How to Properly Tighten Lawn Mower Blades
O.K… let’s not over engineer this… the sharp side should be closest to the grass. Almost all blades have wings at each end, which lift the grass up and help blow it into the bag or out the side. The wings should be closer, pointing up towards the mower’s deck. Think of it like a fan that’s blowing air up towards the underside of the deck.
Most mowers use the standard right-hand thread rules, but be aware there are some riding mowers and even fewer older 2-cycle walk mowers that don’t. If you have a riding mower that discharges out the back between the wheels, use caution as you may have one blade that is right-hand and the other is left-hand, because one blade will be turning the other way. Toro made some zero-turn riders like that, and there are other manufacturers that do as well.
Modern mowers use a class 5 USA or similar bolt with right-hand pitch. As the shaft spins, the motion tightens the bolt. For safety, remember to torque properly.
In layman’s terms, angled sides should be upward towards the engine, with the flat side down towards the ground. Referring to the actual cutting section of the blade, unplug the spark plug when touching anything underneath the mower. Turning the blade can accidentally start the engine, so be cautious. Also, place the mower on its side where you put in the oil to avoid leaking gas from the other side.
Most mowers have "wind wings" at the end of the blades, ie tabs that turn up. Those should always be turning up when installed. The reason is that they create suction under the mower deck which makes the grass stand up for a smoother cut. As far as I know, all mowers turn in the same direction, so this is of no concern.
It’s a great question and you would be amazed at how many people get it wrong. It gets confusing when you have the mower on its side and the other problem is multiple blade decks found on riding mowers, this really confuses people, and even me, once and a while. You just need to find which way the engine turns—clockwise or counter-clockwise. The attached video shows it extremely well.