CAR & BIKE

2025.05.02

The man who turned his passion into a job, "SHAFT" Representative Mr. Kazuya Kurihara Interview

Turning what you love into a job. Kazuya Kurihara, an unconventional tuner who chose a simple yet challenging path. What does a person who has worked on performance tuning of Harley-Davidson and four-wheeled injected vehicles say about how to carve out a path?

Kazuya Kurihara
Born on May 22, 1967, in Gyoda City, Saitama Prefecture. Representative Director of SHAFT Co., Ltd. After working as a chef, he founded Kurihara Workshop, focusing on motorcycle maintenance at the age of 30. Later, he renamed the business to SHAFT. Using a chassis dynamometer, he excels at tuning and maintenance, extracting high performance from Harley-Davidson ECMs (computers) and four-wheel EFI systems.

Transitioning from a chef to a beloved world, becoming an unparalleled tuner

—A large building packed with Harley-Davidsons and various vintage cars. Kurihara-san of SHAFT, who specializes in injection (※) American cars, is among the top in the country for precise performance measurement and tuning know-how using a chassis dynamometer. He is particularly skilled in intake and exhaust systems, but is unique in the industry. How did he venture down this path?
※Injection = A device that injects fuel using digital signals.

―When asked about the trigger for this.

Kurihara: "Originally, I was a Chinese cuisine chef. To put it simply, a cook. I did it for about 10 years and reached a point where I owned my own restaurant. At that time, sales were not bad, but one day I suddenly thought, 'This is not what I want to do.' So, after considering it, I figured, 'As long as I can pay off my debts, I can manage.' In that moment, within 5 days, I quit my job as a cook completely."<

—Was there no anxiety about leaving the stable job as a cook?

Kuriwara: "I tried my best for 10 years. However, the thought of simply increasing the number of stores never crossed my mind. In short, it's the feeling of a craftsman. However, isn't authentic Chinese cuisine something that is completed by dozens of chefs? I thought that wasn't something I should do and wanted to delve deeper into what I could do alone."<

The trial and error of Harley customization, which was a hobby during the founding period

― The time when I decided to become a craftsman in the world of motorcycles coincided with my sudden start on Harley customization. The circumstances of the early founding phase seem to have been quite original.

Kuriwara: "At that time, I was riding a 66 early shovel (note). It was really fun, and I started customizing it in various ways, welding and trying different things. Back then, around the year 2000, when you thought of vintage Harleys, you thought of choppers, right? So, I thought about it and made various things, but when they were completed, they weren't what I had imagined. There's something that you just can't surpass. In short, I learned that simply being from that era is not enough to make it good."

Kuriwara: "While I was doing that, one day someone asked me, 'If I pay you properly, can you do something for me?' That was my first job." <

Note: Early shovel refers to Harley-Davidson motorcycles produced and sold from around 1966 to 1969.

"Why did you transition from customizing vintage Harley choppers to tuning like you do now?"

Kuriwara: "While I was customizing my own Harley, I realized that I was just fixing broken things. There was a time when that was the kind of work that existed in the world, but I thought that it would be better to just move on to new models, which are surely faster, more durable, and more interesting. So, I decided to transition to tuning the current model (injection bikes). Kuriwara: "Once I changed the concept of the shop, I wanted customers like that to come in. But then, without understanding anything, customers would attach something like a sub-computer and say, ‘It got better!’ But then they would also say things like, ‘It only gets 7 km per liter.’ When I thought about doing supercharging and wondered, ‘What do we do about this?’ I finally realized that I needed a chassis dynamometer (※) to measure things."

※Shashidai = an abbreviation for Shashidai Dynamo (Chassis dynamometer), a large mechanical device used to measure the power (horsepower, torque) of cars and motorcycles.

- In this way, the current flow that SHAFT excels at, leading to tuning shops, is being established.

A new perspective on Harley tuning.

―At that time in Japan, tuning Harley had been a trial-and-error process. It seems that this went on for a while…

Kurihara: "There were many people who wanted to ride old bikes or Harleys, and it felt like it was normal for them to break down. It would take a hundred kicks just to start the engine, and they’d be like, 'I'm already tired.' And there were people who were frustrated, saying they didn’t want to ride bikes or cars that would break down anymore."

―Not only tuning injection cars but also old carburetor cars can become easier to ride with just the right setup. Mr. Kurihara had the intuition that by mastering the chassis dynamometer, he could unlock the engine's performance and transform it into a more enjoyable bike. However…

The beginning of using the chassis dynamometer, which is essential for tuning at SHAFT

Kurihara: "So, from there, I tried to hear various opinions, but there were only people pretending to know, those who were ignorant, and those who denied it. I bought a used machine (the chassis dynamometer), but that alone didn’t solve the mystery."

Kurihara: "These days, everything is injection, right? When that started to become more common in society, I thought I would go in the direction of using that machine for adjustments. At that time, I wanted to study Harley's performance custom, so I went to various places around Japan, and when I asked everyone how they were using it, they would say, 'That's not a Harley.' No one gave me a proper answer. Really, no one had ever done it. I first bought a used machine (chassis dynamometer) and learned by going to places where people seemed willing to teach me, but no one had ever measured a Harley; it was that kind of world."

- "Harleys are slow, have poor fuel efficiency, and break down." Seeking a world different from such common beliefs, Kurihara immersed himself in tuning. This eventually connected to the world of craftsmanship he aspired to be a part of.

In his student days, there were many wrecking companies in his hometown

- "What I happened to love were bikes and cars," says Kurihara. However, his passion is extraordinary. Otherwise, he wouldn't have been able to establish himself in a field that involved expensive machinery where no one had ventured before. Where was his identity formed? He talked about his hometown during his student days.

Kurihara: "When I was a child, I might have had a family member who loved cars. At around 7 years old, I had a childhood where I rode my uncle's Honda Monkey. Also, during my middle and high school days, in Gyoda City, Saitama Prefecture, where I grew up, there were about 300 to 400 wrecking companies. I only knew wrecking companies. Growing up in such an environment was, in a way, a fortunate experience."

―Certainly, it's a fortunate environment to fix and ride, and that makes knowledge and know-how valuable.

Kuriwara: "So for things that I can't really handle myself, I think I want to acquire whatever technology I can, no matter how much I have to humble myself. I’ve been raised in that kind of environment."

The prices of tools that I impulsively buy are on a different level

―Not just bikes, but Kuriwara has also included four-wheel chassis dynamometers. It's not just a characteristic of the shop called Shaft; in a sense, it should be something like a commitment. It's a machine that is excessively high-performance and costly for a privately operated shop. When I ask Kuriwara about such "large investments," I’m left surprised.

Kuriwara: "Even for work that would probably be considered 100,000 yen for ten days at a daily rate, if I want to, I'll buy the necessary tools for 20 million yen or 50 million yen. It's just my nature. I often end up buying tools worth 30,000 yen for a job that pays 3,000 yen. So, the calculations just don’t add up (laughs). That kind of business is probably not ideal, it wouldn't work. However, I have always felt that maybe, in a little while, the world would definitely catch up, right?"

―By the way, the chassis dynamometer for four-wheel vehicles that we are currently using costs about 30 million yen, and the full set of the Dynojet for two-wheel and three-wheel vehicles is 8 million yen. Additionally, there are many requests from businesses for two-wheel setups, including a spare set available for 6 million yen. Just this system of three sets alone represents quite a significant capital investment.

As a car and bike enthusiast, I've ended up buying even a military Hummer like this. Even though it's a replica, it properly features a gun mount.

The school bus in the back actually belongs to my wife. There are many stories about the efforts to operate this bus on public roads. Moreover, we receive numerous inquiries for it to be used as a movie prop vehicle.

We have a great selection of machine tools. This space has everything, like a toy box.

In a world where decent people can ride.

The consideration of intake and exhaust control inevitably led to the creation of an original muffler. “Even if you say you're an outlaw on a Harley, you’re just an ordinary person, right? So I thought I’d make a legal muffler that you can take for a vehicle inspection.”

- Mr. Kurihara is a craftsman who does the necessary work with the necessary tools, but it’s quite difficult to come across such shops. Ultimately, the encounter with a good shop and a good craftsman should also be an issue for the user themselves. I asked Mr. Kurihara about his encounters with customers.

Kurihara: "I think it's most important to have a place where you can enjoy the process. Because it's a machine, it will definitely be fixed. However, there are time, money, and processes involved. So I want to be with someone who can enjoy the process together. Yeah. It’s not so much that I want an immediate answer; simply put, a job that costs 1 million yen will take 1 million yen, so we need to enjoy the process together." <

- Doing what you want to do, bringing in good machines. You’ve grown up in this local area, but have you been able to face customers more broadly and meet like-minded people?

Kurihara: "Yes, that has been fulfilled. I think customers who don’t resonate with me should go where they fit better. The way I ensure I can meet people who need me is, after all, about how I approach performance tuning. If you do something unnecessary for someone who doesn’t need it, it doesn’t necessarily go well. However, if I’m asked whether doing something like this (investing in a chassis dynamometer) was the right choice, I would say I’m completely right. It was worth the trouble."

Kurihara: "From back in the heyday of shovel choppers to the new Harleys, meaning the era of injection vehicles, I have been leading the world of tuning for more than a decade up until now. If you ask me where I’m facing now, it's a world where truly decent people can ride. I can control fuel with tuning, manage fuel efficiency, and control exhaust emissions beyond that. I can also make mufflers that can pass vehicle inspections and take the same tests as manufacturers and pass."

―Mr. Kurihara continues to create Harleys and American cars that you can proudly and enjoyably ride in today's era.

For a while after its establishment, the company was called Kurihara Workshop. "I thought it was half-hearted. When you decide on a name, you start heading in that direction, right? I think it was around 2015 when I settled on the name SHAFT."

SHAFT (Tuning Lab)
3-16-14 Kamishiba-cho, Fukaya City, Saitama Prefecture, Inside Shibasaki Warehouse No. 8
048-501-7893
https://shaft-labo.jp/

Photography / Hiroshi Nose, Interview & Text / Masashi Kaki

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