Zinc

An assortment of 1920s-era Cracker Jack toys made of zinc. This picture courtesy of www.timepassagesnostalgia.com.
Remember the candy Cracker Jack? Each box came with a toy inside. The toy, especially in early years (early 1900s), was made of what many people would call "pot metal." However, the metal was in fact zinc. Zinc was used in earlier times by manufacturers who did not possess the best casting facilities. Being a relatively heavy metal (similar to lead) and one that flowed more easily and consistently than most, together with its low cost zinc was a natural for very cheap trinkets such as the Cracker Jack toys and those of similar companys during that era. Zinc also showed up in early Japanese powersports application in the form of carburetor castings. Prior to the mid 1980s most motorcycle carburetors were cast of an aluminum alloy very rich in zinc. You can spot these carbs easily. Their surface finish is darker than that of the later carburetors, and the carb castings themselves are quite a bit heavier, and I mean quite a bit. As in twice as heavy. A GL1000 carburetor body stripped of everything save the throttle shaft weighs two pounds, exactly double the weight of a similarly-prepared GL1100 carburetor body. Zinc is a highly reactive metal, even more so than is cast aluminum, which itself is chemically fairly dirty and prone to electolytic reaction. Even more so zinc. So reactive is zinc that in the marine world it has traditionally been employed as a sacrificial corrosive annode, giving itself up to save the aluminum and other metals of marine engine and powerdrive assemblies, for example. As far as carburetors are concerned, zinc is so reactive that special cleaning methods for older carbs need to be practiced to avoid darkening the metal, and older carbs that sit for too long exposed to the weather tend to corrode internally quite rapidly and irreparably. They are also difficult to repair by welding. As already alluded to, the reason older carbs are high zinc in metal content to begin with is because their manufacturers practiced lower technology casting methods, for whatever reasons. Most likely the metal mold and vacuum casting methods of today just were out of reach cost-wise. Though carburetors eventually would be made of higher grade aluminum having much less zinc content (witness the GL1100), the use of zinc continues to be observed, demonstrably in such motorcycle parts incorporating key tumblers, such as keyswitches and fuel tank caps, as well as in fuel pumps and fuel petcocks. These parts are surprisingly cheaply made and very prone to severe corrosion. Another place zinc has popped up in recent years is in aftermarket float needles, the plunger half of the carburetor's float valve assembly. The manufacturers of zinc float valves are making very inexpensive products retailing for a fifth of their OEM counterparts. Zinc float valves self-destruct in very short order. Their manufacturers attempt to slow the part's disintegration by plating the valve with chrome, but all that manages to do is delay the inevitable, and make the failure more sudden when it does finally happen. I avoid the use of chrome-plated float zinc valves whenever possible.

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