My notes to friend who has a gauge like this, incase he needs to give it a clean etc.

Here are some photos of tools etc to repair the sunnen gauge. The wedge shaped piece of 3/16 rod is to undo the 2 latches that hold both sides of the cases together. These go into the case through the 2 top capscrew holes, from the head end. The side with the long taper, follows the outer wall of the case, while the shorter taper is against the latch. The turned section of the 3/16 rod is 3.8mm diameter(Ø0.15 inches) and about 1/4 inch long. Carefully insert the rod until it reaches the latch, and gently push. You will hear it click and clunk. The LH side latch is a bit restricted by the shaft for the setting lock. Both latches can be undone. Before the shells can can apart, quite a bit needs removing first.
The spring that runs through both sides needs to be undone or removed before separating the shells. 2 counter sunk screws in the base, and the end plates and the rods removed as well as the side tool holder. 2 screws securing the inner to the front only need to come off,if the smaller side shell is to be removed or the mechanism is removed completely. I used the 4 long capscrews as handles to remove the large side cover. There are 4 dowels in each corner, so may need a little wriggle to seperate.Great care is needed not to damage any of the very fine wires or the indicator arms/needles or the flexure spring assembly. Not shown in the pictures is the long piece of 3/8 unc threaded rod, to hold the mechanism as it comes out of the small side shell and is used to place the mechanism back into the small shell side.
Putting back together, after service/clean of the mechanism.
When the mechanism is mounted on the wood block and suiteable 3/8 screw,(for easier clean and service) eyeball the front 2 screws and see if the size adjustment handle and 1/2 bushes look like they will line up with the case.I used the 2 long screws to assist in the position. Adjust the handle until it does.
I used wood blocks to elevate the gauge to protect the gauge fingers from the bench for assembly and taking apart.
Using a bit of 3/8 rod as a handle, carefully place the unit into the case.Be mindful of the sprung flexure side on indicator arm side. Try to hold onto the solid part of the mechanism frame. It has to sit on the ledges in the bottom of the shell and you will see the other areas that need to align, like the key holes for the markers and the 2 front frame capscrews. Insert base mechanism screw loosely. Next stand upright and insert and just nip the front frame scews.
Set the latches to the open position, and insert the 4 corner dowels.
Place second shell on and loosely insert base mechanism screw.
Close the latches with a allen key or suiteable rod from the threaded insert side of the top two screw holes.
Insert the 4 long capscrews for the case and initailly just nip up. Tighten the front 2 mechanism screws and then 4 case screws. Then tighten the base mechanism screws . Check that the gauge moves from the finger.
Place the long spring in between the 2 floating plates. I used a piece of wire to draw it through after 1 side was attached. Then proceed to assemble all the other parts that need to be added.

All photo's are in my album Sunnen.