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Accordingly, how do you remove the blade from a Briggs and Stratton lawn mower?

Use a piece of wood to deep the blade from rotating. Remove the bolt (B, Figure 21) that holds the blade. Briggs & Stratton engines that are used on walk behind lawnmowers use a right hand thread and would be loosened by turning counter-clockwise with a 9/16" wrench.

Also, can you start a lawn mower without the blade? Yes you can run an engine with out a blade . No problem though as posted it is surprising how much flywheel effect the blade adds and most of the noise is from the blade.

Subsequently, question is, how do I loosen a lawn mower blade?

To remove the blade, you must turn the blade's retaining bolt to the left, or counterclockwise. Before undoing the blade, tip the mower on its side with the engine air cleaner facing up and block the blade with a piece of wood so it can't turn.

Are lawn mower blade bolts reverse thread?

Most lawnmower blade nuts are reverse-threaded in order to secure the blade on the drive shaft of the engine. This ensures that the blade's rotation does not loosen the nut. However, not all blades use a reverse-threaded nut to secure them to the engine's drive shaft.