I have a craftsman lawnmower (with a Honda engine) and I'm trying to remove the lawnmower blade to sharpen it. It seems really tight. I'm wondering if I'm turning the bolt the wrong direction. Does anyone have a trick to determine if it's a 'normal' bolt or a reverse threaded bolt?
The torque on the bolt is highest when the blade is engaged. Be careful when thinking of this since as the blade is engaged and accelerates, the blade itself—due to the acceleration (or resistance of that acceleration by the blade itself)—wants to "slip" in the opposite direction. This "slip direction" should be the direction that tightens the bolt. So then loosen the bolt in the same direction as the blade rotates.
If you can see the threads on the bolt, then look for them to rise away from the nut in the direction of loosening the bolt. To use webcam photo art, looking at the bolt from the side with a nut below:
... you would turn that nut to the right to unscrew it.
For something like a lawnmower where you're exposed to lots of dirt and moisture, I'd hit the threads with some WD-40 to break down any rust. Let it soak in for a while.
And for many devices where there's a fast spinning blade, it's not uncommon to have the threads reversed so that the motion of the blade tightens the nut instead of slowly working it loose. This isn't to say that yours is reversed, just that it's possible and wouldn't be unusual.
Standard approach is to try the standard direction (counter clockwise) to loosen. If that doesn't work and there is reason to think it could be reverse threaded, try the other direction. I would liberally spray the threads and let soak with WD-40 before doing much more.
If there is still doubt because it is stubbornly stuck:
Usually a lawnmower blade nut is open. If the stud bolt sticks through the nut, or there are exposed threads on the nut, look carefully to determine the actual loosening direction. For example, this is conventional CCW loosening (see how the threads at back left are higher than back right?):
If the bolt stud and the nut align so that there are no exposed threads, the simplest technique is simply to be more aggressive in stages. Increase the trial force by like 33% first in one direction and then in the other. That should be a pretty rugged system able to take some abuse.
If, somehow, it still has not yielded, I would Google the mower make and model for a parts list and see if the nut description indicates whether it is standard thread or reverse thread.
A guy down the street has several lawnmowers. It seems most of them use a bolt threaded into the end of the tapped driveshaft. In that case, looking is not going to identify the direction of the thread, so skip the first bullet point.