I have an older fillet-brazed steel MTB frame that has 130mm spacing. I would like to respace it to 135mm. What's the best way to go about this?
There are a few ways to go about this. A good way is to take a fine piece of string or strong thread and tie it to the right rear dropout somewhere. Bring the string toward the front of the bike and around the head tube and back to the left rear dropout. Pull the string quite tight and tie it to the exact same spot on the left dropout that you tied it on the right dropout. You now have a reference line to gauge frame alignment.
Next you need a decent metric ruler. Use it to measure the distance between the inside surfaces of the rear dropouts. If they're perfect, they'll measure 130mm inside to inside. Continue checking the alignment by measuring from your string to the seat tube on both sides. These measurements should be exactly the same, too. If all the measurements check out you know that the dropouts are the correct width and the rear end of the bike is aligned dead-center with the frame.
It's your choice how you want to spread the dropouts to 135mm. You're only adding 2.5mm to each side, which isn't a lot so you might be able to simply stand behind the bike, grab one dropout in each hand and pull outward to achieve this. It really depends on your strength and the type of tubing, but I'd say about 75% of the time that's all it takes. You can check your progress by measuring with the ruler to see if the 130 is now 135. If you can't do it yourself, you could have a friend help with each of you standing across from each other with one hand on one dropout for pulling and the other hand on the opposite chainstay for bracing. With 2 people you will want to start with easy pressure, check progress and pull a bit harder. But, be careful not to pull too hard and overdo it. Ideally you'll only bend it once to just about 135 and then fine-tune from there.
If you go over, you can squeeze the dropouts together to bring it back. In most cases it should only take a couple of tries to get it right. Then you'll check to ensure that both sides moved an equal amount by measuring the distances to the seat tube from the string. If they don't match, you pulled too hard on one side and you'll need to move the rear end again to center it and maintain the 135mm spacing.
Other ways to bend/align the rear end include using a turnbuckle that fits between the dropouts. You may need to add blocks of wood to the turnbuckle, but once it fits in there and will stay put you can turn the center of the turnbuckle and it will put pressure on the dropouts and spread them and that's a way to do it evenly and carefully. Just check frequently to see how you're doing. You'll have to go past the 135 because the frame will "spring" back.
If the bicycle is stripped down to the frame you could also clamp the frame by the bottom bracket in a sturdy bench vise. Supported this way, you could use a long lever like an oar to pull first one side over and then the other. Just be sure not to rest the end of the oar on anything that can dent or get damaged when you start pulling on the oar. You could put a block of wood between that end of the oar and the bike if you're pushing against the frame.
One thing to keep in mind. The dropouts need to be exactly parallel to each other too. There's a special tool for checking this that shops carry. When you're resetting the rear end be careful not to exert too much pressure on the dropouts themselves. None of the techniques I described would do this, but if you got creative you might bend the dropouts out of alignment with each other and you'd then want to visit a shop and have them realign them, and probably check your cold setting, too, since if the dropouts are crooked, the spacing is probably not as exact as it should be.
Happy frame aligning!