Hollow Knight Silksong's Cogheart Pieces are clockwork keys fans must collect to unlock late-game progression and, ultimately, Act 3's true ending. The three pieces fuse into the Cogheart, powering the clockwork Second Sentinel as part of the journey forward. Each piece lives behind a bell memory puzzle within the Citadel's depths, typically near the end of the area’s progression and surrounded by enemies, traps, and hazards.
The Cogheart Piece in the Choral Chambers is technically the first players access if the area is fully cleared on arrival. It sits in the far western part of the Chambers, above the Ventrica and northwest of the Citadel Spa. Access comes from either navigating the chamber or traveling east from The Slab. The piece is inside a room on the left side of the tall chamber's western end. To obtain it, solve a bell memory puzzle by pressing seven bells in three sequences. Number the bells from right to left as 1 through 7. The correct sequences are: First sequence: 1-5-4-6. Second sequence: 3-6-5-1-7. Third sequence: 2-4-6-3-5-1. Enter all three sequences in order, and the Cogheart Piece is yours. The puzzle doesn't fully reset when you solve one sequence; solving two keeps the progress for the third intact, sparing players from starting over.
The Cogheart Piece near the Whispering Vaults sits near the end of that progression path, in a side chamber near The Stage where Trobbio is fought. It cannot be reached via The Stage itself; instead, descent from the Songclave in the upper-right portion of the Choral Chambers is required, or access via the Underworks if arriving from the Exhaust Organ. The Vaults piece is in the bottom-left room of the tall central chamber. Here, the bell memory puzzle follows the same seven-bell layout as the first piece. With seven bells, the correct order is: First sequence: 1-2-7-4. Second sequence: 7-5-6-4-5. Third sequence: 2-7-3-5-6-1. Enter all three sequences to move a step closer to completing the Cogheart.
The Memorium Cogheart Piece is technically the easiest to locate, but reaching it requires the Faydown Cloak double jump. Upon entering Memorium, travel east past the big enemies, and after grabbing the map on the ground floor, take the next right-hand passage above. The memory puzzle here uses a clock-like layout, so the bells are read from the one o'clock position and proceed clockwise around the ring to seven, which sits at the center. The correct order is: First sequence: 3-1-7-4. Second sequence: 7-1-6-7-5. Third sequence: 6-2-4-1-3-5.
After collecting all three Cogheart Pieces, they fuse instantly into the Cogheart, a mechanical engine used to power the clockwork Second Sentinel. To reach the Clockwork Core beneath the Cogwork Dancers boss fight, players should travel to the western edge passage. Provided the double jump is available, the inert Sentinel can be reached by leaping into the opening above the spinning gear hazard, then traveling as far to the right as possible. The Cogheart is placed inside the Sentinel, prompting a brief conversation between Hornet and the awakened machine before it departs for parts unknown. Expect to see it again on the journey ahead.
With the Cogheart Pieces now known, it is worth considering a return to lower Pharloom to purchase any mapped locations from Shakra or to grab Silkshot if it has not yet been acquired. There remains a substantial journey ahead in Silksong, and more discoveries are likely to follow.