shooter71,
The o-ring trick is brilliant! Wish you had posted that last summer when I was mitre-ing 1-1/2" and 2" stainless 90's every day for 2 months! Have to agree with DaveBoyer about the larger sizes. On bigger bores (over 6"), I prefer to use band saw blades or steel banding (the good stuff is 1090 steel and is used for damascus knives). On one particular project where we were mitreing pipe (12" up to 36") to fabricate 30's, 45's and 90's, a sheet of plywood with a "U" shaped slot cut out (like a mouse hole or dog house door) was found to be more accurate than wrap-a-rounds.