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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
31 pieces in 13 part and color combinations.
LEGO counts 137 pieces. This inventory comes from a separate parts catalog and lists 31; the two do not always count the same elements, so their totals differ. 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 (107) and each piece's element ID, both listed above.
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Technic Tread Crawler
×2
Tyre Offset Tread 24 x 8
×4
Tyre for Train Wheel for Electric Train Motor 12V
×4
Train Wheel Spoked for Motor
×4
Wheel Old with 4 Studs
×4
Wheel with Split Axle hole
×4
Battery Box, 4.5V 6 x 11 x 3 1/3 Type 3 [Complete Assembly]
×1
Motor, Train, 4.5V 12 x 4 x 3 1/3 Type II
×1
Electric Connector, 2 Way Male Rounded Narrow Type 1 with Cross-Cut Pins
×2
Wire 12V / 4.5V with Two Leads [Undetermined Length]
×1
Technic Axle 6
×2
Plate Special 6 x 16 with Motor Opening Wide
×1
Technic Brick 2 x 4 with Top/Side/End Holes and Hollow Studs
×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.
