Rolling Wrench
GY6 171cc / 172cc Scooter Big Bore Piston (Fits QMJ157)
GY6 171cc / 172cc Scooter Big Bore Piston (Fits QMJ157)
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This Rolling Wrench kit is the result of years spent testing other big bore kits on the market — keeping what worked, fixing what didn't. It's built with premium rings, gaskets, springs, and metals, and it's a true no-case-boring big bore kit — the largest you can run on a GY6 A motor (QMJ157) without boring your cases. As the saying goes: there's no replacement for displacement.
🔍 Does this fit your bike?
- 171cc / 172cc, 12:1 compression — for the GY6 A motor (Chinese GY6 QMJ157) only.
- 54mm stud spacing (GY6 A).
- Drop-in fit — no case boring required. Many competing 61mm kits require boring your engine case; this one doesn't.
- Will NOT fit a 161QMK "200cc" engine.
- Buggy/go-kart owners: fitment hasn't been fully confirmed across every GY6 engine variant used in buggies, and some may need case modification to accept the larger cylinder. If you're running this in a buggy or kart, this is a buy-at-your-own-risk purchase — see below.
⚠️ Before you order
- No returns on performance parts — this applies especially if you're installing on a buggy/kart given the fitment note above.
- Forged is recommended for you weekend warriors. Cast is for you everyday riders.
- The piston ring end gaps are pre-measured and pre-installed into the cylinder for you. Do not remove the piston from the cylinder — this can bend the rings during reinstall and require you to buy new ones. If you need to install the piston pin, slide the piston down just far enough to do that without exposing the rings.
- Use super unleaded fuel only.
WATCH THIS VIDEO TO VERIFY FITMENT
✓ What's in the box
- Piston rings
- Head gasket
- Base gasket
- 2 piston pin circlips
- Piston pin
- 61mm drop in cylinder
- piston
Want more out of it? A performance camshaft and big valve head are the two upgrades we'd recommend for real gains. Going further, a forged piston option is available for extra strength and durability. A performance carburetor pairs well too, though it's not required — CLICK HERE. A wideband O2 sensor kit also makes tuning any of this dramatically easier — CLICK HERE.
You'll find similar-looking kits online for half the price — those are made in China. This one's made in Taiwan, and the quality difference is real.
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Installed the new cylinder and piston. Turned it over by hand and the original valves were leaking. No surprise really with a higher compression. Cleaned and ground the valves and we are getting close to 180 psi and it’s running great! Idles perfectly at very low RPM. Love it.
Good quality parts good service, the ruck definitely has more power and torque. Now it’s time to gear up.
I noticed the power difference instantly
The Big Bore Kit that I ordered works flawlessly. Installation went smoothly and customer service is Top Notch. She even advised me that it may not fit in my GY6A Buggy, but I ordered anyways and it was perfect. Matt's videos are always helpful and on point with all aspects of the install. Will definitely purchase more from them for my new to me GY6B buggy. Possibly do a 232 Big Bore in the near future.
Quality product.