Thanks for the answers but it seems to be a lot more complicated than I thought.
- Rocky you measured a "Flowkooler/Robertshaw" thermostat with the reference 333-180 at 0.680 so very close to what is indicated on the packaging (11/16 = 0.6875). But it would apparently be the one that Larry says he does not match OEM by pass plate.
- JB1490, you are talking about 0.672 to 0.677'' but your photo shows 0.75'' (12/16'') on a "Robertshaw" whose reference is not visible.
- The one I bought from Summit with the Summit reference 333-180 SUB (I admit I didn't pay attention to the mention SUB when I ordered) but in a FlowKooler packaging reference 333-180 only measures 17.10 mm (0.673'') so a little less than the 11/16 indicated on the packaging.
- I also have a Motorrad whose hat is even smaller: 16.60 mm (0.654'') ........
I visited the FlowKooler website and, If I understood correctly, they say the 333 series is made for the OEM bypass plate but only available in 180 and 195°F and indicate 11/16'' diameter; it is only if you want 160°F that you have to choose the "substitute" which is in fact a 330 which must be installed with an "alternative sized by pass" but without specifying the diameter, just "alternative sized".
FlowKooler would therefore sell 333-180 and 333-180 SUB which would be different and Summit would sell 333-180 under the reference 333-180 SUB????? So the error would be more at Summit than at FlowKooler?????
TimMeyer gives him no indication of the diameters!
Furthermore, the hole in the bypass plate measures exactly 19.23 mm = 0.757''.
This leaves a circular space 1.05 mm wide, i.e. an approximate section of 60mm² = 0.093 SI. On a total surface of 0.45 IF it represents 20%, it is not negligible and would prove Larry is right, all the FlowKooler would be SUB ?????
The one measured by JB1490 seems more suitable but this time with a very small clearance of 0.0035'' all around which seems very little especially if we consider the "precision" of the centering.......
It is therefore impossible to know with certainty what it is exactly and
it would therefore be really interesting to know precisely the exact diameter of the Ford OEM or TimMeyer thermostats. Or the diameter of a FlowKooler substitute bypass plate.