It would be helpful to know what mower you have so we know how it is designed. It might be as simple as installing a lock nut, new hardware or using thread locker. Or maybe a better description. Does it thread into an arm that attaches to the main housing or does the bolt thread into a hole directly in the deck etc.

Sorry, took me a while to get back with you. I took pictures so troubleshooting would be more easier.

The model is Craftsman 917.378501 21" Hi-Tunnel Mulcher EZ Walk.

Here are the pictures.

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I lifted the mower so I can take a picture of the inside. However, the wheel fell off. Hahaha. LOL. :D

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The screw threads are fine. I don't see any damage. However, it's not circle anymore. It's a bit triangular in shape. Not sure if that's normal.
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OUCH! Looks like the threads are damaged. And looks like that this thread is part of the whole deck.
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From the pictures I see the Adjuster nut and bolt needs to be changed. I would add a fender washer also. Hopefully there will be enough thread left. As for the nut I would suggest a nylock and not use a lockwasher to give you more thread.
If it was in my shop I would weld a washer to the frame.
Believe it or not there is a lot of stress in that area.

It looks as though you would have a few ways to repair this. The easiest and most direct would be to install a nut on the backside along with a new shoulder bolt if need be. Source a new bolt that isn't self threading and a bit longer. A good hardware store or ope dealer should be able to help.
ANother way would be to remove the handle bracket and secure the bolt w/ a nyloc nut to the back of that. Bore out the old worn out hole to accept the nut so the bracket can be bolted flush. You could bore a hole above in the bracket where the bolt head wouldn't be in the way and zip the old self thread or another bolt and nut for extra support in holding the bracket to the deck.

Hi rosemallow, I don't see any bolt. I do something but is welded.

The easiest and most direct would be to install a nut on the backside along with a new shoulder bolt if need be.

What about the bump(8th pic) that looks like the bolt on the backside? I think the backside space is too tight also.

Also, can you please explain to me what you mean by "zip"?

I also found the diagram of the exact mower I have. So am I correct that I need to buy the following:

1. #38
2. #39
3. #28

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Forgot to ask. What does "isn't self threading" mean? Do you think, Ace Hardware will have this kind of shoulder bolt you described?

Bore out the old worn out hole to accept the nut so the bracket can be bolted flush.

I got it now. :D The outside red piece of metal will prevent the nylock from getting detached since the existing hole is actually on the other metal. :)

I will have to make sure that the hole I will be making will provide enough space so that the nylock will flush well.

Awesome!!! :D

The 'triangular' bolt you have is a 'Tri-Round' thread-forming bolt. I hate these on mowers because they always seem to go bad and the threaded hole gets destroyed in the process. I would tap the existing hole with a slightly larger thread and use a different shoulder bolt OR replace the bracket - part #15. Another alternative is to remove the bracket, take it to a machine shop to be welded-in and re-threaded for the original bolt. Considering the value of the mower, which is about $0, a guy with a welder might just weld it in place.

Done! It's fixed. Thank you for all those cool ideas! I only spent $0.25 for the nylock and some electricity for the drilling. I'm so happy that I will be able to mow confidently now. I mow really slow before and lift it really slow. LOL. :D

I'll post pictures later.

Thank you so much! PRICELESS!!! :D

Here are the pics after I fixed it. I also used the mower and works pretty well. :)

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That was a good suggestion and a good fix! I didn't think there would be clearance for the nut.