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Rolling Wrench

GY6 171cc Big Bore Kit + Head + Cam (Fits QMJ157)

GY6 171cc Big Bore Kit + Head + Cam (Fits QMJ157)

$505.00 USD
$505.00 USD
Sale Sold out
Shipping calculated at checkout.

All the performance you need in one kit. We've spent years testing other big bore kits on the market — keeping what worked, fixing what didn't. Built with premium rings, gaskets, springs, and metals, this is a true no-case-boring, drop-in 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 (QMJ157) only. 54mm stud spacing.
  • Fits most QMJ157 engines, but not all. Watch the install video on this listing before ordering — we walk through the specific engines this won't work on and why.
  • Drop-in fit — no case boring required.
  • Buggy/go-kart owners: fitment hasn't been fully confirmed across every GY6 engine variant used in buggies. This is a buy-at-your-own-risk purchase — see below.

⚠️  Before you order

  • No returns on performance parts — especially relevant given the fitment note above.
  • You'll need to choose forged or cast piston in the dropdown — that's the one decision this kit asks you to make. 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.

✓  What's in the box

  • Cylinder kit (forged or cast piston — your choice)
  • Big valve head
  • ProBld performance camshaft (built for low-end and mid-range power)

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.

Add a wideband O2 sensor kit to make tuning this a breeze. For torque specs once you're ready to assemble, check our GY6 torque settings guide.


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