What is the procedure for straightening out a side rod on a small tank engine? What precautions are necessary to prevent cracks from forming in the future?

A lot has to do with how damaged the rod may be and what the material in the rod is. Generally, minor kinks in forged steel rods can be straightened by basic "heat and beat" blacksmithing, but you do want to be careful not to make it longer in the process by drawing it out. The little tank engines I've had a lot of fun with weren't made with odd alloys, just fairly common carbon steels that don't require a lot of specialized controlled cooling, etc - just let them ease their way back to room temperature and you probably won't need to worry about annealing or whatever, the blacksmithing process will do that as part of itself. If the rod is so badly torqued that it is staring to split in cracks radiating from the bend, then you might need to make a new one, in which case simply mill it from a billet and a bit of normalization and you're good to go. Imagine you are a shortline railroad blacksmith in someplace like Cimarron in 1882......

dave

Depending on the problem, a hydraulic press may be able to straighten things for you. Along with some heat this may work better than using a sledge hammer IF you don't know much about blacksmithing.
Show us some pictures & we can give you better advice I think.
I was a blacksmith for 20 years before I had bad carpal tunnel problems. I am now just starting to get back in to smithing & hope to set up a shop to do repairs & making new parts for the railroad preservation community.

Dave Huffman

How did they get bent in the first place?