This was lucky but shows that usually the Japanese are honest in business (my experience at least).
But a shame... the sanding is not really the solution plus the Japan solenoid acts much faster:
Necessary force to pull the plunger:
100g (0,98N) (the strongest ever 120g) Japan Solenoid
200g (1,96N) (strongest ever 250g) China Solenoid
With a powersupply based on a simple microcontrollerboard one can test how quick the solenoids releases.
Tested with 7.2V from the orig. Pentax Li-Ion battery D-LI109:
Releasetime Japan Solenoid: 10ms (and it still functions with an shorter impuls of 4ms!)
Releasetime China Solenoid: 100ms! It won't function at all with 10ms, forget 4ms.
But if one runs the China-Solenoid several times with 100ms/7,2 Volts and then tries with 10ms it does work 1, sometimes even 2 times
but then collapses back into its previous "stronger position".
But worse: If one uses it many times with 100ms and then just lets it stand for a few hours it won't even once release anymore with 10ms but only 100ms!
Now changing over to AA-Eneloops instead of using the Li-Ion D-LI109 things do change:
The releasetime is now between 50ms for the Chinasolenoid and 8ms for the Japansolenoid, which even can at times be switched within just 2ms!
But that the solenoid is sticking and not releasing is not only due to the fact that the plunger is held with more force!
The plunger "wiggles" more in the body of the green solenoid. The body acts as a bearing. The more precise a bearing is, the more presice the function.
This is partly due to the fact that PET (green solenoid) is a cheaper material (and softer) while the teflonbody of the white Japan solenoid can do this job way better.
Teflon (PTFE) is a great bearing material, for example all "Well Tempered turntables" (some of the worlds best) use teflon bearings since about 1988 and the run and run and run without any failing. If such a bearing would be made out of PET... it wouldn't last 1000 hours and soon the shaft would wiggle in the bearing, the platter wobble, you could see it (worse, hear it!)
Next problem: the plunger is made out of an alloy and has shown several times rust.
All this can be read
HERE.
The problem is, post like yours can encourage others to go "the cheap way", the result can be final failure.