Starting in the mid 1970's, my first real 'career' was as a factory trained diesel fuel injection repair tech. The shop owner had a square one about the same surface area as the 6" dia plates. We used it to lap internal sections as well as the nozzle tip, when rebuilding unit injectors for Detroit Diesel engines (which were two stroke in those days). Anything that required a metal to metal seal would be hand lapped on that little cast iron block. Muscle memory still allows me to do the figure eight pattern, without hesitation...I sometimes wondered if I did it in my sleep!

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