172cc Forged Piston Cylinder - ProBld

The strongest and most reliable GY6 big-bore kit available. The ProBld 172cc kit is designed specifically for riders who want real power gains without sacrificing long-term durability. It’s the largest true drop-in cylinder kit for the QMJ157 engine—no case boring required.

Unlike standard big-bore kits that use low-grade cast pistons and soft aluminum cylinders, this kit uses a 4032 forged piston for high strength and long service life. It’s daily-ride safe, heat-stable, and built to handle hard throttle and extended rides.

This kit doesn’t just make your GY6 bigger. It makes it stronger, smoother, and more reliable than stock.

Expect real performance gains: stronger midrange, quicker acceleration, and noticeably more torque across the entire RPM band.

Why This Kit Is Different

• 4032 Forged Piston — Stronger than cast, built for heat and abuse, long mileage reliability.
• Pre-Measured & Installed Piston Rings — No ring filing, no guesswork, no risk of incorrect end-gap.
• True Drop-In Fit — No case machining required for any QMJ157 engine.
• Improved Cylinder Wall Hardness — Better heat dissipation and wear resistance.
• Ideal for Daily Riding or Full Builds — Works with stock or performance cam + head.

If you want a setup that pulls harder, runs cooler, and lasts longer, this is it.

What’s Included

• ProBld 61mm Cylinder
• 4032 Forged Piston
• Pre-Installed & Pre-Gapped Piston Rings
• Wrist Pin & Circlips
• Base Gasket & Head Gasket

Everything required for installation (no additional machine work needed).

Fitment Notice

You must verify that your engine is a QMJ157. This kit will not fit other GY6 variants.

Click here to learn how to decode your engine.

Buggy / Go-Kart Owners: Stud spacing varies. We recommend testing with the 61mm gasket test kit before purchase.