Originally Posted by
garage sale GT
...I reasoned that the two sides of the rear frame were pretty much the same, so if I simply pulled each dropout by hand in opposite directions, each side would bend the same amount. It did....
Older steel frames usually have a big crimp on the drive side chainstay, as a result it move easily than the non-drive side. Most frames I find already spread are offset. 4mm offset doesn't make much difference in handling (assuming it was square from the factory...) so most people never notice.
BCRider has it right, do one side at a time with the BB clamped in a bench vise.