One of the first important steps in grand piano action regulation is the adjustment of glide bolts. The glide bolts sit at four points distributed lengthwise along the center rail of the grand piano action. They have a tuning-pin-shaped protrusion towards their top, a central threaded portion that allows for up-down adjustment, and smooth rounded bottom. The bottom of the glide bolt both provides support for the center rail of the action and allows the piano to easily glide from left to right when the una cord pedal is depressed.
When glide bolds are properly adjusted the action will securely rest on all four glide bolts equally. No glide bold will be adjusted too high (thus not in contact with the key bed) or too low (thus supporting too much weight in comparison to the other glide bolts). If glide bolts are not properly adjusted the center rail will not provide proper support to the keys.
There are two popular methods for adjusting glide bolts. One involves removing all of the keys in the piano and then inserting thin strips of paper between the glide bolts and the key bed to detect for proper glide bolt height. The trick is to slowly lower the each glide bolt with the paper beneath it until there is a slight resistance when trying to pull the paper from between the glide bolt and the key bed. The second method of glide bolt adjustment can be accomplished with the keys still in the action. We’ll share some video clips below to demonstrate how this type of glide bolt adjustment goes.
First you must raise all of the glide bolts so that they are not in contact with the key bed. This involves turning them counter clockwise until they are high enough. You’ll probably need less than a full turn to achieve this.
Chances are that a glide bolt is not in contact with the key bed if you do not see the nearby key fronts move down as you rotate counter-clockwise or up as you rotate clockwise. Be careful though, the pressure from your hand on the tuning hammer can also cause some downward movement of the keys when the glide bolt is not in contact with the key bed. Try to imagine what is happening. For example, the bottom of the glide bolt may touch the key bed as you move too far clockwise, thus raising the balance rail and consequently the local key heights.
Once all glide bolts have been raised enough so that none are in contact with the key bed you can begin your adjustment. You will adjust one bolt at a time and should not have to readjust any bolt much after deciding its initial position and moving on to others. You will adjust the bolts in the following order: treble, center right, bass, center left.
You want to lower each bolt to the point at which it just begins to bear weight. In order to find this point you need to tap the action above the center rail while slowly turning the glide bolt clockwise. Just before the glide bolt bears weight it will begin to make a clacking sound as you tap. This is the glide bolt clacking against the key bed. Just after the glide bolt bears weight the tapping will cause a more solid thumping sound instead of a clacking sound.
Similarly you can check for proper positioning of the glide bolt by listening as you pull up on the glide bolt with one hand and tap the keys above the nearby center rail with the other. Now you should hear a clacking sound as you lift the glide bolt and tap. The clacking should turn into the more solid thumping sound as you tap after releasing the glide bolt.
You may have to alternate between these two methods of checking the glide bolt position until you are satisfied. The next video shows the process of finding the right position for one glide bolt. We alternate between using the two methods described above.
Be careful not to adjust the glide bolts too low. It’s okay if the key fronts move upward when you rotate clockwise slightly past your chosen glide bolt position. But if they move counter clockwise slightly past your chosen position it may mean that your chosen position is too high. Again, you can imagine what is happening with the glide bolt and the key bed. You want that glide bolt just where it comes into contact with the bed. If it is any lower than that then it is pushing the center rail and thus the keys upward. And then if you rotate counter clockwise the keys fronts will move downward as the glide bolt releases its extra force on the key bed.
Special thanks to the Peruvian North American Cultural Center in Arequipa, Peru for the use of their piano for this demonstration. You can read more about el Centro Cultural Peruano y Norte Americano (CCPNA) in our posts about the FPF Ambassador Program.