Although a grandfather clock is an imposing and attractive piece of furniture that can dominate a room, it also is a timepiece. If your grandfather clock is not keeping accurate time, you can adjust it to put it back on track. A weight-driven clock keeps the pendulum in motion by an impulse from the clock's mechanism. As the pendulum swings back and forth, it regulates the timekeeping ability of the clock. You can adjust the pendulum to improve the timekeeping of a grandfather clock, but it takes patience.
Wind the grandfather clock with a winding key or crank.
Set the correct time on the grandfather clock. Use an accurate source such as a computer clock as your reference. Check again after 24 hours.
Stop the motion of the pendulum by placing your hand in its path at either end as it completes its arc. Hold a soft cloth in your hand as you bring the pendulum to a halt as gently as possible. Allow the pendulum to hang freely.
Locate the rating nut directly below the pendulum bob. The bob is the large round disc mounted on the pendulum rod. The rating nut controls the position of the bob, which increases or decreases the speed of the clock mechanism.
If the clock is fast, turn the rating nut one full turn counterclockwise for each minute the clock is fast in a 24-hour period. When adjusting the rating nut in this direction, make sure the bob is lowered and remains in contact with the nut. Use the cloth to grip the bob when lowering it.
If the clock is slow, turn the rating nut one turn clockwise for each minute the clock is running slow in 24 hours.
Set the pendulum in motion by pulling the bob near the extent of its arc and releasing it. Avoid twisting the bob in any way as you pull. Grip the bob using the cloth to cover your hands.
Check the clock again in 24 hours. Continue the process of turning the rating nut in smaller increments until the clock is keeping the correct time.
The amount you need to turn the rating nut might vary depending on the clock, so you might need to experiment. The key to success is to be patient and observe how the clock responds to your adjustment every 24 hours. If the pendulum has two nuts, the lower one is for finer adjustments of the clock's timekeeping ability. Once you get the time as accurate as you can by moving the bob, you can move the lower nut up or down to "fine tune" the grandfather clock mechanism.
To avoid corrosion, handle all brass parts with a clean cloth.