The Japanese Gennou & Handle Part 8 – Head Style & Weight

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A comparison of two styles of hand-forged gennou heads. Top: A Yamakichi gennou head by Hiroki. Bottom: A classical ryouguchi head by Kosaburo with the antique “swollen eye.” Both handles are made from American Osage Orange, an excellent wood for hammer handles. The top handle still exhibits the neon yellow color typical of OO, while the bottom handle has been exposed to sunlight for a few weeks and turned a nice but unusual brown color.

Better a bald head than no head at all.

Seamus MacManus

In the previous post in this series about Japanese hammers we examined a feature found in all modern hammer heads: the essential, unblinking unseeing eye. In this post we will touch on the style of heads recommend for using with Japanese chisels. We discussed this subject in this post as well.

Gennou Head Shapes

The most common head shapes commonly available in Japan nowadays are: ryoguchi, daruma, funate, yamakichi and various hybrids thereof.

Ryouguchi

Ryouguch is the most common style of head, at least in Eastern Japan. It has two faces: A flat one for striking chisels and nails, and a slightly domed opposing face for kigoroshi and setting nails below the surface of boards.

While a simple design, this style of head has a relatively high moment of inertia, making it is more stable than other styles and therefore less likely to twist out of alignment during the swing, or twitch upon impact, a positive thing if you are a card-carrying member of NBA (Nail Benders Anonymous). (ツ)

Face designs in this style vary widely including round, oval, square, rectangular (usually with corners removed for a more octagonal shape) true octagonal, and the “Ichimonji” style with roundish sides and a flat top and bottom. We prefer the rectangular shape with cut corners best, but one style is no better than another. We don’t recommend, however, faces with 90 degree corners as the corners are counter-productive during kigoroshi operations and are structurally weaker.

If you are worried about pulling nails, we encourage you to use a nail bar to reduce the number of broken hammer handles wandering the world sad and lonely as a cloud.

A 200monme/ 750gram/ 26oz Modern-style ryouguchi gennou by Kosaburo. Notice the symmetrical shape, slightly flared ends, and the polished “hachimaki” band near each striking face.

The Daruma

Named for a famous buddhist priest of oval stature who lost both arms and legs through excessive meditation in his quest for “satori,” an intensely spiritual obsession that no doubt consumes the attention of some of our more enlightened Beloved Customers, the daruma (pronounced dah/rhu/mah) style gennou head is a stubbier version of the ryouguchi gennou, always with a round face.

This style of head is more popular outside of Japan than it is domestically, for reasons your most humble and obedient servant fails to understand. From a physics viewpoint, at a given weight it is less stable than any other style of gennou, but because it has a bigger face, and is intended to be used at constantly differing angles such that stability is not so much a virtue, it is preferred by carvers. Joiners like it too for cutting repetitive mortise and tenon joints, but it is not favored by most trades and may invite remarks at jobsites from other workers about the owner being unable to find his derriere with both hands and a GPS. That said, your humble servant frequently uses daruma heads for cutting precise mortise joints. Wait a minute…. where did I set down that darn GPS tracker….?

An 80monme/ 300gm/ 11oz daruma head with an rock maple handle.

The Funate

A funate gennou with bubinga handle.
The tail of a funate gennou. This point can be sharpened for creating pilot holes for nails when shipbuilding, or left as a rectangle of starting and setting nails. The face is slightly domed, but still flat enough for striking chisels. A good multi-purpose head that favors nails more than chisels.

The funate gennou is closer in appearance to Western hammers with a skinnier neck behind the striking face, but without the split-tail “piano chisel” a foreman from my misspent youth named Jack Frost called the claw on his 28oz waffle-face framing hammer. It is more commonly seen in the Western Japan than Eastern Japan where I learned Japanese woodworking.

This gennou is useful for finish work involving nails and for tapping-out plane blades, but less useful for wacking chisels.

The Yamakichi

Yamakichi was the name of a gennou blacksmith working in Fukuoka on Kyushu Island that originated this style of head and gave it his name. “Yama” 山 means “mountain” and “kichi” 吉 means “luck” or “lucky.” Kosaburo introduced this style to Tokyo in response to customer demand and with Yamakichi’s permission, we are told, improving the design somewhat.

This style is a heavy-duty stubbier version of the funate with a slightly domed face and a kinda sorta pointy tail, perhaps better suited to driving/setting nails than the ryouguchi head, but certainly better for striking chisels than the funate style.

Better with nails than the ryouguchi style, this head makes an excellent all-round hammer for working in the field, and can even handle tapping-out and kigoroshi tasks.

The design has a unique and interesting appearance which reminds this humble scribbler of a 1956 Ford F100 truck in that, while neither sleek nor smooth, it has a sculptural quality not seen in the other styles that “grows on you.” It feels good in the hand.

There are other in-between head shapes, but these are the four basic styles generally available for woodworking today.

Another view of the Yamakichi gennou pictured at the top of this article after the color has mellowed through exposure to sunlight. This is 300monme/375gram/ 11oz head by Hiroki has an American Osage Orange handle. (The decorative twine was added at the tool’s request. It has a thing for the color red).

Weight

The subject of gennou head weight was examined at some length in a previous post.

Regardless of the type of gennou head you select, weight is a critical factor that will depend on what you plan to hit, your height above the thing you are hitting, how hard you need to hit it, and how precisely you need to hit it. Your own practical experience is the best basis for selecting the genno weight for a particular job, but some guidelines can be suggested.

To begin, the traditional measure used for gennou in Japan is the “monme,” with 100 monme equaling 375 grams or 13.2 ounces (1 ounce = 28.35 grams). 

The standard middle-of-the-road weight for genno used by carpenters in Japan is 100monme (375grams/ 13.2 ounces). The most common hammer used for finish carpentry in the United States weighs 16oz = 120monme, a size commonly available in gennou too. So if you are going to buy your first gennou, and you intend to use it for general finish carpentry or furniture making, a 100 or 120 monme genno is a good place to start. 

For finer work, an 50-80 monme (11-7 oz) to gennou is a good choice. If you intend to make furniture or joinery, one in this weight range is a must-have.

For cutting deep mortises in heavy timbers with large chisels, as in timber framing or boat work, a 200monme (26oz) hammer is frequently used, but 250 (33oz) and even 300monme (40oz) heads are available. I own and use them when necessary. Some factors to consider when selecting a heavy gennou are that with greater weight comes greater impact force, and greater penetration, but heavier gennou are more tiring to swing and harder to control precisely.

Other factors to consider are the width of the chisel blade being used, since a wider blade requires more force to cut to a given depth, and the hardness/toughness of the wood being cut. Only experience can tell Gentle Reader what weight will work best in a given situation. Just be aware that, unlike bobby socks and government health care, there is no such thing as one-size-fits-all.

Conclusion

We hope this article has answered some of Gentle Reader’s questions on the subject of selecting a gennou head. If you have additional questions or need clarification, please use the “Leave a Reply” form below.

In the next post in this metaphysical adventure series we will discuss the differences between mass-produced and hand-forged gennou heads. We will look at woods suitable for making handles, and gennou design in much greater detail in future posts, promise.

YMHOS

If you have questions or would like to learn more about our tools, please use the questions form located immediately below. Please share your insights and comments with everyone by using the form located further below labeled “Leave a Reply.” We aren’t evil Google or incompetent facebook and so won’t sell, share, or profitably “misplace” your information.

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