and

Well, if you know the second, why do you do the first - you are sure to move them different amounts and the whole back end will be crooked.
I never use the long screw and nuts/washer method.
Use a length of wood and a chair or stair or something.

Assuming the back end is (still) symmetrical, do it this way
a) measure the frame
b) move one stay 1/2 the difference between what you have and what you want.
c) move the other stay.

The frame is aluminum. You don't use any bending method on aluminum.

Clint in KY are you certain it's a 2004 Trek 3700 mountain bike. Do you know that the person you bought it from can be trusted? Perhaps they took a frame and threw on some parts they had laying around and used wheels that weren't the correct OLD. And then just suggested to you that it was a 2004 Trek 3700 mountain bike

In your picture, is the origin of the scale on the inside or outside of the other drop?

To clear up some information: I misjudged the distance before i used the ruler. It was 150 mm which was 15 mm too wide.
I had the bike upside down when I measured it, and at that time I thoroughly cleaned the bottom of the BB and checked where the chain stays joined the BB. There is no cracks or bends or any other obvious damage.
It did not take a lot of effort to compress the chain stays the bit over a half inch to get the axle to seat in the dropouts.
I thought long and hard about what kind of damage could have caused the chain stays to bend outward. Nothing came to mind.

Thanks for all the input other than the snarky replies.

And you are asking because id all those 150mm QR hub bikes out there?

Because depending on where the origin of the scale is it can make a difference. And there are bikes with almost 150mm wide and even wider spacing. So wouldn't you want to know better if you can get better information than was given in the picture.

Or must we just assume it's a 2004 Trek 3700 and that the stays were spread by some undetermined means?

Maybe it bears repeating that AN ALUMINUM FRAME SHOULD NOT BE BENT OR RESPACED!

I've had had 20"x4" fat tire bikes with 150mm drop outs. Maybe this is a good opportunity to add some cheap rear suspension with an extra fat hub and tire back there. 🥳

everything I have ever heard, seen posted, etc is that you should not cold set aluminum frames.

I thin OP needs to get a new frame to be safe

and i think 1/2 inch isn't enough movement to worry about Resetting it.
riding it sprung that far isn't a big concern, either.
the main concern is getting it reset in the right place, side wise.

got any Actual source of that idea?
resetting a minor amount of variance of an aluminum frame?
or is it just over-reaction by armchair "engineers"?
how much reset is "too much? 3mm? 5mm? 30mm?
i've reset dropout angles on several aluminum frames. not one problem.
i reset/recentered the rear triangles of a buddie's fuji MTB multiple times(double bends to either side!)... it took five years of his insanity on that bike to create a minor crack at the BB shell... he weighed about 200 lbs at the time, and regularly flung that bike off of major jumps. He SMASHED seat rails into new and creative curves (OUCHY!).. one seat was so badly bent, i was forced to CUT the rails to salvage the post and clamp.
let's hope Clint isn't that crazy.
the unjustifed paranoia about aluminum frames harkens back to the first duralumin frames, and the improper heat treating they endured, and the Way too skinny aluminum Campy cranksets... that paranoia endures, much like the paranoia around tiny finish scratches in the surface epoxy on Carbon Fiber bikes... we see those posts frequently.

there's a pair of trek 3700s floating around this area... i've recabled and rechained them several times now... one got a host of nice upgrades 2 years ago, too.
and the frames both have been reset due to a few hard crashes...
they're fine.
both are 2005 or so models... smalls, in the "Dirt Jumper" frame style.

Road frames are made of a lighter tubing, and they can have problems.. MTBs are stronger for a reason.. i've seen a few Jamises break.. i've seen WAY more new CF bikes do Ugly things, but folks sure buy a lot of new ones, eh?
i've heard several people continually trash talk stout aluminum frames because they AREN'T a fancy brand name... i just talked to a Very high end Ti frame builder that said "all frames, regardless of material, get cold set after the welding is done."
etc.

i think clint's bike will be fine after he cold sets that 15mm away... unless it moves too much, all at once.. THAT is a problem.

I agree because aluminum work hardens so quckly and you don't know where the bending is actually happening. To bend something that stiff 10mm you have to flex it 30mm or so. After whatever already happened to it, that strain could mean that a spot next to a weld fails on your next ride.

What MTB came with QR wheels and hubs wider than 135?

If there isn't one, then instead of acting like a ass, why don't you inform and teach? Your responses to other members tend to only be your self interest in flattering yourself and ridiculing others.

My responses come from the shear mass of utterly uninformed posts that people with seemingly no knowledge decide to post anyway.

I already asked you once what you were talking about with QR hubs that wide, and despite the fact that you know what i know about, you decided not to double check what you're asserting.

If I had to teach everytime someone created a completely erroneous factoid, that's all I'd ever do.

Why don't you take responsibility to not talk out of your bottom, and look stuff up when you're given a hint?

Just with my hands, I’ve squeeed across the dropouts on my Masi, Mondonico, and Woodrup, all steel lightweight road frames. All were hard to compress, no way I could cold set one with my hands. I later tried the same thing when I had mt ccousin’s classic Cannondale in my hands - it was somewhat stiffer. So not surprised, but it would be really hard to cold set a quality Al frame without massive effort. If his OLD is now 15mm too wide, that is most likely a sign of an already damaged frame.

But I’m preaching to the choir!

To the Thread Starter: Strip down the old frame, and trash it. Keep the fork, headset bearing set, Bottom bracket, and all the derailleur wires, wire guides, and brackets. Label the parts so you have some hope of re-using them. Get a better frame or frame + fork, have it checked at a quality bike shop or with a local guy who really understands frames and alignment, and build it up into a bike that you can ride. Let the old one go.

If you buy a steel frame and spend probably less than $1k for tools and a bike repair book or two you can become able to maintain a steel frame in excellent condition for a long time to come. Once the frame is straight and has brakes bolted on, you can use the frame as a wheel truing stand, then use the wheel to support derailleur alignment, then use the main frame to detect misalignment in the fork then perhaps get the for aligned by a shop with old frame tuning equipment.

Just to put an end to the nastiness, I intend to ride the bike! I ride paved and hard packed trails, and do not do jumps or downhill.
I have put some effort and very little money into this bike because I am living on a retirement budget. I would like to have a bike that I can ride for cardiac rehab to get off the treadmill and outside once in a while. I have another bike but it is marginally too big for me. The bike I bought is a 13” frame that accommodates my short inseam. A longer stem and a riser handle bar make the reach acceptable.
Some time ago I suggested a Budget Bike forum and it was rejected because some were of the opinion that it would attract too many “experts” who would disagree with other “experts”. I can see that happening. While I appreciate those who offered genuine advice, the snarky comments are very disturbing.

I bid you all goodbye.