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Dear Fabio, although The GT2 16t can fit exactly, I'm afraid they will not stand with the heavy load/pressure.
The cheap Chinese idler will fail after 10-20 hours of work, while a good gt2 can handle much more but less then 623zz bearings.
If you will hear a squicking sound - you should know that the idlers need to be replaced.
In case that you decides to use the 623zz please flip the belts on the rear so the flat side of each belt will touch the corner bearing (and not the teeth side).