Stopwatch maintenance. A watchmaker explains repair points with photos.

Yutaro Iizuka, a watchmaker at Chrono Theory Tokyo, will convey the importance of watch repair and maintenance through actual work. This time, he will focus on stopwatch maintenance. He will revive a Minerva "Rally Timer" from around 1950, explaining the causes of malfunctions and key repair points. It is rare to have the opportunity to take a close look at the internal mechanism of a stopwatch mechanism. Join us as we experience the view seen by the watchmaker after maintenance.

Stopwatch repair and maintenance

　This time, I'd like to introduce a stopwatch equipped with a chronograph that I received. It was in good condition except for the oil, but I'll be writing about the Minerva Rally Timer, and the scene where the hands are removed during maintenance.

　In the case of a chronograph, the start and stop of the chronograph second hand is determined by whether or not the chronograph mechanism is linked to the time display mechanism. In other words, the balance wheel must be constantly moving in order to control the movement of the second hand (starting and stopping). This is one of the difficulties in making a chronograph work.

　In contrast, stopwatches are extremely simple, with the movement of the balance and its stop directly controlling the second hand. Naturally, they have fewer parts, and the parts themselves are thick and durable. Rally timers also have a special design that takes maintenance into consideration.

Major Features

　The Minerva Rally Timer is a stopwatch from around 1950, and what makes it stand out is its impressive coil spring. The sound it makes when you press the push button is unique, and the vibrations are transmitted to the spring, creating a mechanical movement where you can even hear the faint sound of the spring vibrating. You can get a sense of its power from the photo below.

　The mechanism is simple, but each part is thick, giving it a reassuring design.

　Gathering information is an important element in maintenance, and these days, you can gather a lot of information online. I use it to check the case structure and disassembly method, and also to look for information on oiling the movement. Since this is a historic watch, I looked up a catalog from that time.

　The catalog from that time that I discovered it states, "Minerva's new patented 'coil spring' structure is the result of 20 years of technological research and is unbreakable. This mechanism guarantees outstanding safety and durability. In our factory, we regularly test timers that have operated one million times, and the results show no signs of wear (according to the author)." The number of tests was an astonishing one million times.

　Generally, springs are called "leaf springs," and the strength of the spring is determined by the shape of the leaf. In this case, adjusting the spring's thickness is extremely critical.

　To give a simple example, simply increasing the thickness from 0.15mm to 0.3mm increases the strength of the spring by eight times. It is necessary to find the appropriate strength within this dimension range, so it is easy to imagine how difficult the adjustment is.

　According to the Minerva catalog, leaf springs generate functional friction and are always at risk of wear and tear, but coil springs, on the other hand, do not require adjustment or repair and will never break.

　The article said that using coil springs reduces the difficulty of maintenance, making it possible to meet the "heavy duty" requirements (usage) required in the sports world and other areas. Heavy duty!

I never get used to removing the needles.

　According to information on the Internet, the Rally Timer movement does not appear to have any special mechanisms or eccentric features.

　I'll start disassembly right away.

　Perhaps to meet the demands of "heavy duty," the hands used on stopwatches were attached very strongly. This is already one of the most nerve-wracking and stressful aspects of watchmaking. Forcing the hands would damage the dial, and on dials made of enamel or other materials, the hands were so hard that they would easily break.

　Sweating lightly, I continued observing, taking deep breaths to keep my hands from shaking, muttering to myself, regulating my breathing, and concentrating on my fingertips and the tool while observing. When it came out safely with a 'sniff', I cried a tear of relief.

　It is necessary to put another cover on top of the vinyl cover to prevent even the slightest pressure from being concentrated on the dial. You cannot let your guard down. In some cases, you may have to remove the dial along with the hour hand, leaving only the hour hand. It always requires optimal judgment, preliminary research, and thorough observation. I doubt you will ever get used to this work.

Let's look at the cam and screws.

　Here, I would like to show you the parts that we don't usually pay much attention to, the cams and screws, along with images taken during the repair.

　The heart-shaped cam (heart cam), essential for resetting the chronograph second hand and totalizer counter to zero, is generally used to convert rotary motion into linear motion. In the case of a watch, the opposite is true; the linear force (motion) applied to the cam is converted into rotary motion, making it possible to realize the zeroing mechanism for the hands. It is a cool and useful cam with a simple yet complex function.

　Next, this watch uses large-diameter screws, which I think contribute greatly to the aesthetics of the movement. They were originally flat, mirror-finished screws, so I polished them all. While large screws are easy to polish consistently, it takes a long time to achieve a satisfactory finish. I think I'll move on to the next screw once I've removed the scratch here, but usually other scratches become visible before I can do so.

　I continue to polish it patiently, looking forward to when I have finished polishing everything and can assemble the movement.

　In the case of chronographs, the sheer number of screws can make you want to cry, but there are also senior watchmakers with decades of experience who have made screw polishing a part of their maintenance routine. I wonder how many screws they have polished so far. I'm sure I'm not the only watchmaker who reads his blog and is inspired to stop complaining.

This problem point

　Let's summarize what factors caused the malfunction in the rally timer that was serviced this time.

　The first issue is magnetism. In fact, magnetism can be quite troublesome. In cases where magnetism is so strong that parts stick together, as shown in the photo, it may not be possible to remove it even by demagnetizing the entire device. In such cases, it is necessary to demagnetize each part individually.

　Although it will vary depending on the environment and the materials used in the watch, I would like to ask you to be careful to keep your watch at least 5 to 7 cm away from anything that generates magnetism in your daily life.

　Next, a part broke. A ruby ​​stone (pallet stone) is attached to a part called an anchor. It is an important part for transmitting energy to the balance wheel as efficiently as possible. In this repair, this stone was missing. In this case, the only option was to replace it. The pallet stone was ordered from overseas, its length was adjusted, and adjustments were made to ensure it was operating without any problems.

　Finally, damage occurs over time. Parts that are subject to manual force tend to develop metal splinters known as burrs over time. These can have a negative impact on the surrounding metal. In this case, the gears had worn away at the bridge, leaving it covered in brass powder. If this wasn't removed, it could one day require major surgery, or it could be used as a donor movement for a watch of the same model. In addition to removing the burrs to prevent further wear on the bridge, I also performed a simple chamfering to prevent burrs from forming.

　Since the part was large, about 1/4 the size of an index finger, chamfering it was relatively easy.

After maintenance

　The owner who requested the repair from me wanted to place the Rally Timer on his desk and measure work time while listening to its low-beat sound of 5 vibrations (18,000 vibrations per hour). It's quite an enthusiastic way to enjoy it, but it certainly makes a great sound. Towards the end of the maintenance work, I left it running on my work bench to check the daily rate, and the sound really helped me concentrate on my work.

　The movement is large, and in a quiet room the ticking sound can be heard loudly without having to get close to the ear, so I think the customer is very satisfied. I took a photo of the movement, showing how the polished screws made it look even more impressive, and gave it to the customer as a gift.

　Apparently, this watch was purchased at an auction. If you are thinking of bidding on it at an auction, it might be a good idea to consult with watch enthusiasts in advance, such as at a local watch shop or on social media. There are many people with deep insight on social media, so you might enjoy just listening to their stories. If you have decided to purchase it after consulting with them, there are likely technicians nationwide who can repair it, so I recommend taking it to your local watch shop for inspection or repair before using it.

　I hope you will broaden your enjoyment of watches and find a watchmaker to look after you. Each watchmaker has different experiences, so the more watchmakers you get to know, the more you will be able to hear different stories, which will lead to the opportunity to discover fascinating watches that you have not yet encountered.

Author Profile

Yutaro Iizuka
A watchmaker currently employed at Chronotheory. While studying at Hiko Mizuno College of Jewelry, he participated in the 2018 Walter Lange Watchmaking Excellence Awards. He won the award for an acoustic indication triggered by a bimetallic thermometer. After graduating, he worked at a repair company before taking up his current position. His Twitter account is@khronos_. "