Counterweighted crank discs, crank webs and bolt-on counterweights — dimension sheet Source: Xem Studio, https://xemstudio.com/objects/counterweighted-cranks/ (CC BY 4.0) Basis: Single-cylinder balancing practice: counterweight = all rotating mass + ~50% of the reciprocating mass placed opposite the pin at the pin radius; discs cut away on the pin side and thickened opposite; bolt-on arcs for trimming on the engine Catalog version: 2026-10-20 Sizes: see table below - Why: the piston, con-rod and crank pin all pull outward once per turn; without a counterweight the engine walks across the table and the bearings hammer. Put mass opposite the pin: m_cw × r_cw = m_rotating × r_pin + 0.5 × m_reciprocating × r_pin. Over-balance toward 60–70% if the engine shakes mostly along the cylinder axis. - The discs here remove material beyond the pin and add a 150° thickened sector opposite it (about 50% balance for a printed con-rod + syringe plunger); the bolt-on arcs trim the rest: drill the flywheel rim at the three M3 positions and move the arc until it runs smooth. Crank webs are the bar-crank alternative: two webs + a Ø3 pin make a throw in the middle of a shaft so the con-rod can sit between two bearings. - Check: spin the bare crank on its bearings; it should stop anywhere (static balance). Then run and feel the stands. Running speed of a hobby gamma is 200–600 rpm, so static balance is enough; dynamic balance only matters past ~1500 rpm. - LES: counterweighted discs on the 2 in gamma; bolt-on arcs on the Ø100 flywheel for the final trim. stroke_mm pin_offset_mm disc_dia_mm parts 10 5.0 32 disc + web 15 7.5 32 disc + web 20 10.0 34 disc + web 25 12.5 39 disc + web 30 15.0 44 disc + web 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, 13 parts): https://xemstudio.com/files/hardware/counterweighted-cranks/index.json API: https://xemstudio.com/api/geometry?object=counterweighted-cranks