I've ruined two hex wrenches trying to remove my pedal. I actually got one off after blasting it with a hot-air heat gun for a while. The other one remains stuck and Yes, I'm turning it the right way.
I tried penetrating oil as well as the heat gun. (Side Question: Can my heat gun hurt the steel crank arm?)
What is my next step? I've got an extra crank arm somewhere so maybe I cut my losses and go with that new crank arm.
Lesson for all of you: use anti-seize compound when assembling!
Stuck 6mm-only pedals can be a very tough situation, unfortunately.
Presuming an aluminum crank, heat, either from a heat gun or torch, is a method some mechanics go to quickly here. The idea is the different materials expanding and contracting at different rates thermally shocks the corrosion free. Personally I'm squeamish about this and never do it on other people's bikes, out of fear of metallurgical consequences (ruining the heat treatment etc) that might not be obvious. That said, it's a pretty common practice and probably unlikely to bite you in any meaningful way.
I tend to soak it in some kind of penetrating oil for at least a day. Lately I've been using Finish Line Chill Zone, which thermally shocks it using cold. I put a plastic bag around the pedal, rubber band it around the crank, and position it so the threaded part is being bathed in oil. This also contains the mess and smell, which is nice.
To do the actual breaking free, what I do if possible is clamp a 6 poking straight up in the vise, put the crank on top of that, and try to find some way of applying massive leverage to the crank, usually via a Park FFS. I've also clamped the cranks using smooth jaws and rags, but this is pretty likely to do some kind of marring to the cranks anyway given all the force involved.
The heat could potentially alter the tempering of the alloy, but I'm not sure it's going to be catastrophic. As for the pedal, have you tried extending your leverage with a breaker bar of some kind? Aside heat and penetrating oil, there isn't much more that can be done.
General use grease on the pedal threads should always be used. Always.
Pedals are extremely tough to get off in my experience. A normal hex wrench won't give you enough leverage, you need a special pedal wrench such as those found by this search:
https://www.google.co.uk/search?q=pedal+wrench+hex
They seize up, but are often tightened with a high torque. The right tool will help you get it off. I too have ruined several hex wrenches until I got a proper pedal one.
Also remember, the left pedal spindle is reverse threaded, so turn clockwise to remove the pedal when facing the crank arm. The right side is normal, so turn it anti-clockwise to loosen it.
I've not tried this, but another solution might be to use an impact driver and a medium hammer.
A manual impact driver takes impact forces and translates them into rotational forces, plus the impact helps keep the tool's bit deeply in the hole.
From https://en.wikipedia.org/wiki/Impact_driver
Just be sure its set to "undo" not "tighten" ! Any hammer will do - you don't need a Park branded one, but they have the HMR-4 and HMR-8
This tool is not to be confused with a rattle gun or impact wrench. They tend to look like cordless drills, and I have no experience with them. Probably overpowered for a bike.
I used an 18V impact driver with over 200Nm torque on mine after a good soaking in WD 40 and they still won't budge. My bike is only 18 months old!
The impact driver and 6mm hex did not shift it and my open-ended 15mm spanner started to round off the flats.
I solved the problem by dismantling the pedals off the shafts and got a ring spanner on the pedal nut and blocked the crank against the frame with a block of wood. Then my putting my heel on the spanner and applying all my weight in a downward force I managed to crack them off. I was replacing the pedals anyway but beware if you dismantle the pedals the bearings will probably fall out.
