The Ko-ken Z-Series 3/8" SAE sockets were an immediate purchase for me. They arrived yesterday, and the first question on my mind was: "Is the 3/4 socket the same as the 19mm?". I've heard that many tool companies simply re-label their 3/4" sockets as 19mm (or vice-versa) since 3/4" = 19.05mm. But would my beloved Ko-ken, maker of "high-performance tools of convincing quality", belittle themselves with such underhanded tactics? Or would they free their engineers to design a true 3/4" Z-series socket? The question gnawed at my consciousness as I tried to sleep: I had to know. As I lay awake, I devised a plan that would convince me of Ko-ken's quality.
Due to a (likely-unhealthy) fear of not having the right tool for the job, I have amassed a large assortment of 6-point 19mm Ko-ken sockets. Using this collection, I could establish the typical flat-to-flat opening for Ko-ken Z-series and normal 19mm sockets. I could also infer a manufacturing "consistency" of their machining process: a perfect socket should have the same measurement across all three pairs of flats. I'm defining "consistency" as the largest flat-to-flat measurement minus the smallest one. With my trusty Mitutoyo calipers in hand, I set about measuring the flat-to-flat opening of all three sides of my 19mm Ko-ken socket collection. Three times, just to be sure. I also measured the outer diameter and the length, because why not.
Ko-ken claims that Z-series sockets have a tighter fit on fasteners by reducing the flat-to-flat opening. The data supports this. Anyone who has used the Z-series knows they fit tighter on fasteners. Looking at flat-to-flat consistency measurements, the data does not support that the overall manufacturing tolerance is better on the Z-series; the normal sockets are just as good. Note that my copy of the 3300MZ 19mm had one flat-to-flat measurement substantially larger than the other two opposing sides; this is likely skewing the data. Without that "defect" (using that term lightly), the Z-series would have had an average size of 19.166mm and a consistency of 0.023mm, which is better than the normal sockets.
This data establishes that Ko-ken usually has about a 19.17mm opening for a Z-series 19mm socket. Their consistency (for both normal and Z-series) is about 0.03mm. Given that 3/4" is 19.05mm, I would expect a true 3/4" socket to be about 0.05mm larger than 19.17mm. And, the 3400AZ 3/4" socket is indeed larger; it is 19.27mm, (a full 0.05mm more than needed!), with a very good consistency of 0.01mm.
So, did Ko-ken design and build a true 3/4" Z socket? My conclusion is that this is not simply a re-label of the 19mm Z. What I think they did was use the same forging and machining process for the outer dimensions (the length and diameter are nearly identical), and then used a dedicate broach for machining the flats. It is unlikely that a normal series 19mm socket broach would have created the 3/4" Z-series, since the inside geometry of the Z-series is not the same as their normal sockets.
I can now rest easy knowing that Ko-ken is not taking short-cuts in their products. I'm sure the rest of you Japanese tool zealots will also appreciate this is a true 3/4" socket, not some re-labelled metallic swill dumped on the market to appease Americans.
Below you will find a table of my measurements. The "average size" and "consistency" are the average of all three sides measured 3 times. I've highlighted the normal Z 19mm socket in blue and the new Z 3/4" socket in red. I've also included a Ko-ken 13401M (3/8" impact socket, standard length) and a Nepros 1/2" 19mm socket, for comparison.
Due to a (likely-unhealthy) fear of not having the right tool for the job, I have amassed a large assortment of 6-point 19mm Ko-ken sockets. Using this collection, I could establish the typical flat-to-flat opening for Ko-ken Z-series and normal 19mm sockets. I could also infer a manufacturing "consistency" of their machining process: a perfect socket should have the same measurement across all three pairs of flats. I'm defining "consistency" as the largest flat-to-flat measurement minus the smallest one. With my trusty Mitutoyo calipers in hand, I set about measuring the flat-to-flat opening of all three sides of my 19mm Ko-ken socket collection. Three times, just to be sure. I also measured the outer diameter and the length, because why not.
Ko-ken Socket Series | Average Size | Consistency |
| Z-Series (3300MZ, 3300XZ, 4400MZ, 3400MZ) | 19.174 mm | 0.037 mm |
| Normal (4300M-110, 4300M, 3300M, 3400M) | 19.301 mm | 0.033 mm |
Ko-ken claims that Z-series sockets have a tighter fit on fasteners by reducing the flat-to-flat opening. The data supports this. Anyone who has used the Z-series knows they fit tighter on fasteners. Looking at flat-to-flat consistency measurements, the data does not support that the overall manufacturing tolerance is better on the Z-series; the normal sockets are just as good. Note that my copy of the 3300MZ 19mm had one flat-to-flat measurement substantially larger than the other two opposing sides; this is likely skewing the data. Without that "defect" (using that term lightly), the Z-series would have had an average size of 19.166mm and a consistency of 0.023mm, which is better than the normal sockets.
This data establishes that Ko-ken usually has about a 19.17mm opening for a Z-series 19mm socket. Their consistency (for both normal and Z-series) is about 0.03mm. Given that 3/4" is 19.05mm, I would expect a true 3/4" socket to be about 0.05mm larger than 19.17mm. And, the 3400AZ 3/4" socket is indeed larger; it is 19.27mm, (a full 0.05mm more than needed!), with a very good consistency of 0.01mm.
So, did Ko-ken design and build a true 3/4" Z socket? My conclusion is that this is not simply a re-label of the 19mm Z. What I think they did was use the same forging and machining process for the outer dimensions (the length and diameter are nearly identical), and then used a dedicate broach for machining the flats. It is unlikely that a normal series 19mm socket broach would have created the 3/4" Z-series, since the inside geometry of the Z-series is not the same as their normal sockets.
I can now rest easy knowing that Ko-ken is not taking short-cuts in their products. I'm sure the rest of you Japanese tool zealots will also appreciate this is a true 3/4" socket, not some re-labelled metallic swill dumped on the market to appease Americans.
Below you will find a table of my measurements. The "average size" and "consistency" are the average of all three sides measured 3 times. I've highlighted the normal Z 19mm socket in blue and the new Z 3/4" socket in red. I've also included a Ko-ken 13401M (3/8" impact socket, standard length) and a Nepros 1/2" 19mm socket, for comparison.
Model | Length [mm] | Diameter [mm] | Average Size (mm) | Consistency [mm] |
4300M-110 | 110.38 | 25.80 | 19.278 | 0.013 |
4300M | 77.11 | 26.44 | 19.289 | 0.023 |
3300MZ | 55.07 | 24.87 | 19.202 | 0.103 |
3300M | 55.11 | 24.94 | 19.337 | 0.057 |
3300XZ | 35.03 | 24.84 | 19.207 | 0.017 |
3400M | 30.11 | 25.78 | 19.301 | 0.037 |
4400MZ | 29.15 | 25.30 | 19.146 | 0.030 |
3400MZ | 22.10 | 24.79 | 19.141 | 0.027 |
3400AZ (3/4") | 22.12 | 24.83 | 19.272 | 0.010 |
13401M | 32.09 | 26.02 | 19.143 | 0.010 |
| KTC Nepros NB4 | 34.53 | 25.69 | 19.211 | 0.010 |
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