For a 12" spincaster, the bottom plate is probably 12.25" diameter and the piston/clyinder @ 5 or 6". Maybe source your O ring and design around that? The O rings are probably about 3/16 - 1/4" thick.
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Thanks very much for that information Harry, I'm working on locating and making the components for pneumatic operation. In the meantime I did the three threaded rods and will use threaded knobs from Enco until I can switch over.
Below are a few update pics showing the control panel with tach. I still need to label the controls. Thanks again to Joe S. for his coding work on the tach.
Also I have the electric melting furnace built, just waiting on the infinite control to arrive but have fired it up and it seems to works very good. I'm ordering a zinc ingot today from budget casting supply and have to order the silicone from JBJ too. So I should be able to do the first test castings very soon.