Our version of the Fairbanks Morse RE engine has 4 valves. Intake and exhaust are identical and the seat diameter is about 6 inches, with the seat angle being 45 degrees. We do not have a seat grinder anywhere near this big so one was invented.
The first step was to build a wheel dresser. This was done by using the angle dresser from our surface grinder. The wheel is mounted on a custom arbor with a 1/2 inch through hole. This arbor fits onto a short length of drill rod about 4 inches long that acts as a spindle for the arbor. All of this is in turn mounted onto an aluminum plate and held in place with toe clamps. The fit between the arbor and the spindle is very close and there is no chatter or vibration when the wheel is being dressed.
There is another short length of drill rod in an adapter that is pressed into the valve guide. This worked very well except for one hitch. We don't do very much surface grinding and don't have a very good selection of wheels. The wheel we tried was a very fine grit wheel and it loaded so quickly that more time was spent dressing the wheel than grinding the seat. It became pretty obvious that the wheel would not last through the job. We will buy (yes buy) a 60 grit wheel with an open bond. This should solve the problem. All in all this worked very well.