If I recall correctly, in the 1920's, die casting really took off. This was due to the need for mass produced parts for automobiles, which were also "coming into their own". One big firm, known as Doehler-Jarvis, made a lot of the die castings for the automobile as well as other industries. D-J had plants in Detroit, possibly Toledo, a few other midwest locations and one in Brooklyn, NY. D-J used to run ads showing how they eliminated auto parts made by sand casting & machining or by machining from solid stock and replaced them with zinc-alloy die castings. D-J's adds used to show things like hand throttle and spark advance lever assemblies, ignition system parts and carburetor bodies. The other thing that kicked the development and use of die cast zinc alloys into high gear was the rapid growth in the production and use of home appliances. In the 1920's, as homes got electricity, things we take for granted like vacuum cleaners, toasters, radios and washing machines all began to be mass produced. Die casting made a lot of this possible. Look at an upright vacuum cleaner from the 1920's or 30's, and there is a mix of sand cast aluminum for the main housings and a lot of die castings. I've parted out a few of these old upright vacuums to re-use the blower for forge blowers. I am always intrigued by the way those vacuums were built- so much better than the plastic disposables of today. The sand cast casings are heavy enough, and the lighter and more decorative or intricate parts are die castings. The vacuum cleaner makers or the die casting companies must have figured things out, because 80-90 years later, the parts are all in good shape with no degradation or breakdown of the metals used.
Thinking back on the use of die cast zinc alloys, I know I took it for granted. On the older cars and light trucks, parts like carburetor bodies, fuel pump bodies, along with things like windshield wiper gear cases and gearing (or the old "Trico" vacuum wiper motors) along with door handles, knobs and so much more were all made with die cast zinc alloys. For the most part, these parts held up well over time. I do recall some old car parts that were turning kind of black and crumbling, and being told that "it's pot metal... it breaks down over time". At Brooklyn Tech HS, in our freshman year, we took a course called "Industrial Processes". We studied, amongst other things, most of the common metals- where and how the ores were mined, concentrated and produced into usable form. This included die casting. I learned quickly enough that "pot metal" was something of a misnomer, since die cast zinc alloys were (usually) carefully made from known metals in known quantities.
I think the Zamak parts used on those unmentionable "A" lathes were, in many cases, too light duty for the service they were put to. Where other manufacturers used cast iron or steel gears or bronze half nuts, the unmentionable "A" used Zamak. It eliminated a lot of machining and allowed their lathes to be sold for a very low price.
Unfortunately for the owners of such machines, what worked well for gearing and parts in home appliances or light-duty parts of automobiles was not going to hold up in service on a machine tool given any hard use.
In the case of Magnetic Anomaly's tractor oil filter base and auto parts, my guess is the fact these parts were as old as they were might have meant two factors were at work: early attempts at die casting might have been done by a firm not willing to pay a licensing fee (which would have been typical) to the firm(s) holding the rights and "secret formulas) to Zamak, or similar alloys. This resulted in an attempt to duplicate the alloys and it may have missed the mark. The other factor is weather and electrolytic action. Die casting alloys contain zinc. On a Farmall engine, much of it is out in the open or partially so. The weather can get to it. Snow-melt water trickling down or rain blowing onto the engine might collect between the engine block (cast iron) and the filter base (die cast zinc). The high zinc content in the diecast part would then act as a "sacrificial anode" and begin to break down. Paint that was on the engine block and filter base might have weathered and peeled, exposing bare metal. On auto door handles, the makers sometimes used a plastic shim or insulating gasket between the handle and the steel car body. My Dad had a 1952 and 1954 Chevy, and I cannot recall the insulating shims between the exterior door handles and hood ornament and the steel body. The result was after a few years, the chrome plated door handles got kind of cratered pitting. I have a Mack Bulldog off an old Mack truck tractor mounted on top of a big Cummins diesel piston crown. The Bulldog was given me by a trucker who probably logged a million miles looking down the road between that Bulldog's ears. The Bulldog is well travelled, and his skin is pocked with those same kind of pits and craters, but he is still quite intact otherwise. I tend to think a combination of road salt and water formed a strong electrolyte that did its work on dissimilar metalsin the form of die cast zinc and steel or iron parts, whether in the case of the Mack Bulldog or automobile parts.
Attempts to replicate Zamak without exact knowledge of the alloy contents and possibly some data as to the process of making it might account for the "crumbling" of older die cast parts.
