You'll need to know the diameter of the center hole in the blade and the shape of the hole, plus the length of each of your current blades.

to add to baymee's notes, make your blade length measure on the diagonal (not the straight length). Also, determine if your center mounting hole is a basic round hole or has a "star" pattern on it's edge.

Another way is to find the full model number of your machine - usually on a plate/decal under the tilting seat area - go to sears parts webpage - determine the Sears p/n for your blades - use that to cross-reference a Gator p/n in the Oregon Chain online catalogue..

If you were to post the original blade # (on decal under hood) or the 917# from below the seat- we could be sure. OEP actually markets a standard Gator mulcher and a 3n 1 version too.