Print 1 part A, 1 part B to make a pair. For maximum strength, use concentric pattern, 100% infill in all layers.
Each pair requires :
1 M3x8 allen cap screw
1 M3 nut
Orient parts and install as shown in photos so the screw won't loosen as the belt is tensioned.
Place the tensioner as close to the carriage as posible.
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