Traditional Japanese woodworker’s hammer (gennō) with 225 g. head and new handle

This older 225 g. gennō head has been fitted with a new white oak (shiragashi) handle to give it new life. The head is stamped with a maker’s mark which I have not been able to decipher, but it is clearly of an earlier era. The quality of the head is attested to by the fact that it is has maintained its shape and is still in good condition long after its original handle was in need of replacement.

The hammer type known as gennō is the tool of choice for users of Japanese chisels, but it is also a good option for other tasks, including the mundane one of pounding in nails. While there are single-faced genno, the double-faced variety is more common and versatile. This is because of the configuration of the two faces. One face is flat while the other is slightly rounded. The flat end is used for chisel work and for starting nails. The rounded end has two main functions. One is for sinking nails. After starting the nail with the flat end, you can then turn the hammer around to set the nail with the other face. Since this face is rounded, if your final blows hit the nail square on the head, you can set the nail flush with the surface while avoiding leaving hammer marks on the wood. The other main use of the rounded face is for the job of kigoroshi (literally “killing the wood.” This is the process of pounding a piece of wood in order to compress is so that it will fit into a tight space, such as when fitting a cap ring (katsura) on the end of a chisel handle or in preparation for driving a new handle into a gennō head.

Gennō hammers come in a number of sizes, usually based on the weight of the head according to the traditional Japanese system of weights and measures. The basic unit is the momme (3.75 g). The most common head weights (and their metric equivalents) are 60 momme (225 g.), 80 momme (300 g.), 100 momme (375 g.), and 120 momme (450 g.). This gennō with its 225 g. head is best suited for light work with Japanese bench chisels (ōire-nomi) and for sinking nails up to 1½ - 2 in. in length. It is also a good choice for setting or removing the blades on Japanese planes. While all plane blocks and blades will develop hammer marks over time, using a lighter hammer like a 225 g. gennō for these tasks minimizes the damage. This hammer comes in a shop-made box.