The plugs are 1-3/16" - Dorman 550-016
I just measured them - they are all about 1.193 (1-3/16" = 1.1875").
The hole in my 8n the plug goes in measures 1.183" - a bit undersized. So the total interference fit is about .010, no wonder the plug doesn't want to go in.
I'll probably grind or sand down the plug and hope the deformation when I flatten it will compensate for any irregularities in my sanding job (tractor is in the way, got to move it before snow).
Oh, if anyone has a NAPA or New Holland plug they can measure, I would be very interested in hearing how big it is.