I just bought a used table saw and a previous owner extended the table and in the extension put holes to mount a router underneath. There's a 1.5-2" center hole and several counter-sunk holes around it for mounting screws/bolts. I don't know whether the spacing of those holes is fairly standard across router brands or if I'm going to have trouble finding a router to mount in the existing holes.
Best Answer
They are not standard, but there seem to be a few common patterns.
Rockler's website has a guide for choosing a mounting plate for your router. These are the router models that they support:
They also offer blanks that you can drill to fit your router if it's not in the above lists.
On the "Tech Spec" tab of that page, they have PDFs that show the pattern for each of the four above groups, so if you measure the spacing of the holes in your table, you may be able to find one that fits.