The most popular screws sizes in 1/10 buggies such as the X6 you mention are either imperial (unified) 4-40 and metric M3x0.5 (or M3 for short).
The UNC (unified coarse) 4-40 is a #4 (approx 2.8mm) screw with 40 threads per inch.
The Metric M3 is a 3mm screw with a thread every 0.5mm.
Very different screws which really don't mix, but you can usually force a larger M3 into plastic which should be occupied by a 4-40
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