Zlatoust K-43 Caliber: Pocket Watch Service and Mainspring Repair

The Zlatoust K-43: Overhauling a 1955 Pocket Caliber and Repairing Its Notorious Mainspring

"He who wants to, finds a way; he who does not, finds an excuse."
— Socrates

Everyone knows what a "diver's" watch is — a watch for recreational and sport diving. Far fewer people know what a watch for commercial divers is: a watch you can take to a depth of seven hundred metres, intended not for the sport diver but for the professional working in technical and commercial diving. Exotic? Certainly. Yet such watches were once built in the USSR, at the Zlatoust Watch Factory. The movement inside them had to match the task, and it did — the caliber K-43, on fifteen jewels. But this caliber was not fitted only to diving watches. It also powered the splendid "Kirovskie" pocket watches, which in the early 1950s were renamed "Zlatoustovskie." It is those pocket watches we will discuss here. The movement is a fascinating one — though it carries a single flaw, one that did not spare even this example. More on that below. If you would like to see what these and other Soviet watches look like in the metal, the catalogue is a good place to start.

Here is what the factory specification tells us. The Zlatoust Watch Factory caliber K-43 (15-1), 15 jewels, was made to GOST 918—53, with hour, minute, and subsidiary seconds hands. A movement 43 mm in diameter (at the mounting bezel), running on 15 ruby jewels, with a duration from a single full wind of no less than 32 hours and a mean daily rate of ±45 seconds.

As noted, this gentleman wore different clothes over the years. I have never come across him in his diving dress — and had I done so, I would scarcely have dismantled such a watch: an exposed little "thermonuclear reactor," with its radium dial, is no welcome guest in a city flat. But a K-43 in the perfectly respectable dinner jacket of a pocket-watch case is here on the bench. The movement, like the watch, was produced in the first quarter of 1955. The case is elegantly shaped, two inches — practically 50 mm — across, and 17 mm thick at the highest point of its domed crystal. The dial has survived beautifully, having taken on a faintly noticeable ivory tone; rather distinguished, in fact. This style was designated "Design I": a chromed case 49.8 mm in diameter, silicate crystal, a silvered dial fully numbered 1—12 except for the 6, minute and seconds tracks of sixty divisions each, with every fifth minute division numbered 13—24, the seconds scale marked 10, 20, 30, 40, 50, 60, all figures, scales, and lettering in black, and oxidised hands.

Zlatoustovskie pocket watch with a strongly domed crystal, shown from the front under macro lighting — The strongly domed ("domed") crystal is obvious here. Macro photography exaggerates the case marks considerably — in the hand they draw far less attention.

The factory emblem on the dial. Curiously, there is no "Made in USSR" inscription anywhere on it — yet the dial is entirely original. We will examine it more closely later.

Close-up of the silvered dial showing the Zlatoust factory emblem and black numerals — The factory emblem sits on an original, untouched dial. Notably, it bears no "Made in USSR" legend — a detail consistent with its early, domestically oriented production.

As they say, you are met by your clothes but seen off by your caliber. We open the movement cover. There he is — the beauty! A light, elegant decoration runs across the parts. The movement is 43 mm in diameter, roughly a centimetre tall, with bridges 4 mm thick. There is a sense of indestructibility and monumentality about it, as though one had stepped inside a tank or the gun turret of a battleship — everything solid and dependable. Note first the jewels: they sit in gilded chatons (military-inspection acceptance, perhaps?). Look at the quality of the crown and ratchet wheels. They could have been left flat; instead each carries turning, milling, and polishing. No labour was spared. On the barrel bridge are the factory mark, the date of manufacture (1-55), the jewel count (15 jewels), and the caliber number. Remember that number — it becomes very interesting later.

Open caseback revealing the K-43 movement with gilded chaton jewels and engraved barrel-bridge markings — The K-43 laid bare: gilded chatons around the jewels, decoratively turned crown and ratchet wheels, and the barrel bridge stamped with the factory mark, the date 1-55, the 15-jewel count, and the caliber serial. That serial will matter shortly.

This is how the winding wheels and the jewel-count marking appear.

Detail of the winding wheels and the engraved jewel-count marking on the bridge — The winding train and its engraved "15 jewels" marking — finished to a standard well above what a utilitarian movement would normally receive.

Let us look more closely at the bridge of the pallet and seconds wheels. We will return to it during disassembly, but for now note the construction of the jewel chatons and how the wheels sit in their jewels. See how the wheel pivots are made extra-long, protruding from the tunnels of the through-jewels. Getting ahead of myself: this is done to ease assembly. The bridges seat quite tightly, so they are first slipped onto the pivots by their jewels and only then pressed fully home. There is no need to chase the train into place during reassembly, and capping the wheels with the bridge becomes far easier.

Bridge for the pallet and seconds wheels showing chaton-set through-jewels and elongated wheel pivots — The pallet-and-seconds-wheel bridge. The wheel pivots deliberately project through the through-jewel tunnels — so the bridge can be threaded onto the pivots by its jewels first, then pressed down, sparing the watchmaker the usual hunt to align the train.

Disassembly

We begin taking the watch apart. Here is that cherished little screw which we "… loosen by a turn or a turn and a half, then withdraw the winding stem."

The setting-lever screw that must be loosened to release the winding stem — The stem-release screw — backed off a turn or so, it frees the winding stem for withdrawal.

Yes — the stem here is a substantial thing indeed. In keeping with the caliber.

The large winding stem withdrawn from the movement — The withdrawn winding stem is sized to match the movement — robust and generously proportioned, as the rest of the caliber.

We remove the bezel together with the crystal. The case is a snap-fit "khlopushka," so we simply lever the bezel off at the flat.

Bezel and domed crystal levered off the snap-fit case at the filed flat — The crystal bezel snaps off at the case flat — a press-fit front, like the back, with no threads to contend with.

Now we cover the dial with film and lift the hands with my favourite curved tweezers. The seconds hand I removed with needle tweezers — there is no other way to reach it, as it sits deep and tight.

Hands being lifted from the dial through a protective film with curved tweezers — A protective film guards the dial as the hands come off. The deep-set seconds hand needed fine needle tweezers to reach and free it.

Here are our hands. The movement can now be freed.

The three removed hands of the pocket watch laid out — The hour, minute, and seconds hands, removed and set aside.

To free the movement we unscrew two retaining screws. Here they are.

The two case-retaining screws that hold the movement in the case — The pair of case clamp screws that secure the movement within its case.

Then we carefully lift the case away, and the movement transfers onto a clean sheet of paper laid over a soft microfibre cloth.

Movement transferred onto clean paper over a microfibre cloth after lifting the case — The movement is decanted onto clean paper cushioned by microfibre — a stable, lint-free surface for the work ahead.

Now we remove the dial. In this watch it is held by three screws, and the dial has three feet. Its thickness is about a millimetre and a half — a millimetre and a half of brass. They did not skimp on good things back then, they did not.

Dial fixing being released, held by three screws engaging the dial feet — The dial is retained by three foot screws. At roughly 1.5 mm of solid brass, it is far heavier-gauge than economy would dictate.

Screws loosened, dial removed — and with it the hour wheel and its washer. The wheel here is a fine piece too, with a boss and a thrust seat for the washer.

Dial lifted off, exposing the hour wheel with its spring washer — With the dial off, the hour wheel and its washer come away. The wheel carries a boss with a thrust seat for the washer — another quietly over-built detail.

The dial from the reverse. Three millimetre feet. Quite a watch for the professionals of the underwater trade — and for the modest gentleman as well.

Reverse of the dial showing its three mounting feet — The dial's underside with its three sturdy feet — the same no-compromise construction carried through even to the dial.

So — we set this "disc" on the holder and begin demounting the parts.

Movement mounted on a bench holder, ready for parts removal — The movement secured on its holder, ready for systematic disassembly.

The Balance and the Matching Serial Numbers

As always, "in the opening lines" we remove the balance. How do you like that cock? Yes, indeed. The balance is a screwed type, not shock-protected, the stud carrier is fixed — but the hairspring is a Breguet overcoil. And here I ask you to look at the figures stamped on the balance cock. Remember I asked you to memorise the movement number? Well — this is that very number! There it is. Every part, as we shall have occasion to confirm, carries the same serial. Yes. Everything here is serious.

Balance with Breguet overcoil removed, its cock stamped with the matching movement serial — The balance lifted away: a screwed, non-shock balance with a fixed stud carrier and a Breguet overcoil. The serial stamped on the cock matches the movement number — every component on this caliber is numbered to the watch.

And since the balance is off, let us take it apart and lubricate it. Moebius 8000 oil. The assembly is large, and everything is fairly simple.

Balance assembly being disassembled for cleaning and lubrication — The generously sized balance is broken down for service — its scale makes the work straightforward. Lubrication is with Moebius 8000.

Balance components separated on the bench during servicing — The balance components laid out — staff, roller, and hairspring assembly ready for cleaning.

Further view of the dismantled balance parts — A closer look at the dismantled balance, the Breguet overcoil clearly visible.

Now the pallet bridge. Yes — a column of tanks could drive across this bridge. And a jewel in a chaton. It too carries the serial, finely stamped on the reverse.

The massive pallet-fork bridge with a chaton-set jewel — The pallet-fork bridge — massively over-built, with its jewel set in a chaton and the matching serial stamped discreetly on the underside.

The pallet fork and its bridge are removed. It too is finished with an anti-corrosion coating — and you can imagine which kind. These watches were built for serious people.

Pallet fork and bridge removed, showing protective anti-corrosion finishing — The pallet fork and bridge lifted out, both protectively finished — fitting for a movement designed with military and underwater service in mind.

Now the train bridge, the escape wheel, and the seconds wheel. Can you imagine how much material and milling such a part demands? They did not spare it. For a good thing, it is not a pity.

Train bridge with the escape and seconds wheels beneath it — The train bridge over the escape and seconds wheels — a heavily milled component that consumed far more metal and machining than strict function required.

We do not dismantle the motor yet. While the bridge that retains the center wheel is still in place, we remove the cannon pinion on the slow side.

Cannon pinion being removed from the dial side while the center bridge remains — With the center-wheel bridge still holding things steady, the cannon pinion is drawn off the dial side first.

There. The cannon pinion is off. Now we can turn to the mainspring motor.

Movement with the cannon pinion removed, ready to address the barrel motor — The cannon pinion removed, attention now shifts to the spring barrel and its driving wheels.

Here are its component parts: the center and intermediate wheels; the crown wheel (a genuinely "crowning" wheel); the barrel wheel; and the parts of the sliding clutch. The mainspring. It looked sound, and I was too lazy to take it apart — for which I would pay dearly later. But let us not get ahead of ourselves. Everything in order.

Motor components laid out: center and intermediate wheels, crown wheel, barrel wheel, clutch parts, and mainspring — The motor's parts: center and intermediate wheels, the imposing crown wheel, the barrel wheel, the sliding-clutch components, and the mainspring — which looked fine, so I left it intact. A decision I would regret.

The Keyless Works

Next in turn comes the keyless works. The system is highly practical and utterly simple: two levers — the intermediate and the setting lever — both made by stamping; the minute wheel and the setting wheel. The setting-lever spring, however, is splendidly done. It is held in a special slot by a screw; it is flat, and impossible to lose during disassembly. For that, a separate thank-you to the caliber's designers. Incidentally, the caliber was designed in 1930 and first appeared in the "Kirovskie" watches in 1935. It was of course refined over time, and what lies before us is one of the examples made at the apogee of production.

Keyless works exposed: stamped intermediate and setting levers, minute and setting wheels, and a screwed flat setting-lever spring — The keyless works — two stamped levers, the minute and setting wheels, and a flat setting-lever spring fixed by a screw in its own slot. Captive by design, it cannot spring loose and vanish during a strip-down.

We clear the plate of the keyless parts. The spring need not be removed — it will wash clean along with the plate, it is hard to damage, and we are careful. That is all. Into the cleaning bath!

Main plate stripped of keyless parts, the captive setting-lever spring left in place, ready for cleaning — The plate is stripped, the captive spring left undisturbed to wash with it. Everything now goes into the cleaning bath.

And here is our clean fellow on the holder. The plate is ready for assembly. While everything is open, note the depth of the barrel-arbor bearings — over a millimetre. The watch is 65 years old, yet there is not a trace of wear. This is what happens when you dress a "diver" in a dinner jacket: it simply will not wear out.

Cleaned main plate on the holder, showing the deep barrel-arbor bearing seats — The cleaned plate, ready to rebuild. The barrel-arbor seats run over a millimetre deep — and after 65 years they show no wear whatever, a testament to the caliber's robust proportions.

Reassembly: Starting With the Barrel

We begin reassembling the movement somewhat unconventionally. One usually starts with the keyless works, but that will not do here. The reason is that the tunnel of the sliding clutch is made in two halves — one in the plate, the other in the barrel bridge. Until those halves come together, the stem and clutch have nowhere to anchor; you can see by the arrow where the half-tunnel for the stem is. So we begin by setting the barrel and the intermediate wheel onto the plate.

Barrel and intermediate wheel placed on the plate, with an arrow marking the half-tunnel for the clutch — Assembly starts at the barrel, not the keyless works: the clutch tunnel is split between plate and barrel bridge (arrowed), so the bridge must be on before the stem and clutch can seat.

Like so.

Barrel and intermediate wheel seated in position on the main plate — The barrel and intermediate wheel settled in their places.

Then we take the center (minute) wheel — here it is.

The center (minute) wheel held ready for installation — The center, or minute, wheel — next to be fitted.

We set the center wheel in place, not forgetting to lubricate its bearing seats.

Center wheel installed onto the plate with its seats lubricated — The center wheel goes down with its bearing seats oiled.

Now the bridge. But before fitting it, one simply must admire this work of engineering art. Look how much labour is in it — and how much metal went to waste in the making. Again one recalls the remark about the Luger. This is the view from below.

Underside of the barrel bridge, intricately milled — The barrel bridge from beneath — a dense piece of milling that turned a great deal of stock into swarf to achieve its form.

From above it is interesting too. The number of processes is, without doubt, considerable. Moreover, a hard-alloy washer is placed under the crown wheel. Here it is worth saying that the mainspring in this caliber is very stiff — it winds hard, with noticeable effort. It is for this reason that so much attention was paid to the winding mechanism: the crown wheel, the powerful stem, the sizeable sliding clutch. But a strong spring has its flip side. Remember the snag I mentioned at the start? Here it is: such a powerful spring tends to destroy itself. The one serious failure in these watches is a broken mainspring. Alas.

Top of the barrel bridge with a hard-alloy washer seated under the crown wheel — The bridge from above, with the hard-alloy washer beneath the crown wheel. The K-43's mainspring is so stiff that the whole winding train was beefed up to match — yet that very strength is what eventually breaks the spring, the caliber's one chronic fault.

The heel of the center wheel — one millimetre. Again, no wear at all over so long a time. Simply a pleasure to look at.

Center wheel heel pivot, one millimetre, showing no wear — The center wheel's one-millimetre heel pivot, unworn after decades — the kind of detail that rewards a close look.

Here is the crown-wheel assembly. A complex milled form, with its hard-alloy washer.

The crown-wheel assembly — an elaborately milled profile riding on its hard-alloy washer, sized to cope with the heavy winding effort.

Ah yes — most important of all, before fitting the bridge we must not forget to set this little thing: the screw for the intermediate setting lever of the keyless works.

The intermediate setting-lever screw that must be fitted before the barrel bridge — An easy step to overlook: the intermediate-lever screw of the keyless works must go in before the barrel bridge traps its position.

And so we send the barrel bridge home and secure it with three screws.

Barrel bridge fitted and secured with three screws — The barrel bridge seated and locked down with its three screws.

The Motion Works and Keyless Reassembly

Next we move to the slow side…

Movement turned to the dial side to rebuild the motion works — Over to the slow side for the motion and setting work.

…and install the cannon pinion. It is always better to do this before fitting the minute wheel, though that is not always possible. Here it is.

Cannon pinion being refitted on the dial side before the minute wheel — The cannon pinion goes back first — the preferred order where the layout allows it, as it does here.

After this we can install the minute and setting wheels and assemble the sliding clutch.

Minute and setting wheels fitted and the sliding clutch assembled — The minute and setting wheels are placed and the sliding clutch built up on the stem.

Then we set the keyless levers — the intermediate and the setting lever…

Intermediate and setting levers of the keyless works installed — The two keyless levers go back into place over their captive spring.

…and cover the parts with the keyless cover spring. Along the way we lubricate the assembly with "B-1" oil. That is done. It remains to service the balance bearing.

Keyless cover spring fitted over the works, lubricated with B-1 oil — The keyless cover spring caps the works, lubricated with B-1 oil — the keyless and barrel components getting a heavier oil than the jewels.

Servicing the Balance Cap Jewel

We remove the cap jewel and clean the setting of everything that has no business being there. We wash and clean the cap jewel.

Cap jewel removed from the balance setting for cleaning — The cap jewel is lifted out and its setting cleared of debris, then washed clean.

We oil the cap jewel. Moebius 8000 suits here…

Cap jewel being lubricated with Moebius 8000 oil — A charge of Moebius 8000 is applied to the cap jewel — the oil reserved for balance, pallets, and jewels.

…and set it back in place. Look closely and you can see a little ring of oil through the jewel. By that ring, incidentally, one judges the parallelism of the cap jewel to the hole jewel — which matters — and at the same time whether the quantity of oil is right. With correct oiling the ring should not reach the edges of the cap jewel; it should sit like an inscribed circle. Here all is well.

Cap jewel refitted, the oil ring visible through the jewel as an inscribed circle — The cap jewel reseated, the oil showing as a neat ring through the stone. A circle that stays clear of the jewel's edge confirms both correct parallelism with the hole jewel and the right amount of oil.

Reassembling the Going Train

So, we are ready to assemble the train.

Plate prepared and ready for the going-train wheels to be installed — The plate prepared and ready to receive the going-train wheels.

We install first the escape wheel, and after it the seconds wheel.

Escape wheel and then the seconds wheel placed onto the plate — The escape wheel goes in first, followed by the seconds wheel.

We fit the train bridge.

Train bridge being installed over the escape and seconds wheels — The train bridge is lowered onto the wheels — the long pivots making the job painless, just as the author promised.

I want to draw attention to the pivot of the seconds wheel. When working with this caliber one must take care not to damage that pivot during handling. For this reason the caliber must, without fail, be set on a stand in such a way as to protect the pivot.

The vulnerable seconds-wheel pivot that must be protected during handling — The seconds-wheel pivot is exposed and easily damaged; the movement must always be rested on a stand that keeps this pivot clear.

The Mainspring Motor — and the Failure

Now the motor can be assembled.

Beginning to assemble the mainspring motor — The barrel and winding wheels are brought together to rebuild the motor.

Here it is, the crown wheel. A powerful crown, a powerful rim.

The substantial crown wheel of the winding mechanism — The crown wheel — heavy crown and heavy toothed rim, built to transmit the considerable winding force.

We lubricate the parts, set them in place, and fix them with a left-hand-thread screw. The screw has an ordinary slot, not the triple slot one expects for left-hand threads — worth remembering, so as not to strip it by mistake.

Crown wheel secured with a left-hand-thread screw that has an ordinary single slot — The crown wheel is fixed with a left-hand-thread screw — but its head bears a plain slot, not the usual three. Forget that, and you risk stripping the thread.

We install the barrel wheel, check the run-down, and return the pallet fork, having first oiled the pallet stones with Moebius 8000. And yes — not only the stones. Unlike the small wristwatch calibers, in this one we also oil the jewels of the pallet-fork arbor. The lever and the pivot diameters here are large, so lubrication improves the working of the assembly. The oil is Moebius 8000. After fitting the fork we check the impulse. For that the spring must be wound. We make one turn of the crown, a second, a third… and suddenly — "click!" An unpleasant, unexpected click, and the crown began to turn freely with no resistance. Well, there we are. Where did it break — at the start, the middle, or the end? This is the price of laziness. I did not strip it, did not check the spring, I trusted to luck. So — "our song is a good one, begin again from the start." We take everything apart anew…

Barrel wheel installed and pallet fork refitted just before the mainspring broke on winding — Barrel wheel in, pallet fork back with its stones and arbor jewels oiled in Moebius 8000 — and then, on the third turn of winding, the telltale "click" of a snapped mainspring. The price of skipping the barrel strip-down.

Well — disassembled. What do we have? We have this: the classic K-43 problem — the loop has broken off at the tooth of the kore, the barrel arbor.

Mainspring with its inner loop broken off at the barrel arbor hook, the classic K-43 failure — The verdict: the inner loop has torn away at the arbor hook — the textbook K-43 failure mode, brought on by the spring's own strength.

That very little loop.

Close-up of the broken inner loop of the mainspring — A close view of the failed loop — the small inner hook that anchors the spring to the arbor.

Reforming the Broken Mainspring End

It is repairable, but a fair amount of fiddling lies ahead. We will try to restore the spring. We bend back the broken end and release its temper. I do this over the flame of burning plexiglass — that is how I was taught. We heat it to a dark-cherry colour and let it cool. Now this part of the spring is soft, pliable, and workable.

Broken mainspring end being annealed over a flame to soften it — The broken end is annealed — heated to dark cherry over a plexiglass flame and left to cool. Softened, it can now be reshaped without cracking.

Next, in the softened part of the spring we need to make a loop like the one that was there, to wrap the spring around the arbor. We take a drill of suitable diameter, set it in a universal collet holder that will act as a hand micro-drill. If there is no suitable drill, the hole can be punched with a centre punch or even a nail, laying the spring on a wooden base, and then given the necessary elongated shape with a thin square needle file. Ah — the other day someone asked me, with a touch of mockery, what a watchmaker wants with needle files. Yes, yes. This is exactly what for!

New hole being drilled and shaped in the softened mainspring end with a collet-held drill — A new anchor hole is drilled in the annealed end with a drill held in a collet, then shaped with a fine square needle file — the very tools whose place at the watchmaker's bench is sometimes doubted.

The hole is drilled. Now… now I am cross, do not touch me. It may not come out right the first time, so I set the camera aside — it will only get in my way now — and settle down to the routine work. When I finish (if all goes well), I will show everything assembled. In short, off I went to the grubby fitter's table, for this is not done at the watchmaker's bench — too much mess. The caliber, of course, is superb. But the springs let it down. Yes, I will try to find an unbroken one, but for now I just want to get the watch running.

The new hole drilled in the mainspring end, ready for forming into a loop — The hole drilled and ready. From here the work moves to the bench vise, away from the watchmaking station — forming the loop is messy, repetitive work.

The next day. On the third attempt I managed to form the loop properly. I washed the watch and rebuilt it again — there was no other way; once the wheels are off, the lubrication is disturbed. But "the old clock is running." And the fact that the spring is now three centimetres shorter… well, never mind, it will certainly be enough for thirty hours. The main thing is not to despair and not to give up. For "he who wants to, finds a way; he who does not, finds an excuse."