Chosen Solution
I changed out the motor on my Murray, the pulley diameter is different then what was on original motor. (cannot remove pulley) How do I size the correct belt?, everyone i try is either too big or too small. I can’t seem to get a match.
Since you changed both the motor and the pulley, you’re going to have to extrapolate. Use the old belt and put it on. If it is to long, cut it, Then put it back in place and see how much overlap you get. Then cut that to size, tape it and take it to your belt store and get one the same size. If it is to short. measure by how much and use those figures to get one that fits.
Use a piece of string and go around the pulleys following the pattern that the belt would normally travel. Take the string to your local parts dealer and use it as a pattern to find the right belt.
