STD

My Snapper Hi-vac rider has 2 bolts holding the blade on the center mandrel. They are the ones you have to remove. The blade isn't held on by the center bolt.

On a walk-behind the center bolt holds the blade, it is standard thread.

As others have stated, it is a standard right hand thread for walk behind mowers.
My only suggestion is to ensure that the blade is securely on the blade "support" and above the "curl" of the underside of the deck before tightening. I seem to always start tightening the blade, then find that it is binding up in the deck. The fact that I don't have a stock blade might be the problem, but something to check.
Good Luck
Ralph

It's 9/16 hex (3/8-24 thread) and as a couple of other guys said it's standard right hand thread.

You'll need a good socket (preferably 6 point) and a long breaker bar for leverage. Even better would be an electric impact wrench capable of 100ft/lbs or better. I would not recommend an air impact, because you might then twist the head off (been there, done that). However, when that happened, I was able to twist out the broken threaded stud with my fingers. Turned out that the bolt head "welded" itself to the large belleville washer and bolt-to-washer-to-blade just wouldn't turn.

Thanks for the replies, especially Bill_Kapaun. I've tried to get the blade off previously but with the doubt in my head (Am I turning it the right way?) I never gave it my all. This time I blocked the blade and since the socket kept slipping off I eventually removed it with a monkey wrench. It's pretty beat up now, obviously, so it'll have to be replaced too. I got a 21" universal blade with an adapter for a 3/8" bolt so I'm back in business. I found the identical spark plug with no problem but had to clean the filter since they didn't have the right size available. The oil gets changed tonight & I'll have to go to a Snapper dealer to get the filter; there isn't one that close to me.

Again, thanks!

"eventually removed it with a monkey wrench." Do you still have your knuckles? ggoyeneche has a much better method.

"but had to clean the filter since they didn't have the right size available"
If your mower has the foam air filter, be sure to oil it before use.

I don't know what engine that has, but I would expect if you wrote down the engine #'s, you shouldn't have much problem getting the filter at most any small engine shop.
For a Briggs, you'd want model & type.