I went with an electric tach. The original tach rotated counter-clockwise so because I was using a more modern V-8 that rotated clockwise, I went with a new electric gauge that Mark created with a spider web face. The switch on the lower left was something a previous owner had added. It now runs the blower.
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1930 Chris-Craft Model 100 20' "Moxie"
1935 Chris-Craft Deluxe Utility 18'
1938 Old Town canoe
1940 Chris-Craft Red and White 25' "Old Paint"
1942 Chris-Craft Deluxe Utility 18' "Blue Moon II"
1946 Chris-Craft Sportsman 25' "CinCity"
1952 Century Seamaid 18'