QMB139/QMJ157/Minarelli Engine Case Boring Tool

This case boring tool is designed for 2-stroke Minarelli engines, 50cc 4-stroke QMB139 engines, and 150cc 4-stroke GY6 engines, making it easy to open up your cases cleanly for big bore cylinders.

The Details

When you upgrade to a larger cylinder, the stock engine cases often need to be opened up to clear the new sleeve. This case boring tool centers on the existing bore to keep everything straight and true, helping you remove material evenly and avoid hacked-up cases. It’s sized for common small-bore scooter engines, including 2-stroke Minarelli, 50cc 4-stroke QMB139, and 150cc 4-stroke GY6 platforms, so you can tackle multiple builds with one tool.

Important Things to Note

• For 2-stroke Minarelli, 50cc QMB139, and 150cc GY6 engine cases only
• Cases must be disassembled — crankshaft and bearings removed before boring
• Always measure your cylinder skirt and bore to the correct clearance before cutting
• Use light, controlled passes and proper cutting lubricant to avoid chatter

We strongly recommend this tool be used by experienced DIYers or builders comfortable with precision engine work.

What's Included

• Case boring tool assembly for 2-stroke Minarelli, QMB139, and GY6 engine cases