I was putting together my home treadmill and most of the bolts screwed into the frame and tightened nicely, but a couple of them will tighten somewhat and then suddenly become loose again when you try to further tighten them. Obviously the frame's thread is damaged; is there some way to still get these bolts to tighten without buying a whole new frame (unfortunately the bolt hole is drilled directly into the frame and is not a separate component)?
Over-tightened nut, bolt broken
bolts
Related Solutions
If there was only one washer, it generally will go on the nut side as the nut has less surface area in contact with the thing being connected than the bolt side.
Don't underestimate what that little wrench will do, or how much torque you need to hold the bed rails together. With the lock washer, it only needs to be compressed, plus maybe another half turn at the most. The force created by the threads drawing through the nut create an incredible amount of pressure on the joining faces.
The included wrench has 2 distinct ends, one has the legs of the wrench equal lengths more or less, the other end has a shorter leg on one side. Because of the shape of the wrenches, I will assume there is little or no space to get a box end wrench around it, if there is any room behind it, then a box end wrench can be ground down enough to fit around it. A rail bolt wrench may work too, they are made to fit into even a deeper, hole. They are used for handrail assembly.
Back to the offered wrench, the one end with the short leg, in my opinion is the workhorse, the short side is always on the leading side of the turn, as in if you are turning to the right, or clockwise, the short side is on the right side. That side is shorter so it does not bottom out as quickly in the shallow hole the captured nut is in. The wrench with the equal lengths are for the nut when it is in the midway point, where the nut is being turned where the legs of the wrench will still clear the bottom of the hole. As soon as one side of the wrench touches the bottom of the drilled hole, turn it around, and use the end with the short leg to complete the turn, so to speak, then swap ends again, turn that a little, then change ends of the wrench, and repeat.
You may have tried these techniques already, but it wasn't stated in your question to what extent you tried to work with the wrench. The picture of the wrench only shows a little distortion on the short leg of the one end that would occur if the wrench is turned the wrong way. Even though there a slight distortion, having the proper orientation of the wrench to the nut, that won't matter.
Best Answer
Remove the bolts and inspect the threads of both the bolt and the tapped (threaded) hole in the frame.
You have three possibilities here:
Either A) the frame threads are damaged, B) the bolt threads are damaged, or C) both threads are damaged.
If A) or C), you're unfortunately out of luck without drilling and tapping a new hole as Tyler has said (this won't work if the frame is plastic with threaded inserts). This is the most likely because your bolts are probably steel and your frame is probably aluminum, or plastic with inserts.
If B), take the bolts to a hardware store (Ace Hardware usually has a great selection of individual bolts and are usually more helpful, but a place like Home Depot or Lowe's will also work) and find new ones that match. Then take care not to over-tighten them!
General rule: if there isn't a torque specification on a bolt, run it down "snug" (until you start to feel resistance, then give it another quarter to half turn). "Snug" is subjective but it should get you pretty close. And it'll help when assembling cheaply-made parts.