diff --git a/designs/drum_tension_lug/family.json b/designs/drum_tension_lug/family.json
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+{
+  "family": "drum_tension_lug",
+  "standard": null,
+  "base_plane": "XY",
+  "description": "Drum-shell tension lug: a radiused cast body that bolts to the shell with two centreline screws and receives the tension rod in a threaded insert bore at the head end (both ends for a double-ended tube lug).",
+  "source": "SD Sparedrum L-series lug catalogue (body, hole spacing, mount + rod bores); Pearl spare-parts catalogue rod threads; proportion for corner radius",
+  "contributor": "BenchCAD-org"
+}
diff --git a/designs/drum_tension_lug/part.py b/designs/drum_tension_lug/part.py
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+"""drum_tension_lug — the parametric part.
+
+A drum-shell tension lug: a compact cast/chromed body that bolts to the outside
+of the drum shell and receives the tension rod that tightens the drumhead. The
+body sits on the shell (z=0, extending radially out in +Z), is mounted by two
+screws on its centreline into the shell, and carries a rod-thread insert bore at
+the head end (both ends when it is a double-ended tube lug). Corners are radiused
+for the cast look.
+
+Interface + examples: docs/DESIGN_SPEC.md
+"""
+
+import cadquery as cq
+
+
+def build(body_len, body_h, body_w, hole_spacing, mount_hole_d, rod_bore, fillet_r, double_ended):
+    # cast body, sitting on the shell face (z=0), extending radially out to z=body_h
+    result = cq.Workplane("XY").box(body_w, body_len, body_h).translate((0.0, 0.0, body_h / 2.0))
+    result = result.edges("|Z").fillet(fillet_r)
+
+    # two mounting screws on the centreline, blind up from the shell face
+    result = (
+        result.faces("<Z")
+        .workplane(centerOption="CenterOfBoundBox")
+        .pushPoints([(0.0, hole_spacing / 2.0), (0.0, -hole_spacing / 2.0)])
+        .hole(mount_hole_d, depth=body_h * 0.45)
+    )
+
+    # rod-thread insert bore at the head end (+Y), on the body axis
+    result = (
+        result.faces(">Y")
+        .workplane(centerOption="CenterOfBoundBox")
+        .hole(rod_bore, depth=body_len * 0.45)
+    )
+    if double_ended:
+        # tube lug: a second rod bore at the opposite end
+        result = (
+            result.faces("<Y")
+            .workplane(centerOption="CenterOfBoundBox")
+            .hole(rod_bore, depth=body_len * 0.45)
+        )
+
+    return result
diff --git a/designs/drum_tension_lug/preview.png b/designs/drum_tension_lug/preview.png
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diff --git a/designs/drum_tension_lug/spec.py b/designs/drum_tension_lug/spec.py
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+"""drum_tension_lug — the benchmark generator spec.
+
+A drum-shell tension lug. 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:
+- SD Sparedrum L-series lug catalogue: body ~42 x 27 x 23 mm (L20BD), mounting
+  hole spacing faceted 25-70 mm, mounting hole Ø 6 / 8 mm, rod thread #12-24
+  (7/32", ~5.5 mm). Single-ended lugs and double-ended tube lugs.
+- Pearl spare-parts catalogue (rod threads #12-24 / M5 / M6); body scale
+  cross-checked against US8759653B2 (~20.6 mm width).
+"""
+
+
+# ── 1. PARAM_SPEC ────────────────────────────────────────────────────────────
+PARAM_SPEC = {
+    "body_len": dict(
+        desc="lug body length along the drum axis (rod direction)",
+        unit="mm",
+        range={"easy": (40.0, 55.0), "medium": (40.0, 90.0), "hard": (38.0, 150.0)},
+        source="SD Sparedrum L20 (42 mm) up to full-length tube lugs (Pearl CL/STL)",
+        askable=True,
+    ),
+    "body_h": dict(
+        desc="lug body height standing off the shell (radial)",
+        unit="mm",
+        range={"easy": (23.0, 29.0), "medium": (20.0, 34.0), "hard": (18.0, 40.0)},
+        source="SD Sparedrum L20BD 27 mm -> proportion",
+        askable=True,
+    ),
+    "body_w": dict(
+        desc="lug body width tangent to the shell",
+        unit="mm",
+        range={"easy": (22.0, 25.0), "medium": (20.0, 29.0), "hard": (18.0, 33.0)},
+        source="SD Sparedrum L20 23 mm; US8759653B2 ~20.6 mm cross-check",
+        askable=True,
+    ),
+    "hole_spacing": dict(
+        desc="centre-to-centre spacing of the two mounting screws",
+        unit="mm",
+        range={"easy": (24.0, 30.0), "medium": (24.0, 50.0), "hard": (24.0, 70.0)},
+        source="SD Sparedrum mounting-hole facet 25-70 mm",
+        askable=True,
+    ),
+    "mount_hole_d": dict(
+        desc="mounting screw hole diameter",
+        unit="mm",
+        range={"easy": (5.5, 6.5), "medium": (5.0, 8.0), "hard": (5.0, 9.0)},
+        source="SD Sparedrum mounting hole Ø 6 / 8 mm",
+        askable=True,
+    ),
+    "rod_bore": dict(
+        desc="tension-rod thread insert bore diameter",
+        unit="mm",
+        range={"easy": (5.0, 6.0), "medium": (4.5, 6.5), "hard": (4.5, 7.0)},
+        source="rod thread #12-24 (7/32 ~5.5 mm) / M5 / M6 (Pearl, vintagedrumreference)",
+        askable=True,
+    ),
+    "fillet_r": dict(
+        desc="radius on the vertical body corners (cast look)",
+        unit="mm",
+        range={"easy": (2.0, 4.0), "medium": (2.0, 5.0), "hard": (1.0, 6.0)},
+        source="proportion (die-cast lug corner radius)",
+        askable=True,
+    ),
+    "double_ended": dict(
+        desc="tube lug with a rod bore at BOTH ends (1) vs single-ended (0)",
+        unit="",
+        range={"easy": (0, 0), "medium": (0, 1), "hard": (0, 1)},
+        source="SD Sparedrum single-ended lug vs double-ended tube lug",
+        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 = []
+
+    # mounting holes must sit on the body with edge material at both ends
+    if p["hole_spacing"] > p["body_len"] - 12.0:
+        bad.append("hole_spacing > body_len-12: mounting holes need >=6 mm edge to each end")
+
+    # the two mounting holes must not merge into one
+    if p["hole_spacing"] < p["mount_hole_d"] + 4.0:
+        bad.append("hole_spacing < mount_hole_d+4: the two mounting holes would break through")
+
+    # corner radius must be under half the smaller footprint dimension
+    if p["fillet_r"] > 0.4 * min(p["body_w"], p["body_len"]):
+        bad.append("fillet_r > 0.4*min(body_w,body_len): corner radius too large for the footprint")
+
+    # mounting hole leaves body wall across the width
+    if p["mount_hole_d"] > p["body_w"] - 6.0:
+        bad.append("mount_hole_d > body_w-6: <3 mm wall beside the mounting hole")
+
+    # rod bore leaves body wall through the height
+    if p["rod_bore"] > p["body_h"] - 6.0:
+        bad.append("rod_bore > body_h-6: <3 mm wall around the rod insert")
+
+    return bad