Many are often intimidated by the AF mechanical reverse units....you shouldn't be; they are easy to maintain. Moreover, proper maintenance is a "MUST" if they are to function properly.

FOUR-POSITION REVERSE UNITS:

DISASSEMBLY: The earliest units were 4-step (F-N-R-N) units, located in the boiler of steam engines, or about in the center of a diesel chassis. That lever that protrudes from it is not an "on-off" switch.... it is a locking lever that prevents the e-unit from functioning through its 4 positions. If you do not want the engine to reverse direction when power is cut, the locking lever is pushed sideways while the engine is running forward. Now, the engine will always go forward in the future, unless you "unlock" it.  Later e-units were moved to the tender of steam engines, and the lever points downward through the chassis.

   Once you have access to the e-unit, disassembly is simple. You will see two fiber "finger boards" on it. The 4 copper or bronze "fingers" make contact with a rotating drum. These fingers do wear out because they are the site of electrical contact; they get dirty, and they will eventually burn holes right through the contact-spot on each finger. They need to be replaced before they reach this condition! To remove them, each finger board is held onto the e-unit by two tiny metal tabs, one on each end. Use pliers to straighten these tabs (which are twisted); just twist them straight, and then lift off the board. Leave the wires soldered to the old board for now. Next, remove the second finger board in the same way.
   You can now gently spread the two sides of the e-unit just enough to pop the drum out....clean the drum well, with a little contact cleaner on a tissue, or even with just some rubbing alcohol on a tissue. Before replacing it, notice that inside the e-unit, there is a "flapper" at the bottom with a brass pawl that sticks upward and out through a hole in the side of the e-unit. Make sure it operates freely; clean the rivet connection on it, and the pawl, of any grease before replacing the drum.

 It is not unusual ifor these 4-position units seem to "stick" in one position because the drum won't rotate (assuming you don't have it locked, as described above). This is usually the result of the tiny brass pawl that sticks out through the side of the e-unit. When that pawl is pushed upward to rotate the drum, it may fail to drop back downward again. Correcting this is trial-and-error, but here are some tips:
(1) Be sure you clean the rivet connection at its lower end with a quick shot of WD40, so that it rotates freely at that rivet. Do not over-spray anything; the quickest possible shot of WD40 is sufficient. Do NOT use oil or tuner cleaner on that rivet point, as it will attract dust and make the problem worse. 

 (Repeating this step in the future for normal maintenance is usually effective  anytime the pawl begins to stick.)

(2) Use a round, rat-tile file to smooth the top edge of the hole through which the pawl extends. Over years of use, the pawl hitting that upper surface can actually create a tiny "notch" in the sheet-metal edge, and the pawl now gets hung-up in it. Running the file over that surface will often correct the problem.
(3) With needle-tip pliers, give the tip of the pawl a very slight counter-clockwise twist, resulting in less metal-to-metal contact when it is in the "up" position. This needs to be done with care however, as you can break the pawl off, or bend it in such a way that it won't spin the drum.

RE-ASSEMBLY: You will need a new pair of finger boards; these are readily available, and inexpensive, but quality is critical....the length of the copper fingers must be precisely right or they will not contact the rotating drum in the proper place!

   Now just move the wires from the old finger boards over to the corresponding fingers of the new boards, and solder them in place. Be sure there is no wire or solder contact between adjacent wires, either on the surface of the finger board, or underneath.....that will create a short.  Do this before you mount the finger-boards so you avoid damaging the surface of the drum !
   Before pressing the new finger boards onto the tabs, give each of the 4 fingers a very slight downward bend; this will ensure that they make good contact with the rotating drum.  Insert the finger-boards onto the appropriate tabs, and use your pliers to twist those tabs just enough to hold the boards firmly in place. Be careful.....don't twist too far or they will break right off! Note that the upper fingers have the fingers offset to the right. The lower fingers are centered. Don't mix them up.

That's it! You've rebuilt and cleaned your AF reverse unit!

Last resort: If all else fails, then you'll need to replace the entire reverse unit. You can do so with another identical AF reverse unit, OR consider installation of a modern solid-state reverse unit. These are very simple to install, reliable and quiet, and easily removed with no permanent modification should you wish to put an original reverse unit back in the future.

 TWO-POSITION REVERSE UNITS:
   Gilbert later introduced a cheaper 2-position reverse unit (no neutral) that operated on an internal cam which was tipped back and forth by a rod that was pushed upward against it and then dropped back downward by gravity.

   Servicing these e-units is a little trickier, although you rarely have to disassemble them. If it doesn't seem to be functioning properly,
Give it a very quick shot of WD40, particularly aiming at the upper surface of the cam, and also at the metal weight if the rod doesn't seem to want to move up and down freely. Then give it a follow-up shot of contact cleaner. Allow time to fully evaporate, and then apply power and see if it has improved its operation.
This will usually solve most problems with these 2-position units.....fingers in these rarely wear out.

PARTS AVAILABILITY (all are found in my SALES CATEGORY for either DIESELS, or STEAM ENGINES):
   (a) Finger boards for 4-position e-units; (upper and lower)
   (b) Drums (not reproduced; limited availability), but rarely need to be replaced unless they have a heat-damaged (melted) surface, OR a chipped "tooth" on the cog.
   (c) Complete replacement e-unit (original/rebuilt); (specify boiler or tender version, and whether it mounts with one screw, two, or three).
   (d) Modern solid-state reverse unit; (Excellent quality, and easy replacement).
   (e) Contact Cleaner: BEWARE:  all contact cleaners are flammable, so give it plenty of time to thoroughly dry and evaporate after use. Most are also NOT plastic compatible, so they may damage plastic, or remove lettering !   Use with caution.