I'd like to have a bobbed hammer on my Colt Cobra but I'm not one for permanantly altering older guns so I bought a new hammer to be bobbed.
Will the new one need to be fitted or is it going to drop in?
In my experience these will be a near drop-in replacement.
I've done this a number of times for people who wanted to keep the original hammer unaltered, and most times it was drop in.
NOTE: The only time I've had to do real fitting was when the customer supplied the hammer, and it was the wrong "vintage". Colt hammers DID have slight design changes through the production life, so buy a hammer from the same general production era, and it should require little work.
Here's what I recommend:
Buy a copy of Jerry Kuhnhausen's book "The Colt Double Action Revolvers: A Shop Manual, Volume One".
This contains more than enough detailed info on hammer fitting that you should have little problem.
Decide if you want to retain the single action capability. If not, carefully grind the single action notch area completely away, being sure to keep the hammer cool.
If you do want the SA capability, coarsely groove or checker the very top of the hammer to provide a secure grip area for your thumb.
DO NOT alter the "vee" mainspring for a lighter pull, since this will often cause misfires. Alter the spring ONLY after the hammer job, then only if you feel it's absolutely necessary.
In any case, TEST FIRE thoroughly, after the hammer job, at least 100 rounds.
Remember: cocking a "hammerless" revolver is something you DO NOT want to attempt under stress. It's much too easy to "blow one away" since you usually have to "start" the hammer back with the trigger, before you can get a good grip on the hammer's remaining small surface.
It's very easy to pull the trigger a little TOO much and fire the gun.