I know a lot of you guys have "a parts unit" my question is do you take out parts or switch them to save the integrity of the parts unit? Say for example you have two GP7s one you use for excursions and one parts unit. Now say your excursion unit needs the compressor replaced. Do you
A. Take the broken compressor out of the running unit and scrap it and take the compressor from the parts unit?
B. Switch the compressors by putting the bad ones in the parts unit and good one in your running unit, leaving a complete yet no functional parts unit?
Even when your parts unit has nothing good left you still have a complete locomotive that can be used for display and public interpretation. However, I fear too many organizations chose option A.

That's called "cannibalizing" in the Army and was forbidden because it was normally done carelessly and you're left with a piece of equipment that was totally messed up. Usually the replacement for the part taken was lost or misplaced, so the whole exercise was a waste. You were better off ordering a new part and leaving the deadlined equipment alone. That said, if that is your only option, then what can you do? Generally I would not recommend it unless you simply plan on scrapping the parts unit. It will end up being junk.

But a museum or tourist road isn't the army, and money is often the "hitch". If the objective is to show what locomotive looked like in a historic color scheme, ultimately you only require the carbody to look complete. If you are trying to tech mechanics hope to exercise a replacement or diagnosis, of even color code the internal parts of a unit, you can cosmetically replace but tag the part to show it's ka-put. As vintage locomotives become rarer, the case is often decided by corrosion damage.
On the other hand, I know of a GP9 being used as a donor of prime mover and generator to restore an even rarer EMD unit. Several Erie Mining RS units are being used as parts donors for FA restorations. Not great deal of purpose in keeping those old birds once the carcass is cleaned, is there? There was a display MLW unit in the last years that was gutted for static display, but vandalized so heavily, it was cut up in spite of the pain it caused.

Thanks Richard, I'm well aware it's not the Army, and I think I wrote that there may not be any other options to robbing parts. On the other hand, an intact GP-7 may have residual value, so it may be better to sell the unit, or trade it for the parts you need. LTE comes to mind for trades. I've restored 2 GP-9's and 2 GP-10's to service, and George and Spiro were helpful, so I have an idea of what I'm talking about.
Ned

Museums and tourist railroads are two different things. The mission of the first is to preserve history, the second is to make a profit. In a perfect world a museum would tag the old parts and put them with the donner unit so that it stays as complete as possible. In the real world hardly anybody bothers.

Ned, I wasn't implying you didn't know what you're talking about. I am simply stating the financial edge for many operations doesn't allow the luxury of ordering parts off the shelf, reconditioned or not. What's left of a donor unit at the end of an operational restoration may not be worth the effort or space to patch it up and make it look good.

Put it in steam terms if you will. Union Pacific maintains Northern #838 in the Cheyenne Roundhouse as a donor for #844. I'd love to see 838 restored to display condition one day, but having visited her in place, I don't see it coming. AOS Roundhouse has CPR 1293 and 1278. The expressed hope is to one day rebuild and operate 1278, but more likely, she'll support 1293. I'm told most of the NKP Berks in preservation gave up significant parts to restore 759 and 765. In those cases, parts were generally replaced to give the appearance of a whole locomotive, and undisturbed. There's a question mark hanging over CNR Mikado 3377 at Steamtown, used as a parts engine. There's a line of thinking which says she'll be rebuilt from the sister recipient locomotive of the same class. I want to see that happen as well. It would be great to restore all of them with newly minted replacement parts, just don't hold your breath.
And I state this in a non-confrontational manner.

Apologies Richard, my bad, I sometimes, maybe even frequently, have a short fuse.
I think the key to answer the question is: what are the two units? If they are older GM or GE units then they have some value, and it might be best to sell the lesser one to fund parts for the better one. That does 2 things (1) It saves from having to remove parts from the donor, and then installing them on the better, and after it's all said and done, it saves from having to dispose of a now sad looking junker, and possible keeping the organization from the criticism of having junk parts laying around their line, annoying the public.
If the units are almost scrap anyway, then yes, cannibalize one to fix the good one, and scrap the other when done.
BTW, I would love to get a tour of the Cheyenne roundhouse.
Ned

This is an all too common issue that I frequently have problems with myself. Text on a screen lacks the tone that I have to use so often.

It seems to me that the easiest and wisest thing to do, both in the museum and for-profit railroad operations would be to swap the bad part from the operational unit for the good part from the support unit. Then you have an operational unit sooner than later, and time to repair the bad part on the support unit at a later time. That's what would make the most sense to me.

Another consideration is do you have a wealth of surplus manpower to replace the parts on the donor locomotive with the unusable parts? Or would the time be better invested on other equipment?

For our USATC Consolidation we were permitted to remove some parts from an identical display locomotive in a park. Our philosophy was that we wanted to respect the donor locomotive and make it look complete and also show appreciation to the government unit responsible for it. (The caretakers of the donor locomotive referred to it as an organ donor.) We fabricated a replacement whistle, injector, safety valves, buffer, and power reverse to replace items that had been long missing, were needed for our overhaul, or that we could use for spares. We believe the extra work was a good investment.

A consideration is do you have surplus manpower to replace the parts on the donor locomotive with the unusable parts? Or would the time be better invested on other equipment?

For our USATC Consolidation we were permitted to remove some parts from an identical display locomotive in a park. Our philosophy was that we wanted to respect the donor locomotive and make it look complete and also show appreciation to the government unit responsible for it. (The caretakers of the donor locomotive referred to it as an organ donor.) We fabricated a replacement whistle, injector, safety valves, buffer, and power reverse to replace items that had been long missing, were needed for our overhaul, or that we could use for spares. We believe the extra work was a good investment.

How did you make them? Metal or epoxy? Rubber moulds?

They look good, good enough for a static display loco.

Weldments from pieces of plate, pipe, and tubing.

There is--well, was--at least one loco preservation group in the UK that bought a sister example of the loco they were specifically preserving just for spare parts (BR Deltic 55.022/D9000 Royal Scots Gray, the first production unit, with 55.016 Gordon Highlander as the spares unit), that ended up restoring the second unit to operation as well. The other Deltic group, the Deltic Preservation Society, started by fundraising to acquire one, ended up buying two, and then ended up buying a third that an individual had purchased but found himself over his head with--as well as seven spare engines and generators and just about every imaginable spare possible, up to unique tools and lifting beams fabricated by shop forces at Doncaster.

And I know of another preservationist bunch that bought a spare EMD E-unit just to have a spare cab/nose in case of the inevitable grade-crossing accident with their mainline-capable E8s.

I might strongly suggest that unless you have lots of storage track, lots of free labor, and sources for replacement parts, anyone pursuing this strategy should plan for dismemberment and cannibalizing immediately after arrival, lest it sit on a back track for years giving false hopes or delusional fantasies to every young passing railfan. (In that reapect, I can hardly think of a better donor unit than a BCOL M420B or one of those BN "B-units.")

Current status updated with input from Frank Hicks. Title Line edited. Here is a convoluted story:

At IRM, Chicago Aurora and Elgin wood car 321 was originally purchased as a parts car for TM interurban 1129. It was gutted and turned into a storage warehouse. Restoration of the 1129 never seemed to get very far, and the restoration crew stopped progress. Later, when CA&E 318 became available, IRM scrapped it for parts to restore the gutted 321. About a year or two before IRM acquired several cars from the Trolleyville collection, 321 restoration halted after determining that it would be a tremendous undertaking to repair the physical damage and weathering to the unit. The IRM wood car fleet expanded from just the two original cars (309 and 321) to a 6-car collection. 321 is now a parts car for the fleet of 5 operating CA&E wood cars. The trucks were swapped out with those from the 319. Generally, I understand, parts are not reinstalled, but stored in the car with tags indicating what it is and where it belongs.
https://hickscarworks.blogspot.com/2008/03/history-of-ca-321_02.html

That's a good looking car.