4" Garage Door pulley This is a 4" pulley with 3/8" bolt and sleeve(strap or sheave fork) that attaches to the end of an extension spring. When the bearings of the pulley go bad, it causes the door to run very rough. It is very jerky in its travel. Replacing the pulleys will cause the door to run more smoothly. Also, check your rollers and make sure they rotate freely because they can have a similar effect on the function of your door.