barrels until the head gasket blew. Unfortunately, once this level of corrosion was reached, chronic head-gasket failure resulted, and nothing short of expensive and risky weld repair work could reverse the damage. Add to this the unpopular style of the cars that housed the aluminum blocks, and it's likely that most met the crusher by 1975.