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Rule out the usual hiding places first. If the piece really is gone, check the set piece by piece and leave with the element ID LEGO asks for, or a link to buy the part.
Checking
52 pieces in 21 part and color combinations, plus 4 minifigures.
The official piece count includes the 21 parts inside the set's minifigures, which this inventory lists separately. Spare parts are counted in neither figure.
Builders who report a missing piece often find it in one of these places. Tick each one off as you look.
LEGO's request form asks for the set number (40374) and each piece's element ID, both listed above.
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Bracket 1 x 2 - 1 x 2 Inverted
×3
Plate Special 1 x 2 with Pin Hole on Top
×4
Plate Special 2 x 2 with Bar Frame Octagonal, Reinforced, Completely Round Studs
×1
Bracket 1 x 2 - 2 x 2 Inverted
×1
Plate Special 1 x 2 with Handles on Ends
×1
Plate Special 2 x 2 with Grills
×1
Arm Mechanical with Clip [Thick Support]
×8
Weapon Sword / Katana / Shamshir with Capped Pommel [Square Guard]
×2
Weapon Throwing Star / Shuriken with Textured Grips
×2
Weapon Sword / Katana / Shamshir with Capped Pommel [Square Guard]
×3
Animal Body Part, Barb / Claw / Tooth / Talon / Horn, Small
×6
Cone 1 x 1 [Top Groove]
×4
Hinge Cylinder 1 x 2 Locking with 2 Click Fingers and Axle Hole, 9 Teeth
×2
Hinge Cylinder 1 x 3 Locking with 1 Finger and Technic Friction Pin with Round Hole
×2
Bar 1L with Clip [Cut Edges and One Side Hole] [Gap in Clip]
×2
Equipment Hose Nozzle / Gun with Side String Hole Simplified
×2
Plate Round 2 x 2 with Axle Hole Type 1 (+ Opening)
×2
Technic Pin 1/2 without Friction Ridges
×2
Tile Special 1 x 2 with Sloped Walls AKA Money / Gold Bar [Ingot]
×2
Brick Sloped 30° 1 x 2 x 2/3 with Dark Red Horizon Screen and Gold Switches and Buttons Print
×1
Plate 1 x 2
×1
Nindroid with Neck Bracket (Legacy)
×1
Nindroid with Short Legs and Back Connector (Legacy)
×1
Zane / Golden Ninja (Legacy)
×1
Nindroid Warrior with Single Sided Head (Legacy)
×1
Tick each piece as you find it, or mark it missing.
LEGO takes requests for pieces missing from a new set through its customer service site. The form asks for the set number and each piece's element ID, the number printed beside the piece in the parts list at the back of the instructions. This checker lists both.
Request a replacement from LEGOMark a piece missing and your list links it to an eBay search for that part, which also covers pieces from older or second-hand sets.
Copy or download your missing list (the CSV opens in any spreadsheet), or print the whole checklist and tick pieces off on paper. Your marks stay in this browser, so you can check a big set over several sittings.
