Oregon® 20-5/8" Blade for 21" RYOBI Lawn Mowers, OEM Part Number AC04025 

Measure near the center of the blade.

Measure near the center of the blade.

Measured diagonally from the corners of the blade

This is the diameter of any holes to the sides of the center hole.

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

If there are outer holes; this is the distance between the centers of those outer holes.

(Sometimes referred to as offset): If you set the blade on a table this is the distance the center of the blade is above the table top (if any) when the cutting edges are flat on the table

Air lift is the distance the lift panel rises above the cutting edge.

Right hand cut blades spin in a clockwise direction, left hand cut blades spin in a counter clockwise direction.