Redneck,
The flywheel were made from Armasteel, that is cast on my flywheel. Read this link. I only pasted the first few paragraphs, more after this.
https://www.thefreelibrary.com/Q+Wha.....-a0175286114
Many years ago, the General Motors (GM) metalcasting division developed an alloy known as Armasteel for use in differential parts, engine crankshafts, automatic transmission stator shafts, connecting rods and other components.
Armasteel served the automaker well in many critical areas. In the GM Corvair, for example, it was used for differential ring gear carrier/cover assemblies. The material also was used for the crankshaft in the high performance 421-cu.-in. Pontiac engine and the connecting rods in the lightweight Buick/ Oldsmobile V-8 engines of the 60s.
However, confusion about the nature of this material has recently emerged in the casting community. Much of this results from GM's marketing efforts to apply a certain mystique to standard engineering materials. First, Armasteel is not steel but a GM trade name for a grade of pearlitic malleable cast iron. As a 1982 GM brochure explains, it is a ferrous alloy with temper carbon in a matrix of tempered pearlite or tempered martensite. It was produced at the GM Central Foundry Saginaw Malleable Iron plant (SMI), which was closed in mid-2007.
A brief history of the plant was given in the article "A History of Innovation" in the Jan. 2001 issue of MODERN CASTING. According to the article, "SMI was one of the first to develop a melting technique in which cupola iron was transported to electric arc furnaces and 'duplexed,' or treated for metallurgical control and held until it was needed at the molding lines. SMI also developed and introduced pearlitic malleable iron (known as Armasteel) in 1936, which was used extensively for military applications during WWII and the Korean War and also for the conversion of crankshafts and connecting rods from steel forgings to cast components in the 1950s."