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Hello! The actual equivalent length of 3/8" screws is 9.525mm, so what you have (10mm) is a slight bit longer, but that should be OK. The tibia sides are indeed assembled to the foot plate, base plate, and spacer tube with the 3/8" screws, and the additional length shouldn't matter for that. Is the plastic splitting when the screws are driven in? Perhaps it's because 4-40 screws are 0.112" in diameter, which converts to 2.8448mm while M3 screws are just a bit larger in diameter than that (the reference I found says they are actually 2.980mm diameter). If you find the M3 screws are splitting the PLA pieces, you may want to consider something smaller in metric, like M2.6 screws for example, if you can't get 4-40's. Good luck!