repairing diecast

My uncle repair many of Lionel cab castings that where bent. The was his trick.

First don't apply heat, this will actually accelerate out gassing which could cause the cab roof to crack or break entirely.

If you have an existing engine with a good cab than trace this profile onto a wood block, cut this out and make a form, male and female side, take the shell off and place the blocks into a vice, male on one side, female on the other, I would use duct tape to hold them in place, Now place the shell with the damaged cab into the vice and slowly and I mean slowly draw the clamp closed, this should reform or reshape the cab roof to a closer profile. You may lose some paint during this reforming process, but you will be surprised that it does work. Repaint was necessary on all repaired shells. 

now that sounds like a good method.

This is a good method to use to avoid making the bend any worse.  I used this technique on an 736 which had a bent cab corner.  I would suggest doing the clamping over a few days - tighten a bit the first day, then a bit more the second, etc, etc.  This way the metal has a chance to return to the original shape.

I use the wood block technique but also use heat. I have an industrial strength heat gun rated at 800F. I heat the casting until its too hot to touch but not affect the paint. I am aiming for 400F, well below the melting temperature of zamac. Then use the blocks clamped in a vise. I have tried it at room temperature noticed the casting was starting to fracture before it was straightened. Never had an issue using heat.

Pete

thanks for all the good ideas.  bertiejoa