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Today I'm reviewing the Ko-ken 3/8" (or 9.5mm) extension set PK3760/6 (eBay US, eBay CA, eBay AU, Amazon, Rakuten). If you aren't familiar with the brand, Koken are a Japanese manufacturer who specialise in socketry and related tools. This particular extension set is the non-wobble variant - the wobble variant is part number PK3763/6 (eBay US, eBay CA, eBay AU). What's the difference? The wobble-fix extensions have a cut-in that allows for 15° of movement for whatever is attached to the male end. Why wouldn't you just buy the wobble-fix? Koken say that "The Wobble-Fix Extension Bar has less strength than normal extension bar (sic) due to it's specific design with a reduced square drive."
The PK3760/6 set has six extensions included - 32mm, 50mm, 75mm, 150mm and 250mm. The entire range of Koken's standard 3/8" extensions is a good deal more expansive than that selection and includes 300mm, 400mm, 500mm, 600mm, 900mm and 1,000mm extensions.
As you can see it comes in an ABS case, wrapped in plastic:
On the back is a paper insert:
Country of origin lower left:
Sizes on the right:
Turns out the knurling is patented:
With the plastic removed we can see the contents:
The knurling feels as nice as it looks:
The grip provided by the knurling is good and works well. It doesn't make it as easy to freely spin the extension as a quick spinner (like the Koken 3756Z) but it's miles better than a smooth chrome extension, particularly with oily fingers.
The corners of the female end are rounded, which is good as it means that the force will be transferred across the flats rather than the corners:
There's a detent on the inside to receive the ball bearing to retain the extension on the ratchet wrench/other extension/etc.:
The brand and part number - the last part of which is the length - is marked on each of the extensions:
...along with the country of origin:
In use the extensions are top notch - they fit very tightly, much more so than the Taiwanese-made Kincrome set I usually use (K2924, slightly cheaper than the Koken on eBay AU). The knurling is nice under the fingers and provides good grip, with the additional benefit of preventing the extensions from rolling away. If you put a socket on it's side I think you expect it to roll somewhere - hard not to leave extensions on their side, though, and it has been a nice experience having them not roll anywhere due to the knurling being separated into six separate sections with gaps inbetween.
The range of lengths in the kit is quite useful for the work I do on motorcycles and cars (largely Japanese). The slop in these is minimal, particularly when paired with a quality ratchet like the Nepros NBRC390L or the Koken 3725Z (eBay US, eBay CA, eBay AU). Speaking of Koken's Z-series, there are a couple of extensions available from it - as of the May 2019 Japanese catalogue they only have 75, 125 and 250mm lengths listed as part numbers 3760Z-75, 3760Z-125 and 3760Z-250. So, if you're set on the Zeal range you'll have a lot fewer choices extension-wise - and you miss out on the knurling!
After using them for a week I can wholeheartedly recommend them as a quality extension set - the range of lengths is useful, the knurling is a nice feature and they're not badly priced (check links above!).
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