Recently purchased an early '80s bike as a project, and I was just now going to pull the cranks off when I ran into some trouble. I threaded the puller in, and it went in pretty well, but it stopped sooner than it felt like it should have. Moderate pressure with a wrench made little progress, so I backed it out and looked at the threads, which appear to be somewhat damaged. As a test, I put the bolt washer back in and tried to thread the puller in again, and it stopped at the same point (i.e. did not contact the washer), so it's definitely not going all the way in. I'm pretty new to this, and what I've read says you should have the puller all the way in or risk stripping the threads.

Is it safe to try pulling the crank arm (I'd estimate the puller is 2/3 to 3/4 of the way in), or if not, is there some way to realign/repair the threads?

Yeah, those sure do look mangled. Any shop worth it's weight will have a thread chaser for those (it's a common problem.) If the thing is actually 2/3 to 3/4 of the way in, I'd definitely pull the crank arm off (while lightly tapping the side of the crankarm with a lead mallet or any soft blow hammer.)

Alternatively, if you don't want to do either of those, go grab yourself a 3 or 4 arm gear puller at your local harbor freight or heck, I think wal-mart carries them. That'll pull the crankarm right off.

I would only use the gear puller if you wanted to get a new crankarm; since they bear on such a small area, they have a tendency to damage aluminum arms. They work great, but if you want to re-use it, try drill-chuck wedges, designed specifically not to damage the thing they are removing. A "pickle fork" style tie-rod remover works great as well, with lower incidence of damage to anything that matters, and can often be found for $10-12 at an auto parts store. The Hozan tool (the exact same thing, shorter!), since discontinued, would set you back 2-3x that!

Also, that's generously 1/2 engagement on that tool, based on where the thread is damaged. Depending on how tight of a fit there is between the original threads/the puller, it may be enough to remove it, but I wouldn't chance it. Bicycle Research makes a tool that many shops have (as the poster above said) to clean messed up threads.

FYI, if you're shooting for destructive removal, hacksaw/rotary tool a notch above the spindle eye and whack it with a cold chisel. Provided you have that stuff already, it's free and quick.

From your photo it looks like the thread mangling starts about or a bit less than one full turn of the thread. If you got it in more than one turn you
likely contributed to the mangling. You can mechanically straighten or remove the mangled part of the thread with a narrow tool tip, eg 1/8" straight
blade screwdriver or cautiously dremel down the mangled part and probably salvage or at
least remove the crank. A google on L hand square taper crank arm brought up Niagra Cycle with cranks for $10-20, Amazon will also do as a source.
This will give you a minimum cost to
look at: 1)chase thread if you can find a shop with a chaser; 2) try my approach (I did this to salvage a DA hub cassette retainer thread, it was Ti,
but a bit of scraping and dremel work got rid of the bunged threads and the cassette retainer has worked well since (about 2 Kmi so far));
or 3) buy a new crank. If there is a coop in town that would be another source. The crank puller would probably serve as a thread chaser
IF you could get it started right, but I doubt that will work without some thread scrape/grind before hand. FWIW the puller will not work if you put the crank fixing
bolt washer in place, the working end of the puller has to push directly on the axle hole. Putting the washer in and torquing the remover will strip out what threads are left in the crank.

First of all, the advice to not try to pull the crank without the puller screwed all the way in is accurate. If you're positive that the puller is the correct thread pitch and made of quality steel you can use it as a thread chaser. Kerosene is the best thread lube to use to tap aluminum but any type of oil would be OK. Thread the puller in a 1/4 turn at a time back it out a little and repeat until it's bottomed out. Of course if you have to use gorilla strength then you'll need a proper thread chaser but try using the tool first.

I've had better luck with the gear pullers than with the tie-rod end separators. I do think the drill chuck removers would work well if the distance between the arm and frame was small enough.

EDIT: As for the suggestion to use the crank puller as a thread chaser? No, that's a terrible idea. That never works right. It's very difficult to thread them on correctly when the threads are even the slightest bit messed up.

An alternative way to remove the crank is to leave the fixing bolt off and go out and ride the bike around a nearby hilly loop until the crank comes off under
pedaling torque. Not the best way but cheaper than a gear puller. Gear pullers need a square surface to pull against which many sq taper cranks don't have
but yours does. Can't disagree with corrado on using the puller as a chaser, but consider that a thread chaser has sharp hard cutting edges and unless started
properly will just cut a new set of threads over the old ones. The puller is dull enough that frictional forces will tell you something is wrong. A very careful
approach is needed either way.

To each their own, huh? I've used a gear puller on several cranks, and it's marred each. To be fair, a couple of those were profiled such that they didn't have a 90 degree edge, which combined with the short lips on the puller, contributed greatly to the damage. They work great (with some minor profiling) for removing Campagnolo ultra-torque bearings, though!

The beauty of a wedge type puller (be it a drill chuck puller or otherwise) is that if the gap is too big, you can build it up with a shim on one side, since the wedge just needs something to push against.

I stripped out a crank arm's threads last summer when I wasnt paying enough attention to what I was doing. It was pretty old and I think the threads had gotten damaged before I got the bike. I ended up having to cut the crank arm off which is of course the last resort. That's all I've got add, lol..

I don't think it's cheaper than the correct puller unless you plan to replace the entire crank anyway. A square taper crank "removed" that way will have the square recess distorted to the point that it will never fit properly again.

Well, it turns out I was able to (carefully) thread it all the way in with the help of a wrench and WD-40, and the arm popped right off, so problem solved!

That was going to be my first suggestion as well; glad it worked out for you...

WD-40 is basically Kerosene (or more accurately, it's refined twin OMS) with a little lube dissolved in it. So, if Kerosene is the tapping fluid of choice for Aluminum, it's what the OP actually used.

My comment was more aimed at using the crank tool to chase the threads than what to use for cutting oil, but thanks...

What brand/model bike is that?

It's a Motobecane Mirage, 1983 vintage.

Saw a Giant RS930 with similar issue and paintjob. Not the same bike though. Phew!