Regulating Steinway's Case-Mounted Sostenuto
By Sofia de la Peña, RPT
Editor’s Note: If you are unfamiliar with sostenuto theory, this article is not the place to start! First, read Sostenuto Regulation to become familiar with basic sostenuto theory, then read the rest of this article. Trust us, it will make a lot more sense!
Previously, Steinway & Sons mounted sostenuto systems to the action. About ten years ago, Steinway developed a case-mounted sostenuto system to accommodate the Spirio player system, which is now used on all Steinway pianos.
The general function of the sostenuto is similar to any other sostenuto system; however, the biggest differentiation between this system and others is the Sostenuto Cross Rod Assembly mounted to the plate at the bass/tenor break:
Here is a diagram of the parts that make up the Sostenuto Cross Rod Assembly (from the side view):
This assembly has several points of regulation that we will reference throughout this article, so check back here for a terminology refresher as needed.
There are three points of regulation involved:
The Up/Down Setting (Height gap): The sostenuto blade must rest 1/16” above the damper tabs.
The In/Out Setting (Width gap): There must be a 1/16” gap between the sostenuto blade and the damper tabs.
The Rotational Setting: The tip of the sostenuto blade must be visible when viewed from above. This ensures the correct starting point of the blade’s travel.
Required Tools:
⅜” open-end wrench
⅜” open-end wrench (thin-walled)
7/16” open-end wrench
Additional 7/16” open-end wrench
Streamlight “stylus reach” (#65618) (AAAA batteries)
5/32” hex key
9/64” hex key
#2 Phillip Heads screw driver
The ability to eyeball 1/16”
Up/Down (Height Gap) Regulation
The sostenuto blade must rest 1/16” above the damper tabs.
First, observe each damper section to determine if the blade itself is 1/16” above the tabs. This gap is much more obvious when you put the Stylus Reach light behind the blade.
The height adjustment is made with two different types of systems: The pivot brackets and the turnbuckle.
Pivot Bracket Adjustment
Begin by adjusting the pivot brackets. There should be three to four of these attached to the belly, depending on the piano’s size:
To make an adjustment, simply loosen both #2 Philips head screws, then manually position the bracket so the sostenuto blade is 1/16” above the tabs, hold it firmly in place, then tighten the screws.
On pianos where this adjustment has never been made before, the pivot bracket may be fused to the bellyrail. There are two ways to release the bracket. First, loosen both screws and place a flathead screwdriver on top of the bracket, then gently hammer it down to break the bond between the bracket and the bellyrail.
The Pianosmiths tapper works great for this, but a small hammer will do. If that doesn’t release the bracket, with the top screw still loosened, remove the lower screw completely, then use a small flathead screwdriver to tap the lower part of the pivot bracket again to loosen the bond.
Because of the awkward angles, this is definitely a two-person job! It’s best to have one person holding the tip of the screwdriver in place in the keybed cavity while another person taps the screwdriver from above. The pivot bracket is soft brass and can be damaged. This also helps to prevent any damage to the bellyrail.
Turnbuckle Adjustment
The second system that controls the up/down position of the sostenuto rod is the ⅜” turnbuckle found on the Sostenuto Cross Rod Assembly:
To release the turnbuckle, grip it with a ⅜” wrench and loosen the lock nut with a thin-walled ⅜” wrench.
(Yes, it must be a thin-walled wrench! A regular wrench will not fit this system.) Once the turnbuckle is free to move, turn the wrench clockwise to lower the sostenuto blade; turn it counterclockwise to raise the blade.
Once the blade is 1/16” above the sostenuto tabs, hold the ⅜” turnbuckle with the wrench and tighten the locknut with the thin-walled wrench against the plate. Don’t use too much force; “tool snug” is plenty of pressure.
To avoid stripping or warping anything, make sure to use both wrenches. At no point during this process should your wrench touch this collar:
In/Out (Width-Gap) Regulation
There must be a 1/16” gap between the sostenuto blade and the damper tabs.
To assess the in/out position of the blade relative to the damper tabs, look through the dampers and strings and assess if there is a 1/16” gap between the blade and the tabs. Again, this is where this Stylus Reach light becomes super handy. If you place this light underneath the damper underlevers, this gap (or lack thereof) becomes visually apparent.
If your height prevents you from seeing from above, you may need to climb onto the keybed to see this gap.
This adjustment is also made in two different places: the pivot brackets and the foot bracket.
Pivot Bracket
First, loosen the 9/64” socket screw, and adjust the bracket for a 1/16” gap between the tabs.
The bracket tends to twist to the side, so make sure to hold the bracket so you don’t warp or strip it when you tighten the socket screw.
At this stage, it’s a good idea to check the bushing cloth inside the bracket. It is not glued to the bracket and can work itself out over time.
If the bushing cloth inside the bracket starts to fall out, you can either push it back in or loosen the set screw under the bracket to reposition it.
You can either loosen the set screw with a small offset wrench, or take the entire hex socket screw out and loosen it with a regular Phillips head screwdriver. After loosening the set screw, place the bushing cloth with the seam side pointing down.
If you do decide to take the whole screw out, be very careful when putting it back in. It’s screwed in at an angle, and the soft brass is easily damaged. As you re-tighten the set screw, make sure the jaws of the bracket are square.
Foot Bracket Adjustment
To adjust the in/out position, loosen both 5/32” hex socket screws on the foot bracket, then move the foot bracket until you see that the blade is 1/16” away from the tabs.
When tightening the socket screws, secure the hex socket screw closest to the plate first. Then tighten the other one. Any in/out or up/down adjustments will affect the rotational position of the blade because the belly is slightly at an angle. These adjustments must be made before adjusting the rotational position of the blade.
Rotational Regulation
The tip of the sostenuto blade must be visible when viewed from above.
The sostenuto blade should rest at about the 7 o’clock position. If the blade rests closer to the 9 o'clock position, it will rotate too far above the damper sostenuto tabs and end up on top of them instead of underneath. If the blade rests closer to the 6 o'clock position, the blade’s travel is too limited to engage the damper tabs at the end of travel.
When viewing the sostenuto blade from above, a 6 o’clock position will not show the blade at all, a 9 o'clock position shows too much of the blade, and a 7 o’clock position shows just a bit of the blade.
To change the rotational angle of the sosteunto blade, you must adjust the sostenuto lift rod nut, which is the lowest nut on the whole case-mounted assembly. The nut directly above is the locknut.
To adjust, hold the sostenuto lift rod nut securely with a 7/16” wrench and loosen the 7/16” locknut with a second wrench.
Turning the sostenuto lift rod clockwise rotates the blade up towards the horizontal plane. Turning it counterclockwise rotates it down toward the vertical plane.
Once the blade is in the correct position, hold the sostenuto lift rod nut still with a wrench, and tighten the upper 7/16” lock nut against the collar. Do not touch the collar above the locknut.
Troubleshooting
Test 1: Evenness of the blade placement
Start by fully depressing the damper pedal and then engage the sostenuto pedal. The damper should rise by 1/32” to 1/16”.
Next, release the damper pedal, then slowly release the sostenuto pedal. The dampers should fall randomly. If they fall in sections, this indicates uneven placement of the blade from the tabs.
Test 2: Gaps too wide
Engage the sostenuto pedal and play a fortissimo chromatic scale while observing from above. If any dampers slip past the blade or hang open, this indicates that the sostenuto blade is too far away or too low from the damper sostenuto tabs. It could also occur if the damper upstop rail is set too high.
Test 3: Functionality
Engage the sostenuto and play each note individually to ensure the blade functions with the key and that no other dampers are lifted. If the sostenuto engages with the pedal and not the key, assess the damper timing. If dampers are lifted, assess blade and tab position.
Service bulletin courtesy of Steinway & Sons:
For Steinway support, email customerservice@steinway.com.
Sofia de la Peña graduated from NBSS in 2020 and has been working as a technician in the Northern Virginia area ever since. Besides servicing her private clients, she also works as the concert technician for the Center for the Arts at George Mason University and is a technician on staff at Levine School of Music.
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