Many blades will have a round hole in the center of the blade and you will need to know its diameter. Blade Length 5 or 7 Point Star Center holes may be 5 or 7 point star shaped. 5 point star on blade7 point star on blade Square or Rectangular The MTD built blades on many walk behind lawn mowers have a 'bow-tie' hole. In this case the Oregon replacement blade will have a rectangular center hole which will fit on the bow-tie pattern on the blade adapter. Square or Rectangular hole on blade MTD Blades The MTD® built blades (Bolens, Cub Cadet, Troy-bilt, White, Yard Machine, and Yardman) may have a 6 point star. Oregon replacement blades will have a 'Y' shaped center hole which will fit exactly on the 6 point star on the blade spindle. 6 point star for MTD blades The two sizes of the 6 point star. If your blade spindle has a bolt and lock washer that holds the blade to the machine you would use the 11/16" star or 'Y' blade. blade spindle with bolt and lock washer. If your blade is secured with a large flange nut then you would use the 5/8" star or 'Y' blade. blade spindle with large flange nut