This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Fri, 12 Mar 1999 16:14:11 PST
> What's the trick to removing pedals? Of the three times that I have
> tried to remove my pedals (I have two bikes and am in the process of
> exchanging/switching pedals) I have only succeeded once. The main
> problem is the pedals have been put on very tightly and I can't even
> budge the damn thing.
Assuming you know that the left and right pedals have left and right
threads respectively, and that a long handled 15mm pedal wrench should
be used, then the pedal probably fits the following scenario.
Pedals are often made with tight fitting threads in an effort to
improve the hold of this poorly designed mechanical interface. The
intent is to prevent relative motion under load but the result is
motion anyway. This is apparent from fretting damage to the crank
where the pedal axle bears on its face. Besides damaging the crank
face, fretting motion depletes thread lubrication and causes galling
(aka welding) so that the pedal often cannot be removed forcefully
without damaging the wrench flats, wrench, or the crank. Forceful
removal often removes part or most of the thread from the crank bore.
To remove "frozen" pedal shafts from an aluminum crank, remove the
crank and pedal from the BB spindle, heat the pedal end of the crank
over gas flame cooking stove until it sizzles to the wet touch. Using
a pedal wrench, the pedal usually unscrews relatively easily without
damage. If the lubricated pedal does not screw in easily, a thread
tap should be run through the crank to prevent galling on insertion.