diff --git a/designs/guitar_tuning_machine_head/family.json b/designs/guitar_tuning_machine_head/family.json
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+{
+  "family": "guitar_tuning_machine_head",
+  "standard": null,
+  "base_plane": "XY",
+  "description": "Sealed geared guitar tuning machine (Grover Rotomatic style): baseplate, sealed gear housing, string post with press-in bushing collar, worm/key shaft out the housing side, and a locating-screw hole. One solid.",
+  "source": "Grover Rotomatic 102 + Gotoh SG381 dimensioned drawings (post/bushing/housing/mount); proportion for housing depth and key shaft",
+  "contributor": "BenchCAD-org"
+}
diff --git a/designs/guitar_tuning_machine_head/part.py b/designs/guitar_tuning_machine_head/part.py
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+"""guitar_tuning_machine_head — the parametric part.
+
+A sealed geared guitar tuning machine (Grover Rotomatic style): a baseplate that
+sits against the headstock rear, a sealed gear housing behind it, a string post
+standing up through the peg-hole with a press-in bushing collar at its base, a
+worm/key shaft out the side of the housing, and a small locating-screw hole in
+the baseplate. Built in one piece (as cast + assembled), a single solid.
+
+Interface + examples: docs/DESIGN_SPEC.md
+"""
+
+import cadquery as cq
+
+
+def build(baseplate_l, baseplate_w, plate_t, housing_w, housing_h,
+          post_d, post_h, bushing_od, worm_d, mount_d, worm_both_sides):
+    # baseplate against the headstock, centred at z=0
+    result = cq.Workplane("XY").box(baseplate_l, baseplate_w, plate_t)
+
+    # sealed gear housing behind the baseplate (-Z)
+    result = result.union(
+        cq.Workplane("XY").box(housing_w, baseplate_w * 0.85, housing_h)
+        .translate((0.0, 0.0, -(plate_t / 2.0 + housing_h / 2.0)))
+    )
+
+    # string post up through the peg-hole, with the press-in bushing collar
+    result = result.union(
+        cq.Workplane("XY").workplane(offset=plate_t / 2.0).circle(post_d / 2.0).extrude(post_h)
+    )
+    result = result.union(
+        cq.Workplane("XY").workplane(offset=plate_t / 2.0).circle(bushing_od / 2.0).extrude(plate_t * 0.7)
+    )
+
+    # worm / key shaft out the side of the housing (one side, or both)
+    z_house = -(plate_t / 2.0 + housing_h / 2.0)
+    reach = baseplate_w * 0.9
+    shaft = cq.Workplane("XZ").circle(worm_d / 2.0).extrude(reach)  # -Y direction
+    shaft = shaft.translate((0.0, baseplate_w * 0.3, z_house))
+    result = result.union(shaft)
+    if worm_both_sides:
+        shaft2 = cq.Workplane("XZ").circle(worm_d / 2.0).extrude(-reach)  # +Y direction
+        shaft2 = shaft2.translate((0.0, -baseplate_w * 0.3, z_house))
+        result = result.union(shaft2)
+
+    # locating-screw hole near the +X end of the baseplate
+    mount_x = baseplate_l / 2.0 - mount_d - 2.0
+    hole = cq.Workplane("XY").circle(mount_d / 2.0).extrude(200.0).translate((mount_x, 0.0, -100.0))
+    result = result.cut(hole)
+
+    return result
diff --git a/designs/guitar_tuning_machine_head/preview.png b/designs/guitar_tuning_machine_head/preview.png
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diff --git a/designs/guitar_tuning_machine_head/spec.py b/designs/guitar_tuning_machine_head/spec.py
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+"""guitar_tuning_machine_head — the benchmark generator spec.
+
+A sealed geared guitar tuning machine (Grover Rotomatic / Gotoh SG381 style).
+PARAM_SPEC declares each build() parameter; check() holds the engineering rules
+a reviewer audits. Nothing is coupled, so there is no refine().
+Spec: docs/DESIGN_SPEC.md
+
+Sources:
+- Grover Rotomatic 102 dimensioned drawing (WD Music): post Ø6, bushing Ø7.8-9.9,
+  M8 bushing thread, housing ~27 x 24 mm, post height ~27 mm, mount screw Ø2.6.
+- Gotoh SG381 drawing: post Ø6, hex bushing OD 14 mm, housing ~27 x 22 mm.
+- Gear ratio (14:1-18:1) is internal and not a CAD dimension here.
+"""
+
+
+# ── 1. PARAM_SPEC ────────────────────────────────────────────────────────────
+PARAM_SPEC = {
+    "baseplate_l": dict(
+        desc="baseplate length (along the string line)",
+        unit="mm",
+        range={"easy": (25.0, 30.0), "medium": (22.0, 38.0), "hard": (20.0, 45.0)},
+        source="Grover/Gotoh housing footprint ~27 mm -> proportion",
+        askable=True,
+    ),
+    "baseplate_w": dict(
+        desc="baseplate width",
+        unit="mm",
+        range={"easy": (12.0, 15.0), "medium": (11.0, 16.0), "hard": (10.0, 18.0)},
+        source="Grover baseplate ~13-15 mm -> proportion",
+        askable=True,
+    ),
+    "plate_t": dict(
+        desc="baseplate thickness",
+        unit="mm",
+        range={"easy": (1.2, 2.0), "medium": (1.0, 2.5), "hard": (1.0, 3.0)},
+        source="proportion (die-cast baseplate)",
+        askable=True,
+    ),
+    "housing_w": dict(
+        desc="gear-housing width",
+        unit="mm",
+        range={"easy": (18.0, 24.0), "medium": (15.0, 26.0), "hard": (14.0, 28.0)},
+        source="Grover/Gotoh sealed housing ~24-27 mm -> proportion",
+        askable=True,
+    ),
+    "housing_h": dict(
+        desc="gear-housing depth behind the baseplate",
+        unit="mm",
+        range={"easy": (12.0, 18.0), "medium": (10.0, 20.0), "hard": (10.0, 24.0)},
+        source="proportion (sealed worm-gear housing depth)",
+        askable=True,
+    ),
+    "post_d": dict(
+        desc="string-post diameter",
+        unit="mm",
+        range={"easy": (5.8, 6.3), "medium": (5.5, 6.5), "hard": (5.5, 6.5)},
+        source="Grover 6.0 / Grover Super 6.3 / Gotoh 6.0 mm",
+        askable=True,
+    ),
+    "post_h": dict(
+        desc="string-post height above the baseplate",
+        unit="mm",
+        range={"easy": (24.0, 28.0), "medium": (18.0, 30.0), "hard": (15.0, 32.0)},
+        source="Grover post height ~27 mm -> proportion",
+        askable=True,
+    ),
+    "bushing_od": dict(
+        desc="press-in bushing collar outer diameter",
+        unit="mm",
+        range={"easy": (7.5, 10.0), "medium": (7.0, 14.0), "hard": (7.0, 15.0)},
+        source="Grover Ø7.8 / Ø9.9 collar; Gotoh Ø14 hex bushing",
+        askable=True,
+    ),
+    "worm_d": dict(
+        desc="worm / key-shaft diameter",
+        unit="mm",
+        range={"easy": (5.0, 7.0), "medium": (4.0, 8.0), "hard": (4.0, 9.0)},
+        source="proportion (worm-gear key shaft)",
+        askable=True,
+    ),
+    "mount_d": dict(
+        desc="locating-screw hole diameter",
+        unit="mm",
+        range={"easy": (2.4, 2.8), "medium": (2.0, 3.2), "hard": (2.0, 3.5)},
+        source="Grover mounting screw Ø2.6 mm",
+        askable=True,
+    ),
+    "worm_both_sides": dict(
+        desc="key shaft exits both sides of the housing (1) vs one side (0)",
+        unit="",
+        range={"easy": (0, 0), "medium": (0, 1), "hard": (0, 1)},
+        source="in-line (both-sides) vs single-side key-shaft variants",
+        choices={"easy": [0], "medium": [0, 1], "hard": [0, 1]},
+        feature=True,
+    ),
+}
+
+
+# ── 2. check ─────────────────────────────────────────────────────────────────
+def check(p: dict) -> list[str]:
+    """Engineering constraints (empty = valid). Each cites its rule."""
+    bad = []
+
+    # bushing must surround the post with a wall, and fit on the baseplate
+    if p["bushing_od"] < p["post_d"] + 1.5:
+        bad.append("bushing_od < post_d+1.5: bushing collar must surround the post with wall")
+    if p["bushing_od"] > p["baseplate_w"] - 1.0:
+        bad.append("bushing_od > baseplate_w-1: bushing wider than the baseplate")
+
+    # housing sits on the baseplate; worm fits inside the housing depth
+    if p["housing_w"] > p["baseplate_l"]:
+        bad.append("housing_w > baseplate_l: gear housing overhangs the baseplate")
+    if p["worm_d"] > p["housing_h"] - 2.0:
+        bad.append("worm_d > housing_h-2: key shaft does not fit the housing depth")
+
+    # the locating-screw hole (near the +X end) must clear the bushing collar
+    mount_x = p["baseplate_l"] / 2.0 - p["mount_d"] - 2.0
+    if mount_x - p["mount_d"] / 2.0 < p["bushing_od"] / 2.0 + 2.0:
+        bad.append("mount hole overlaps the bushing: baseplate too short for the collar + screw")
+
+    return bad