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My husband has a Simplicity Regency mower. Right now, he is having trouble with the blades. They won't tighten up and stay tightened.

Are threads stripped out in spindles? If not purchase 2 new grade 5 or grade 8 bolts same threads & length along with a small tube of blue Locktite to put a small amount on threads, torque bolts to 50 ft. lbs.