2-3cm is a lot bigger than you think it is
a spring guide bearing and sorbo add 7-8mm at the most and it's not likely the spring. most people are running both piston head and spring guide bearings taking 10mm off the spring's length, and the sorbo in front of the piston has nothing to do with the spring's travel
I'd say your best best is the spring guide probably has a hard time fitting in the back of the piston. Open the mechbox and see if the spring guide bearing fits in the back of the piston.
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