Removing lock on a locked-open safe

... I don't know, but it seems probable that it would depend on the model / brand / year. Do you have that info?
Directions for a Stack-On fire-resistant guns and documents safe say the door can be removed, as follows: "Open the door, remove the hinge caps. Lift the door carefully -- you will need a team lift, the door is very heavy and cannot be replaced."

Many safe doors can be removed rather easily if they are open. Usually there are threaded caps on the hinge pins. Remove them and lift the door off the pins.

Have you check to see if you can just lift the door off the safe? Sling with your forklift?
I bought a couple safes way back in the day. Both were a-rated, and with door open it would just lift off... its not light. I could not dead lift them. I had to use a sling and a lift.

[ ... ]
Brand? It might make a difference.
But most that I know should work well with the lock fully removed. The lock usually engages the lever which opens the door to prevent it rotating.
I'll give what I would do if the lock were a Sargent & Greenleaf (which I am most familiar with). How you would apply this to other brands will vary, of course, but most of this should work similarly.
1) Remove plates to give access to the back of the lock. (Or maybe the entire back panel of the door -- I'm more used to security file cabinets.
2) Remove the four corner screws on the back plate of the lock itself, and withdraw the back plate elsewhere.
3) In the back of the spindle, you should see a flat piece of metal in a 'J' shape which engages a groove in the spindle and in the back rotating part which also has a sliding block in it.
Withdraw this.
4) Unscrew the back rotating part from the spindle -- by turning it or the knob. (Depending on the size of the safe, you may need someone on one side or the other to keep a part from rotating.
5) Withdraw the knob and its spindle.
6) Check whether there is a threaded ring on the outside end of the tube and remove it if so. This may retain the index plate behind the dial.
7) Remove the four screws inside the corners of the lock body, and withdraw the lock from the back.
(Save all the parts for later, when you can set the combination, likely to the default and reinstall it.)
8) *IMPORTANT* -- make *sure* that the locking lever works with all of this removed before closing the door.
Now you can close the door and be sure of being able to open it again.
Now -- if the lock is a S&G (as above) you will see a small square hole in the backplate which can line up with square holes in the three discs. There are several of the keys to fit these, which vary with the particular model of the lock. The key looks like a square Allen key with a single ridge projecting from one corner, and cut away in certain places to allow it to rotate. It is possible to tweak all the discs into line with the back in your hand, put the key in, and then use that to start the process of resetting the combination.
If it is a Mosler (at least with the security file cabinet versions), the disks are two part plastic, which can snap apart, be rotated, and snapped in again, with an index pointer lined up with the desired combination for that disc. Maybe, on older ones, the discs might be brass or bronze -- making it easier to X-ray the door (with film on the inside) and get a starting point with finding the combination. (At least, I suspect that is why plastic is currently used. :-)
I don't know what else to expect with other brands of locks.
Good Luck, DoN.

A little followup. I got the job done and the safes are in my place.
The safe that is locked open is great and beautiful and changing the combo should not be a problem. I just do not know how to do it, but plenty of materials are available to learn.
I started opening one of the locked closed safes. I started with an old black Diebold. Someone already tried to open it, but that person did not understand how safes work.
Here are some pictures, more to be added later:

Cut a big hole on the side with angle grinder and, using a big prybar, broke the drywall material between metal.
The safe has drawers inside.
It is pretty obvious at this point that it is likely empty, but I will press on with it anyway as it is fun to learn how to open safes. Has an old adventure style appeal.
Anyway, I can touch the lock from the inside, there is little room to work due to drawers, but I managed to put together a makeshift tool from a 1/4" wrench and a flat screwdriver bit. I removed two screws and had to give up due to darkness falling.

i

Yep, exactly as you said, everything worked out great today. Already cracking one of the two safes that are locked closed.

Well, I have no idea how much such a safe would be worth. But, it seems that opening a safe by just trying all combinations would not take terribly long, if it is just a 3-number combination. (5 number safes would take maybe 2500 times longer.) Some comments on this are in Richard Feynman's book, which is a great read if you haven't read them.
Jon

Both locked closed safes are, in my opinion, worth nothing. One is already ruined as the hinges are cut off. Another is not lookin super great and not worth a few hours of picking. Since we do not know how to do it, it will take a long time.
Plus I want to practice safecracking, if feels vaguely exciting, like riding a rollercoaster. I know that it is not illegal to break into those safes, but it feels a bit like an adventure.
i

On Oct 2, 2016, Ignoramus4179 wrote (in article):
Isn?t it the most fun if you blow the safe with nitro, like in the good old days of safecracking jobs?
Or at least a thermal lance?
Joe Gwinn

Iggy If I saw a locked safe with the hinges removed I would assume there might be something of value inside. I would know that unlocking it was dangerous because the very heavy unlocked door will fall out of the safe. Take care!

I am using an angle grinder and hand tools.
i

Good point. Look at this picture:

you will see that hinges are cut off on one side only. It is a bit safer, the door with the lock will not fall.
It was a safe in their workshop, not the office, so I am hoping that it may hold some expensive consumables like sewing needles or some such. (it was a clothes sewing company)
We all know, of course, that cutting hinges off any half decent fireproof safe will not open the doors.
i

It's a shame it can't be used. A machinist I used to know kept all his expensive tooling such as gear shaping cutters, hobs, and broaches in a similar fire safe so in the event of a fire they wouldn't be damaged. An old boss of mine kept some company hard drives in a fire safe but he didn't know the difference between a fire safe and a data safe, a fire safe is good for paper material but a data safe has to keep the digital media at a much lower maximum temperature, IIRC below 40C for at least an hour.

...
They're also very useful in shop for fire safe for combustibles.
Got the missing hinges, Iggy? Once in it, I'd surely be tempted to keep it around for such misc. uses.

They actually forgot all their company keys in a drawer of a locked-open safe. I found them upon the safe arrival to my place.
I emailed them. ;-)
i

I hope you included the profile so they can know which new blanks to file down.

Igor,
You are missing a great opportunity. Us locksmiths are often in need of a talented machinist or welder. Contact some of your local guys and offer a t rade of services. Not only will they be able to give you a working combina tion, but they can also do a proper service and inspection to make sure the safe will not fail after you lock your stuff inside.
Roger

Roger, thanks. A little update on the safes.
I was able to open one of the two locked-closed safes by cutting a hole in a side close to the lock, unscrewing the lock cover and removing the lock mechanism.
Inside, I found a bunch of meaningless profit sharing meeting minutes, other papers, two old certificates of deposit for $132,000 and $140,000, and $11.02 in cash.
I cut a similar hole in the side of another safe, however I was less lucky in opening it, as the safe had double doors and I had inside access only to the inner door.
However, I was able to reach inside with my hand and look and feel the drawers. I became convinced that the safe is completely empty inside.
I gave away both of these safes to craigslist scrappers.
Cracking them open was fun, but not profitable (as expected).
At least I did not have to pay for their disposal.
i

On Oct 2, 2016, Ignoramus4179 wrote (in article):
Killjoy. We wanted a video, from a safe distance.
Joe Gwinn