[rcktpwrd]

those go into the factory torque links commonly referred to as engine mounts. They take up the extra space in the rubber bushing to help eliminate movement in the mounts. They will add some additional vibration, it not really noticeable with a manual transmission. I installed two sets of them, one in the upper mount and one in the lower mount. The car feels a bit more solid on take off and especially during shifting as the engine doesn't rock back and forth quite as much.
I hear that doing both upper and lower with an auto trans is not advisable due to the increased vibrations, which is only noticeable at idle...