NCY 50mm Big-Bore Cylinder Kit - 139QMB 50cc to 81cc

This NCY big-bore cylinder kit is the displacement upgrade for compatible 139QMB / QMB139 four-stroke scooters, taking the original 50cc platform to 81cc with a 50mm bore and the standard crankshaft stroke. Give your scooter more go with a cast-iron cylinder and matching piston assembly built for your next performance project. This is a big-bore conversion, not a standard-bore replacement, and the cylinder head is not included.  No case cutting required!

Key Features

• More Displacement: A 50mm bore produces 81cc using the standard stroke, without requiring a stroker crankshaft to achieve that displacement.
• 139QMB Performance Upgrade: A bolt-on cylinder upgrade for matching GY6 50 / 139QMB four-stroke builds.
• Cast-Iron Cylinder: Replacement cylinder with cooling fins for the air-cooled engine.
• Matched Piston Components: Includes piston, piston rings, wrist pin and retaining clips.
• Gasket Set Included: Head and base gaskets accompany the cylinder and piston components.

Technical Specifications

• Brand: NCY
• Part number: 1100-1237
• Manufacturer part number: 603C02GY650
• Kit type: Big-bore cylinder conversion
• Engine platform: 139QMB / QMB139, GY6 50 four-stroke
• Bore: 50mm
• Displacement: 81cc with standard stroke
• Cylinder: Cast iron, air-cooled
• Included: Cylinder, piston, rings, wrist pin, retaining clips, head gasket and base gasket
• Cylinder head: Not included

Compatibility

• Compatible with 139QMB / QMB139 (GY6 50) four-stroke scooters being converted from the original 50cc configuration to a 50mm big-bore setup.

Head compatibility matters: Most stock cylinder heads will not fit this kit. We recommend the NCY 50mm big-valve performance cylinder head, 1100-1239, sold separately. Confirm the installed engine configuration and matching head before assembly.

Recommended Performance Pairings

Pair the kit with the NCY Performance Camshaft, 1100-1250, sold separately, to help the bigger cylinder reach its full performance potential. The original performance pairing targets 50–55 MPH; actual speed depends on the complete engine, tuning, gearing, rider load and riding conditions, rather than the cylinder kit alone.

We also recommend the NCY Oil Decompression Tube Assembly, 1100-1282, sold separately. It replaces the dipstick and allows excess crankcase pressure to be released, making it a useful companion to a cylinder-kit upgrade.

Not legal for use in California on pollution-controlled vehicles. Check local laws before installing.

This product is intended for off-road use only. This product is not intended for use on public roads or land.

• 50mm big-bore conversion adds displacement to compatible 139QMB scooters
• 81cc with standard crankshaft stroke
• Cast-iron air-cooled cylinder
• Includes piston, rings, wrist pin, retaining clips and head/base gaskets
• Cylinder head not included; NCY 1100-1239 50mm head recommended
• NCY 1100-1250 performance camshaft and 1100-1282 decompression tube recommended separately

Brand: NCY; Part number: 1100-1237; Manufacturer part number: 603C02GY650; Type: big-bore cylinder kit; Engine: 139QMB / QMB139 GY6 50 four-stroke; Bore: 50mm; Displacement: 81cc with standard stroke; Cylinder: cast iron, air-cooled; Includes: cylinder, piston, rings, wrist pin, retaining clips, head gasket and base gasket; Head: not included; Recommended head: NCY 1100-1239; Use: off-road only

Compatible 139QMB / QMB139 GY6 50 four-stroke scooters converted to a 50mm big-bore configuration; 81cc with standard stroke; not a stock-bore replacement; most stock heads will not fit; NCY 1100-1239 50mm head recommended separately