There are several weak points in the Gilbert SIT units. These existed in some AF S-gauge engines from 1946-1947.
Brushes and springs are very small, and subject to breakage, wear, and heat breakdown.
The bakelite brush brackets on the back of the motor also tend to be quite fragile, and frequently crack. Bakelite is not easily repairable.
SIT armatures and fields were wound with very fine enameled wire, and are easily damaged.
Early SIT units used a rubberized cloth bellows. It's not unusual for the rubberized coating to dry out, crack, and become ineffective.
Later units, and repaired units, used a diecast metal cylinder and piston in place of the bellows, but the cylinders typically deteriorate and crumble.
And, of course, the internal wick and heating element can be either burned out or hardened.

Brushes, brush springs, bellows material, and cylinders are available as replacement parts. The bakelite brush brackets are not. Armatures and fields are not available as replacement parts, but can be rewound if not otherwise damaged.

In addition, SIT units can be upgraded with can-motor drives which operate much more smoothly, quietly, and efficiently than the original motors. (See our "Can Motors" page.)

A. DISASSEMBLING THE SIT UNIT:

This will be necessary in order to (a) replace the internal smoke wick; (b) replace the bellows; or (c) install or replace a cylinder.
First, remove the hex-head screw that attaches the piston lever to the piston gear. A 3/16" nut driver will fit this screw-head.
It would be wise to unsolder the wires connecting to the smoke unit cover, as these are very fragile if stressed. (Make yourself a drawing of what wire goes where, for later reference !)
Next, compress the bellows so that you can see two flat-head screws underneath it. These 2 screws hold the SIT unit to the tender frame. Remove them.
Unsolder the wires to the tender truck rivets, so that you can completely remove the SIT unit from the tender frame. Set the frame aside.
No matter whether you are going to do (a), (b), or (c), you need to remove the cover plate to the smoke chamber. If you are careful, you may be able to unwrap and unsolder the smoke wick leads which rise up through the two solder lug holes. If not, and you break them, you will probably have to install a new wick. Quite frankly, I think it is worth doing that anyway, while you have it all apart! You don't really want to go through all this again in the near future!
If (a) is your task, you are now ready to do so. Turn the unit upside-down, and you can remove the screws which hold the smoke chamber to the frame. Then proceed to replace the wick in the usual manner (see Clinic #1).
If (b) is your task, skip now to the next section (B), below.
If (c) is your task, skip now to section (C), below, and follow the instructions there.

When all of that is completed, re-assemble everything in reverse order.

B. BELLOWS REPAIR:
The square fiber board on each end of the bellows is easily removed. Remove all old bellows material. Then clean the edges of all residue, and lightly sand them with fine emery paper.
Replacement bellows material is wrapped tightly around all four edges of one fiber board, after a thin bead of fast-setting glue is applied to the board's edges. I have done this successfully with "superglue", but if you're not comfortable using that, try rubber-cement.
Once it sets up, apply a thin bead of cement to the edge of the second fiber board (be aware of orientation of the tab for later attachment to the gear).
Then carefully insert the fiber board into the bellows, and be sure it is firmly wrapped tight around it. Set it aside to set.
Finally, apply a thin bead of glue along the seam, and pinch it closed to set.
Once all glued joints are dry and tight, re-assemble the unit and test.

A Bellows Jig is available that makes this rebuilding much simpler. The jig clamps both fiber boards in place parallel to each other, and the proper distance apart.
Glue is them applied to all edges, and the bellows is wrapped around both boards at once. The seam is glued, and allowed to set before the bellows assembly is removed and re-installed.

C. CYLINDER REPLACEMENT:
A 5-piece cylinder kit is available which replaces the bellows assembly. The kit consists of a smoke piston, cylinder, attachment clip; piston pin, piston lever, and full instructions. Assembly is quite easy.
This converts the SIT unit into a piston unit similar to the standard smoke units found in most later engines.

SMOKE-IN-TENDER CYLINDER CONVERSION KIT
This kit includes all five parts required to make this conversion.

NOTE: there are two different versions of smoke chambers.
On version (1), there is just a hole in the face of it, through which the bellows plate inserts and attaches with a nut.
On Version (2), the face of the chamber has a notch cut midway down.; on this version, the bellows plate slides down into the notch and attaches with a pressure-clip.
Remove the existing bellows assembly by removing the screw which attaches it to the fiber drive-gear.
Detach the other side of the bellows by removing either the clip or the nut which holds the bellows plate to the smoke chamber.
Assemble the cylinder conversion, which is made to fit version (2).
If you have version (1), you will have to use a Dremel tool, or similar, to turn the hole into a notch, by slicing the chamber face downward from the top until you reach the hole. The notch you cut should be ¼” wide, to accommodate the “neck” of the replacement cylinder.
The smoke piston lever attaches to the fiber drive-gear with the same screw you first removed.
Test with wire jumpers to the two tender trucks before re-assembling your tender.

D. SMOKE WICK REPLACEMENT:
Replacing a SIT wick and element is very little different from replacing one in a later smoke unit. Our instructions for doing so are found in Repair Clinic #1, so we won't repeat it here.

PARTS AVAILABILITY: You should be able to locate all parts referenced in this clinic as "available" in our online AF PARTS CATALOG, under "SMOKE UNITS".