Thinking of trying something crazy....
 

I have a 1985 Cannondale ST500 with a bent rear triangle. It's about a fingertip off on one side, which is to say that I can fit my fingertip in the offset on the non dirve side.
I'm thinking of trying something certianly stupid and quite possibly stupid dangerous.
I know a cold set will just snap it, but what about a hot set?
if I were to perhaps take a torch heat the rear triangle and try and set it, and IF it didnt break, which I know is a massive if, would it be safe to ride?
I feel like I know the answer to this which is HELL NO! but if it sets in place and I see no cracks or fractures or micro fractures, would it be stable enough to ride again? Its shot otherwise but I just love this danm frame so much!

Heating hot enough to facilitate straightening will remove the heat treatment in the frame. Not a good idea. You would be better off trying to cold set it back and watching for cracks.

Noted! I forgot about the heat treat the aluminum gets. Torch is a no go in that case.
has anyone tried to cold set one of these? Anything other than failures?
I do know that cold setting alu frames almost always ends badly, but again the frame is shot eithier way so it may be worth a shot to try...

In the annealed state most aluminum alloys are relatively ductile and soft and can be bent and formed without fear of cracking.
Your frame is in a heat-treated state, hardened to give it strength but not as ductile. This is why people advise against bending frames in this state.

If you are attached to the frame emotionally, the best way to go about it is to have the frame annealed by a professional heat-treater, then align/straighten it, then have it heat-treated again. This will probably be expensive but will be a sure-fire way to get it straight and strong.

...

Here's a little more detail:
http://www.matweb.com/reference/aluminumtemper.aspx

6061 is a common alloy used in bicycle fabrication. The manufacturer of the tubeset will receive the raw tubing in the annealed state and then will form it. They'll draw it to get the right dimensions, put it through mandrels and dies to butt it and taper it. They'll bend it to make S-bend chainstays, etc... Then they will solution heat treat it to raise the strength, followed by another heating process called "artificial aging" which relieves stress in the metal, makes the grain structure uniform, and returns some toughness to the metal. At this point the metal is labeled 6061-T6 (T6 designating the solution heat treatment followed by artificial aging).

The tubing is purchased by the framebuilder. It's machined and welded by these fabricators. The welding process makes the area around the weld somewhat brittle and weak by changing the grain structure, so the frame needs to go through the another heat treatment process to relieve that weakness. With some alloys this involves a full heat-treatment and with others it might just involve artificial aging.

It's in rough shape paint wise. I am attached to it, but not quite emotionally so. Expensive is a no go on this guy.

Aluminum goes from a solid to a puddle, without warning, in the blink of an eye, when heat is applied. My take on the OP's interest is not a good idea.

If it is unusable in its current state, you have nothing to lose by trying.

GO SLOWLY!

Lots of leverage and small displacements. Be happy with less than perfect alignment.

I'd try bending it back cold, to see how it goes. But I'd never trust it again. Even the non-annealed stuff I've played around with tends to break (or at least crack) the second time you bend it.

Just find a new old frame; they're pretty common. Maybe not that exact year, but I don't think they changed much between 1985 and 1989, on that model.