I've got the exact same problem with a
Lawn-Boy 10606 mower. The curved factory blade is a
model 89963.
Since the factory mulching blade doesn't perform well when bagging grass, I recently bought an
Arnold 20" universal super high lift blade to try while bagging. The universal blade is flat unlike the curved factory blade. The universal blade's packaging specifically states it fits Lawn-Boy mowers.
The mower uses
this blade adapter (the curved profile can clearly been seen in Amazon's picture). The blade is then sandwiched between the blade adapter and this
blade support plate as seen in
this video. The blade support plate, factory blade, and blade adapter are all curved to match each other.
Also, the universal blade's installation instructions (see attached image below) say to remove the blade adapter and bolt the blade to the crankshaft using what they call the included adapter, which is really just an insert to make the star-shaped hole in the blade round. The problem with that is the self-propelled drive belt pulley and shear key are integrated into the factory blade adapter.
Is it advisable to use this flat universal blade with this mower at all? If so, what would be the best way? Obviously, the factory blade adapter will have to be used, but it could probably be mounted without or without the blade support.
Alternatively, is it possible that a flat blade adapter (with integrated pulley) exists that would fit this mower and better accommodate a flat blade?
Thanks for your help!