Bolt size It is very important to know the size of the bolts! Screw length: 120mm Thread length: 50mm Non-thread length: 57mm Thread diameter: 16mm START TRAINING LEG MUSCLES Simple installation Install the leather roller (one long and one short) and the weight plate holder on the machine.Use the bolt（M16） to fix the whole accessory on the squat rack(Hole diameter :16mm)Check the height and fit the spacer (don't forget), then tighten the nut. The length of the rack aperture needs to be more than 57mm to fit our machines properly. If your rack is smaller than this size, it may be unstable after installing. you can solve it by using spacers to increase the width of the rack. We have supplied 2 spacers in package DUAL EXPERIENCE Our Leg Extension and Curl Machine can achieve both front and back training, effectively exercise your leg muscle groups.(Please note: You will need a bench or jump box to assist with the workout when doing leg curls!) TARGETED TRAINING Our leg extension attachment can target different muscle groups in your legs, and you can enjoy professional leg exercises without spending extra money on large fitness equipment. KNEE FRIENDLY Our leg extension attachment, under normal use, do not cause injury to the knee. Regular leg exercises to enhance the strength of the leg muscles can effectively protect the knee joints and avoid knee diseases.