The Japanese Gennou & Handle Part 3 – What is a Gennou?

Kiyomizu Temple, Kyoto, Japan

What we have is given by God; To teach it to others is to return it to him.

Gian Lorenzo Bernini

There are as many varieties of hammers in Japan as there are in Western countries. With one notable exception, and in one specific application, Japanese hammers are not especially superior to their Western counterparts. That exception is the gennou (pronounced gen/noh), a hammer intended specifically for striking chisels, adjusting plane blades, and crushing wood (i.e. “kigoroshi” or “wood killing”). This article will provide a further introduction to the gennou hammer.

What Is a Gennou?

A box-stock, garden-variety, economy Japanese gennou hammer with a one-size-fits-somebody handle

The Japanese have different terms for different hammers, of course. A hammer used strictly for driving nails, or banging sheet metal, or driving stakes is called a “kanazuchi” meaning “steel mallet.” A common slang term for this tool is “tonkachi.” The gennou, on the other hand, can of course be used to drive nails, but it’s especially suited to striking chisels and adjusting planes.

The word “gennou” was borrowed from the name of a Buddhist priest who lived, or so the story goes, in the 1300’s and used a steel hammer to destroy a poisonous rock that was troubling the common folk. I’m not sure what one has to do with the other, but there you are.

The Attraction of the Gennou

Many Japanese craftsmen often have an emotional attachment to their gennou. Perhaps this is because, unlike saws, chisels, and planes that are gradually but inevitably sharpened away until almost nothing remains, or squares or marking gauges that loose tolerance or wear out, a quality gennou will last for a lifetime relatively unchanged other than the occasional replacement handle. A good gennou is a simple, reliable, hardworking friend that never complains. It doesn’t have a pigtail; It doesn’t need to be sharpened. And most importantly, it will never ask to borrow your truck, or pose a dangerous question like “do these pants make my butt look huge?”

Technical Matters

The gennou is a simple tool consisting of a steel body of one shape or another attached to a wooden handle. The head has a rectangular hole called the “eye” in English and “hitsu” in Japanese to receive the wooden handle’s tenon.

The steel used for modern gennou is typically a standard, high-carbon Japanese tool steel used for making hammers, axes, and many other tools and designated “SK.” Chemically, it is very similar to 01 steel in the Americas; Not as pure as Hitachi Metal’s Shirogami or Aogami steels, of course, but still completely adequate for hammers. I wouldn’t pay extra for a gennou head made from Shirogami steel, much less purple-anodized unobtanium, and you shouldn’t either

Mass-produced gennou are either cast or drop-forged very inexpensively. The eyes are rough and the handles are secured with wedges. Indeed, the eyes are typically so irregular that the head will not stay on the handle without wedges, but a high-quality gennou with a good eye and a handle made by a skilled craftsman doesn’t need wedges or other silly contrivances that compensate for sloppy tolerances to connect the two. “High-quality” is the key.

A gennou head with a rough and/or irregular eye can create unnecessary problems for the user. “Irregular” has several connotations in this context. One obvious irregularity is an eye that is not truly rectangular. For instance, it may have curved, twisted walls, wonky interior dimensions, or interior corners that are not square. Not only will it be a right pain in the tuckus to make a handle to fit such an eye, but you can bet your sweet bippy it will produce strange vibrations and cause the handle tenon to loosen up sooner.

Another irregularity commonly seen in the eyes of poor-quality gennou is rough interior walls. You would think that rough walls would generate higher friction and grip the tenon better, and perhaps they do compared to highly-polished walls, but rough, uneven walls tend to wear-out the wooden tenon of the handle causing it to loosen over time. Imagine the vibrations the tenon is forced to absorb through those walls and the grinding action between wall surface and handle that results.

An intentional irregularity frequently seen is end walls (versus the longer side walls) that are sloped from each opening towards the center of the eye, essentially making the eye bulge inwards at its center. The purpose of these bulges is to crush the wood of the tenon when it is forced into the eye, increasing friction, while also providing a dovetail-like area for the steel wedge to expand the eye back into. It’s a reasonable solution for rough, irregular eyes in low-cost hammers to be used by amateurs, but one that the craftsmen who truly understands gennou and wants a lifetime tool finds undesirable. We will touch on this detail more in future posts.

Still another irregularity the careful craftsman must watch out for is an eye that is not perfectly centered in both axis in the head. You might think that an eye that is a little skewampus wouldn’t make a big difference, but it does because, not only is the balance and center of mass of such a head also skewampus so that the head tends to twist during the swing and wiggle on impact, but because making handles for such a head is unnecessarily troublesome. A clean, uniform, straight, properly-centered eye is worth every penny it costs, especially if you are a professional and consider your time and sanity of any value.

A difficult question I am frequently asked is “how much irregularity is acceptable?” The answer is simple: If you think it is too irregular, then it is, because the work to correct the defect or compensate for it will all be on you.

Please understand that correcting major defects in hammer eyes is hard work. It takes time, concentration, a good eye, a flashlight, and a deft hand with skinny files and rulers to remove just the right amount of metal in just the right places inside that narrow eye, a task that is much more difficult than removing metal on an exposed surface because the files are thin and bendy, it’s hard to see what you’re doing, you don’t have much leverage, and consistently making a straight pass is not easy. Blisters will bloom. Patience will be tried. Sanity may quiver. Try it yourself and you will quickly see why.

This is the whole point of high-quality heads like those made by Kosaburo and now Hiroki and why they are worth the high cost: Their eyes are true when new, no adjustment necessary, saving the purchaser many hours of tedious work and blisters over a lifetime of hard use. Every time you make a handle for a high-quality head it saves time and leaves you with a good feeling. It’s a true friend, one whose dog never craps on your lawn.

On the other hand, a poorly-made head is a curse, a money-pit (if your time is worth anything), and a frequent source of irritation (especially when it inexplicably loosens and soars free as a hardened-steel bird (ツ)。)

I hate to say it, but Beloved Customers and Gentle Readers should beware of one last defect when purchasing an expensive handmade gennou head. A perfect eye is truly a difficult thing to make, certainly more difficult than making a head cosmetically beautiful. Unfortunately, one or two famous blacksmiths (who shall remain unnamed in this series of articles, so don’t ask) have earned a reputation among knowledgeable professional woodworkers in Japan for occasionally making gennou with skewampus, eyes. Caveat emptor, baby. She may wear high-heels, a short skirt and be beautifully made-up, but if she has a curly tail and oinks she’s probably a pig, unless she’s a trans-boor.

If you cannot hold and inspect an expensive gennou head in-person before concluding the transaction, at least make sure you purchase from someone with a solid guarantee, one without weasel-words and that reimburses you for return shipping, like C&S Tools’ guarantee does. A guarantee that you must argue about and then spend more money to benefit from is less than half a guarantee IMO.

We will delve further into the tempering and differential hardening of gennou, as well as laminated gennou heads in future posts in this series, same bat time same bat channel.

Why Use a Gennou for Chiselwork?

This is a questions we addressed in a previous post, but which we also examine further here.

Almost any striking tool, from steel hammer to leather mallet, can be used to strike a chisel. The problem is that, unless one is either gentle or the handle of the chisel is reinforced, a steel or even brass hammer will eventually destroy the chisel’s handle. The solution in the West has been to use a mallet made of wood, leather, rubber, or plastic instead to cushion the blow and preserve the handle. Let’s consider this for a moment. 

The purpose of striking a chisel with a hammer is to drive the chisel into and through the wood by cutting it, right? But a soft-faced hammer/mallet, be it made of wood, plastic or rubber deforms when it impacts the chisel cushioning the blow and wasting energy through deformation and heat generation. It may also unnecessarily waste energy through air drag, as we discussed in the Part 2 of this series. Since energy is lost, more mallet strikes are necessary, wasting time. This is demonstrably counter-productive.

Besides being relatively soft, a fat-faced mallet is bulkier, slower to swing, and is therefore less precise than a slimmer steel hammer. While there may be some that are thrilled with cutting slowly and expending extra time and energy in the process of cutting a joint, most people want to cut as much wood as possible, as precisely as possible, in the shortest amount of time as possible, and with the least energy expenditure possible. But if a chisel handle is so fragile that one must expend extra time and energy to keep it intact, then it is only logical to conclude that there is something wrong with the design of the chisel.

Ise Jingu Shrine, Mie Prefecture, Japan

The Japanese are very serious about woodworking, as anyone who has gone to Kyoto or Nara and seen the ancient wooden temples there can attest. When it comes to chisel work, Japanese carpenters don’t tolerate such silly nonsense as a chisel that must be coddled, and so early-on Japanese craftsmen developed a wooden-handled chisel that can be struck hard with a steel hammer all day long without breaking. 

The excellent design of the Japanese chisel combined with the quality of steel, and the forging, lamination, and heat treatment techniques used in manufacturing most Japanese chisels provides a tough cutting edge that stays sharper longer, placing Japanese chisels at the very top of the evolutionary pyramid of chisels, IMHO. And as the Japanese are wont to do, they developed a hammer specifically for striking chisels.

Most hammers intended for driving nails have a domed face which does not work well with the wooden handles of Japanese chisels because it tends to dish in the end of the handle causing the hoop to loosen. This can even result in the handle cracking or splitting. A flat-faced hammer is much better. The Japanese “ryouguchi” double-faced gennou has one face that is forged flat, for striking chisels, and an opposing domed face for driving nails or performing “kigoroshi.”

And while one absolutely could grind the face of a Western claw hammer flat and use it to strike Japanese chisels without any problems, the gennou is designed specifically for striking chisels. In your humble servant’s opinion, it is a superior tool for the intended purpose.

In the next post in this series we will examine three varieties of gennou to help you decide which is best for you.


The Pagoda at Horyuji Temple, registered as one of Japan’s National Treasures. Yessireebob, a lot of hammer and chisel work went making this structure.

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11 thoughts on “The Japanese Gennou & Handle Part 3 – What is a Gennou?

  1. By popular demand, I will in future place my comments here, although I am usually asked (told, actually) to place (shove, actually) my comments elsewhere. it matters not one whit to me.

    i thought a gennou was something like a wildebeest.. but on a more serious note: i have decided to build that toolbox with the loops for poles (the round wooden sections, not the people, although… ) I will then force my apprentices to carry my tool chest from job site to job site while my journeymen carry me around on a palaquin like toranaga-sama all the while burning incense and chanting my praises.

    Stan, would you happen to know a Shinto priest who would accompany my entourage?


    1. Andrew:

      May I be allowed to help bear your palanquin?

      I do know Shinto priests who would probably feel honored to accompany your daimyo gyouretsu… for the right price. We hire them all the time here for jobsite ceremonies. Oh, don’t forget the beer, they must have beer!


  2. Although I do not have the pleasure of knowing either of you gentlemen personally I take comfort in the commonality between trades worldwide of alcohol consumption and irreverence to all HR protocol .


    1. Dear Gav, knowing Stan as I do fir almost 3 postings, i can assure you it is indeed a pleasure. Knowing me however is a chore that can only be regretted. i am, alas a teatotaler, in fact i am bereft of all vices except those on my ancient hobelbank (pronounced „hobelbank“). i accept that swinging a gennou from dawn til dusk makes every hero crave a biro so after consultation with my HR consultants of whom i have none, i shall place the colloquial barrel of suds within easy reach of all those who surrender to their cravings. Living as i do, in a area with more breweries than woodworking shops i shall have no procurement problems. Stan: please ask the Shinto Priests to chant a few „namu amida butsu“ for me, i‘ll let you know when my palanquin is ready, we‘re running out of gold leaf.


      1. Gav and Lord Andrew the Gennou Swinger:

        I wrote a fun reply last night, but the internet was acting weird, lightning was flying around, and I lost the whole thing so went to bed in disgust.

        Adult beverages are a big no-no on American jobsites. I don’t what it’s like now, but the last jobsite I worked in London, everyone had two pints at lunch and were mellow but useless in the afternoon. A lot of rework the next morning.

        In Japan, adult beverages are verboten during working hours, but on large projects the GC puts on a party of sorts on the jobsite once a month for his crew and for the subs. Mediocre food, a little sake, and lots of beer. A cheap price for the goodwill created.

        The last large new-construction project I did in Yokohama we had a several quasi-religious ceremonies. The first was the “jichinsai” conducted by a couple of Shinto priests. Big robes, endless chanting, and a pile of white sand with wooden shovels. It ended with all in attendance (except me, because demon rum shall never touch my lips) taking a ceremonial drink of sake from small dishes called “sakazuki.” It sounds like a groundbreaking ceremony but the purpose is entirely safety-related. The ceremony is intended to placate and calm any malevolent spirits in the ground that would be disturbed by the construction so they won’t cause trouble and injuries.

        One older carpenter died on the jobsite of a heart attack (pre-existing condition), and one AC duct installer cut his hand on sharp metal (you know how that sheet metal gets). We had a fire, started by a welder working in violation of the rules, of course, that was quickly controlled with no injuries or serious damage (the fire department had a good time, I know because I bought them two cases of beer to commiserate them over the lack of roaring fire. They were distraught with dissapointment). So, since there were no serious injuries and no “lost workdays,” I must conclude the jichinsai ceremony was entirely successful.

        You may argue otherwise (it was not an inexpensive gathering), but if we had not had the ceremony and someone was (heaven forbid) seriously injured or killed, then the lack of the ceremony would have been blamed, I promise you. Cheap insurance.

        Another jobsite activity involving spirits (pun intended) was the column-setting ceremony. When the first set of columns on the first day were set on their anchor bolts on the piling caps (big deep piles and huge steel columns of the sort that make structural engineers weep with joy), a small pile of salt and another of rice were placed on the piling cap near each column baseplate, and a glass of sake was poured over the baseplates to purify the columns and appease any unhappy spirits. No accidents, and the columns are still standing (I assume the pilings are still in-place), so I concluded that the spirits like a snort or two now and then.

        And then there was the final grand opening ceremony (plenty grand, but not very open since security was tight) where the company bigwigs and local politicians all gathered around to smash the wooden lid of a big barrel of sake with wooden mallets, something called “kagami ware” meaning “mirror breaking.” A good time was had by all and sundry.

        Like Lord Andrew, I too am a teatotaler (except I don’t drink tea either) so much of the inebriated fun of these things is lost on me, but I do occasionally enjoy watching otherwise diligent and responsible men turn into frikkin idiots after a skinful. I always refrain from taking incriminating photos…. or do I?


        PS: Happy to ask all the Shinto Priests in my acquaintance to pray for you Andrew (Namuyo Houren Gekkyou), but they wouldn’t spit on you or me even if we were on fire without first receiving vast quantities of cashy money. Please send a barrel or three of suds first so we can get a better discount on their holy ministrations. I assume you can get a quality brew at a decent price in your neighborhood.


  3. I have swung my gennou in many countries but when I arrived here in Lower Bavaria I was very surprised to see carpenters (Zimmerer) and cabinetmakers (Schreiner in southern Germany, Tischler in northern Germany) drinking beer. on the job. In Bavaria Beer is legally considered to be a Foodstuff, not a recreational beverage. I have seen masons using a beer bottle as a level (beer – label comparison). In southern France the team of craftspeople would literally set a table, bring out the wine and smoked duck and enjoy a lunch under the shade of a tree. In Italy it was much less formal but just as tasty.

    My conclusion: Artisans – Craftspeople are a breed apart. Looked down upon in many societies as “labourers” we still take pride in our work, respect our craft and its traditions and enjoy life. The sensitivities that make us good at our trade also allow us to celebrate life in ways that others cannot.

    Another big difference: Personal Safety Devices. On Projects in DC, NYC or San Francisco the workroom was covered from head to toe in day-glo plastic protective equipment. In Germany many still wear sandals in the workshop.

    My fondest memory of working in the USA: I managed a large fit-out for a German furniture company in Manhattan. The Showroom was at Broadway and Greene on the third floor above the Armani Exchange. We had imported a bespoke steel sliding door system from germany that had to be lifted by crane through the window. The first attempt had to be cancelled when Manhattan got hit by a hurricane. Sandra Bullock was filming nearby and took refuge in our lobby. We postponed the task and rescheduled for 3 days later. We got a permit to block off the street from 3 AM to 4 AM. Just before 3 AM I heard the unmistakeable sound of Harleys burping through the canyons of SoHo. Promptly at 3 the Millworkers arrived. In V formation, 12 bad-ass looking but very professional and courteous men with nazi helmets 1%er tats and fu-man-chu moustaches. I handled crowd control, they handled the steel. It took under twenty minutes. I had smuggled in a few crates of various bavarian beers and showed my appreciation for a job well done.

    It’s been a long road since then, just as long as the road before that. In a way, i’m always on the road. My skills have taken me all over the word, 65 countries and still counting. I’ll turn 66 in September, and I still don’t know what I want to do when I grow up.

    Stan, if you’re ever in Tokyo Hands Ikebukuro please tell them their Dozuki suck.


  4. thanks Stan, you are indeed a fine fellow. where would you buy a Dozuki? I have been to Japan (Kobe, Kyoto, Edo) on projects a few times. Never had time to peruse the tool shops. my japanese saws were all bought in Canada, the USA, UK or Germany. I did buy a few things at Tokyo Hands 2 years ago, the dozuki i bought there for a friend was really poor. misaligned teeth, not even straight. but ikebukuro was close by.

    I have a folding Dozuki (manufacturer unknown) which has seen much use on job sites. it has been more places than i have been. it even has more teeth than I do. My good tools rarely leave the shop. in fact they are locked away. The blade on the folding Dozuki is very worn, teeth missing (knots, not nails). the holding mechanism is obsolete. I will have to buy a spare blade from another maker and grind it to fit my folding handle.


    1. Andrew:

      In response to your question, I would buy a kaeba dozuki or replacement blade from my neighborhood hardware/tool store. A ten minute walk. Amazon has them too nowadays.

      I like the Zeton (“Z Saws”) best. A simple mechanism, quick to change, and quality is consistent.

      I had my really good dozuki saws hand-forged 30 years ago, and the latest by Takijiro 6~7 years ago. Wonderful saws, but a pain to get sharpened unless you live close to his forge as I do. Hand-forged saws are far superior to kaeba.

      I suspect a folding dozuki would defeat much of the purpose of a dozuki, but to each his own.


  5. Stan, thank you for some greats posts. A question about the irregularities of holes in gennou and what kind of error is reasonable. With other handmade Japanese tools, chisels for instance, I’ve come to accept certain slight irregularities and know what to look for, but what are acceptable tolerances for holes in gennou?

    I have for instance a gennou where the eye is not perfectly centered on the axis of the head, instead its appoximately 1,5mm closer to the flat face than to the rounded face. The hole is also slightly out of square on the one side. It’s handmade by a student of a master craftsman and so is midrange price-wise. Other than the irregularities mentioned above, it looks good and it balances well placed on the rounded side. I’m trying to decide on whether to return it or not.


    1. Silas:

      A difficult question, but the answer is simple: If you think it is too irregular, then it is, because the work to correct the defect or compensate for it will all be on you.

      Let me begin with the statement that properly correcting major defects in eyes is hard work. It takes time and a deft hand with skinny files to remove just the right amount of metal in just the right places. Removing metal inside an eye is nothing like removing metal on an exposed surface because the files are thin, you don’t have much leverage, and consistently making a straight pass is not easy. Try it yourself and see.

      If the eye is offset or skewampus in relation to the three axis (the long direction of the eye, the short direction, and the depth), then correcting with a file may be too difficult. Often it is easier to make the handle to compensate. This is not all that easy either.

      If the eye is offset or skewampus too badly, the balance of the hammer will be irreparable even if you can correct the swing somewhat with the handle. An eccentric head will want to wiggle during the swing and jink on impact. This is a very common defect in consumer-grade gennou heads. But I can’t tell you how much is too much.

      The head you mentioned may be alright since it isn’t skewampus, just light at the front end.

      “Out of square on one side” might be a more serious problem. If the sides are straight and parallel, you can make the handle tenon fit, but more often than not the sides are not parallel. If the eye has internal defects, the tenon tends to loosen and fail sooner.

      Whether to return it or not depends, in my opinion, on how the head was represented. If you paid a mass-produced price, but it was sold as handmade by a master, then maybe not. If you paid a master-blacksmith price for mass-produced quality, then you should return it.

      This is the whole point of high-quality heads like those made by Kosaburo and now Hiroki and why they are worth the cost: They are nuts-on when new, no adjustment necessary, hours of time and many blisters saved. Every time you make a handle for such a head, it saves time and leaves you with a good feeling, while a poorly-made head is a curse, a money-pit (if your time is worth anything), and a source of irritation (especially when the head loosens inexplicable) its entire life.

      It is difficult, indeed unwise, to buy gennou heads via the internet sight-unseen because you can’t check the eye’s tolerances using pictures. Hiroki and (authentic) Kosaburo products are the only ones I would trust sight-unseen.

      In any case, if you buy an expensive head sight-unseen, be sure the seller offers a serious, no weasel-words guarantee, one that includes return shipping, like mine does. A guarantee that you must spend more money to benefit from is only half a guarantee IMO.


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