Need a large wrench to get the bottom bracket off a bike? Disappointed by the assortment of not-large-enough wrenches in your bike took kit? Me too. Here I use two wrenches in an ad-hoc fashion to do the job of a much larger one.
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Step 1: Identify the Problem
So here we have a bike bottom bracket that I want to take apart. However, the lack of standardization means that none of my bike tools are able to get it off. I could have waited a few hours till the shop at work opened, and snagged a larger wench from there, but I was on a mission.
I used the crescent wrench to grab on to the edge of the nut, and tightened it onto the edge so that it wouldn't wander off. I set it up so the handle crosses the rotation axis of nut. This will let me torque the crescent wrench with a large open wrench in the next step.
This has the potential to mess up the interior surface of the nut. It didn't and I didn't care because I was just trying to take it apart, but your mileage may vary.
Quick disclaimer: Every shop safety guide ever published includes "Use the right tool for the job," and most accidents are caused by using the wrong tool, so if you can use the right tool you should. While doing this I was very aware of the risks, and tried not to be any dumber than I was already being.
Step 2: Add Another Wrench.
Now that the crescent wrench is all snug, take a handy massive open wrench and use it to rotate the crescent wrench. Neither of these are designed to do this, but I didn't run into any issues with slippage. As long as the load is perpendicular to the axis of rotation it is a stable arrangement. If you start angling the open wrench up and down you will likely slip and bash the dickens out of your hand. Using a long wrench also give massive mechanical advantage: the nut came right out.
Step 3: Fin.
And now the nut is out. The nasty goopy grease they packed the bracket with kept the threads all nice. Too bad it couldn't save the rest of the bike.
Hypothetically you could apply this method to other stuff. I can't think of any applications offhand, but I'm sure there are others.