Well, I whipped out the digital calipers and a bunch of plugs....

O.S. Turbo, GRP Turbo (C6), Mugen Turbo, and O'Donnell Turbo

Now rather than post my actual numbers, I'll post my findings. Reason being, calibration of digital calipers will vary so numbers may vary. The key is to use the same digital calipers for all the measurements, apples to apples comparison...

I measured total length from where the plugs sits on the button head to the end of the cone.

Length of the cone itself, from the shoulder to the tip or end.
Diameter at the end of the plug (filament end)
Diameter at the shoulder (Where the threads end, and just before cone starts)
Length of plug from where it sits to where the threads end....

In summary; I found the Mugen and GRP to be basically identical (no real surprise).

The O'Donnell was very close to the GRP that I'd have no fear of running it in my GRP. The differences I found were in the .001 range which I can attribute to my error....

The biggest difference was between the O.S. Turbo and the GRP C6...

The two big differences were in the (outside) diameter of the tip/end where the filament is. The O.S. measured .197, the GRP .202. The next biggest difference was in the diameter of the "shoulder". The O.S. measured .256 and the GRP .271

So, the O.S. is smaller by some. Other than that observation, I don't know what that really means....

I'd sure like to hear what others mic'ed out...