Cleaning and servicing a Mamiya Sekor 100mm f2.8 shutter

What are we working on?

• Mamiya Sekor 100mm f2.8
• Mounted on a Mamiya Press
• Slow shutter operation, with visible grease on the blades.
• Grease is also present on the aperture blades.

This operation can do as much harm as good. It requires shutter-speed testing equipment and several suitable tools. I cannot take responsibility for the outcome.

Before starting, I recommend reading the entire procedure so that you do not get stuck halfway through because you are missing a tool, for example.

Dismantling the parts around the shutter

Start by removing the focusing assembly, leaving only the shutter.

Remove the rear cover; it is glued in place. (photos 1 + 2)

Next, use a lens spanner to remove the ring retaining the rear optical group (3). Take care when removing the rear group; use cotton gloves to avoid scratching it.

Underneath is another ring to unscrew with the spanner, using its round holes. (4)

The two parts can now be separated. Be careful: the flash-sync socket wire runs between them. Remove the large, thin washer with holes. (5)

Remove the flash-sync socket and its two small screws with a small flat-blade screwdriver. Then remove the small black aperture-control handle, held by one screw. (6 + 7)

The front optical group then unscrews, revealing the shutter blades. (8)

Before continuing, I recommend checking the shutter speeds with a tester. Later in this procedure, the speed-governing assembly must be removed and the speeds readjusted. Their accuracy depends on the precise position of this assembly inside the shutter. If your speeds are correct, record its position very accurately. A useful English-language tutorial on adjusting shutter speeds is available here: https://www.filmshooterscollective.com/analog-film-photography-blog/sticking-slow-shutter-speedshow-to-fix-it-brett-rogers-1-13

Next, remove the aperture-control ring and shutter-speed selector ring, starting with the three visible screws. Each screw has a washer underneath. The first ring can then be lifted off. (9 + 10 + 11 + 12)

Remove the small screw securing the gold-coloured notched threaded ring, then remove the ring itself. (13 + 14 + 15)

Lift off the screw-carrying plate. Its underside has a notch that locates over a tab on the shutter’s upper plate. The shutter-speed ring can now be removed to expose the mechanism below. (16 + 17 + 18)

Opening up the mechanism

From this point onwards, I strongly recommend taking as many photographs as possible before removing each part. At every step, check what has moved and how the springs behave before and after a part is removed. Make close-up videos while moving each part. You will thank yourself during reassembly.

Start by removing the shutter’s upper plate. Note the positions of the several rods that interact with it. (1)

Underneath, remove the cocking arm, carefully recording how its gear teeth mesh with the wheel. Unhook the spring before lifting the arm. (2 + 3 + 4)

Then remove the toothed wheel of the cocking spring. (5)

Remove the disengagement arm. Its retaining screw also secures the speed-governing assembly below and holds a spring against the arm. Carefully note the spring’s position, the arm’s position and the tab inside. (6 + 7)

Before removing this screw, record the exact position of the speed-governing assembly: its position determines shutter-speed accuracy. You can make a fine mark on the top and measure the gap between the assembly and the outer housing with a micrometer. Without a micrometer, you can insert a small flat-blade screwdriver into the gap and note the exact point at which its width matches the gap. (8) Then remove the screw and the arm. (9 + 10)

The speed-governing assembly (11) is held by a second screw, partly hidden underneath (12). Removing it reveals just how much grease has accumulated below… (13 + 14) It will soak for a few hours in Zippo lighter fluid or isopropyl alcohol, followed by gentle cleaning with a toothbrush. Before refitting, the wheel pivots can be lubricated very sparingly with a suitable lubricant.

Next, remove the release lever. It is connected to a fairly strong spring. Carefully record its position and the positions and movements of adjacent springs. (15 → 21)

Remove the locking lever, noting the spring’s position. (22 → 26)

Then remove the three-armed part, noting the springs. (27 → 29)

Observe the movement from X to M and the part pushed by this movement. Note that the spring we are about to remove sits behind it. (30)

Remove the screw retaining two springs. Carefully record the position of each spring leg. During reassembly, the lower spring goes behind the X/M selector tab. (31 → 35)

At this point, we could move to the next section and lift the entire plate out of the housing. However, refitting it would mean holding back two springs while lowering the plate, which can be tricky. To make reassembly easier, we can remove the part that interacts with the X/M selector.

First remove the top screw. It retains a small plate with a spring underneath. Note the path of the spring. The gear and spring can then be removed. (36 → 40)

Remove the small spacer, then the small toothed wheel. (41 → 43)

Only one removable part remains, but it is awkward to refit. I prefer to leave it in place. Its spring will need to be pushed back during reassembly. (44)

Shutter blades and diaphragm

Before continuing, photograph the positions of the blades relative to the housing. We will now remove the inner plate by undoing four screws at the rear. The X/M selector may hide them. Record each screw’s position individually, as their lengths can differ. (1 → 4)

The blades are greasy and stuck together, and will be removed for cleaning, so I do not need to keep them in place. I lift the plate out through the front, pulling on its centre and freeing the remaining springs. If you want to keep the blades in place, leave the shutter face down and lift the housing away instead. This is not easy if you left spring-loaded parts in place during the previous stage. (5)

When I lift the plate, the first shim stays stuck to it because it is covered in grease. Note its position; it must go back in the same place. (6)

A cotton swab wiped over the plate clearly shows the accumulated dirt. (7) Carefully record how the blades are positioned and overlap. (8)

Remove them and clean with a soft cloth and isopropyl alcohol, wearing gloves to avoid fingerprints. (9 + 10)

When grease has reached the shutter, the diaphragm underneath is usually affected too. Before dismantling it, record the upper plate’s position, noting the notches relative to the housing. (11) Observe how the blades work, then remove the retaining screws. One screw resists, so I apply a tiny drop of WD40 with a syringe to help release it. After 15 minutes, it comes free. (12 → 16)

Try to lift the plate gently without moving the blades underneath. Then record their positions and how they overlap. (17 + 19)

Remove them carefully: they are very fragile and bend easily.

The plate underneath must also be removed and cleaned. Note the small pin on its underside, which fits into the guide controlling the aperture. (20 → 22)

Remove this guide from the rear by undoing three screws and removing a small spacer. There is grease everywhere… 🙁 (24 → 29)

I then remove the X/M lever, which is greasy too. (30)

The housing is now empty. I clean it with a toothbrush and isopropyl alcohol, then clean the other parts with isopropyl alcohol. (31)

Once everything is thoroughly clean, reassembly can begin in reverse order.

Check the blade and plate positions, and the pin relative to the slot. Place the assembly on a support, then lower the housing over it, engaging the plate’s pin in the slot. (32 → 51)

Good luck 🙂