Includes prefiited M4 grub screw. Center hole diameter is 5mm. Length 14mm. The no tooth marks bit is 8mm in length by 12mm in diameter. The tooth marked bit is 16.5mm in diameter 6mm wide. Module 1. Suitable to fit 540 or 550 size electric motors brushed / brushless. Important Note: This gear is for 1/8 models. Please check measurements above before purchasing.