SEIKO S-284 Kojiake – Case Opener for Snap-On Backs, Narrow Blade 4 mm | Original SEIKO Japan

Original SEIKO watchmaker's tool, imported directly from Japan – rarely available outside of Japan.

The Kojiake (こじ開け, "lever opener") is the classic Japanese tool for opening snap-on and press-on casebacks. The S-284 version from the professional SEIKO tool range features a particularly narrow blade with a tip width of only 4 mm, making it designed for cases with extremely narrow opening notches – such as small ladies' watches and vintage cases where wider case knives cannot even engage the narrow caseback notch.

A special feature of the SEIKO Kojiake: The blade is deliberately designed so that it can be resharpened by the watchmaker and adapted to the respective case – as is common in Japanese workshops. The tool thus grows with its tasks and, with good care, lasts a watchmaker's lifetime.

The series includes three blade widths: S-282 (8 mm), S-283 (5 mm) and S-284 (4 mm, this listing).

What differentiates the tool from European counterparts:

European case knives are predominantly designed as wide universal blades. For the narrow opening notches, which are common especially in Japanese watches – including many vintage models from SEIKO and Citizen – the appropriate narrow version is usually missing in European trade: A blade that is too wide cannot get a grip in the notch and slips off, with a corresponding risk of scratches. The S-284 is the tool with which these casebacks are opened ex-factory – from the original SEIKO tool range, which is not regularly distributed in Europe. In addition, there is the resharpenable blade: Cheap openers with soft blades bend at the tip and have to be replaced; the SEIKO Kojiake is resharpened instead.

Dimensions:
Tip width 4 mm
Reference: SEIKO S-284 (SE-S-284)

Condition:
New, original packaging

Note
Improper use of tools can damage watches. Use is at your own risk.