Bolt-on counterweight arcs R15, R20 and R25 — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/bolt-on-counterweights/  (CC BY 4.0)
Basis: 120° ring segment 6 thick, 8 mm radial width, three Ø3.2 holes at R − 4
Catalog version: 2026-10-20

Sizes: see table below
- A 120° arc 6 thick and 8 mm wide radially: R15/R7, R20/R12, R25/R17 (outer / inner).
- Three Ø3.2 holes (M3 clearance) at R − 4 (R11, R16, R21), at the centre and ±40°.
- It bolts to a flywheel or crank disc opposite the pin; moving it along matching holes trims the balance.

file  outer R  inner R  arc  thickness  holes
bolt-on-counterweight-r15  15  7  120°  6  3 × Ø3.2 at R11
bolt-on-counterweight-r20  20  12  120°  6  3 × Ø3.2 at R16
bolt-on-counterweight-r25  25  17  120°  6  3 × Ø3.2 at R21

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Wikipedia: Engine balance — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Engine_balance
- Wikipedia: Crankshaft — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Crankshaft
- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 3 parts): https://xemstudio.com/files/hardware/bolt-on-counterweights/index.json
API: https://xemstudio.com/api/geometry?object=bolt-on-counterweights