Background in Mechanical, Electronic, and Systems Engineering.
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“How do I machine my engine cases for the big bore kit?”
It’s the most common question we get about GY6 Big Bore Kits. Most guys want the extra power, but aren’t sure how to go about actually installing that mouth-watering 175cc or 180cc kit.
Specifications – Cut Diameter and Tolerances
- 58.5mm Kit (155cc) – No machining needed
- 61mm Kit (170cc) – Machine cases to 65.15mm ± 0.075
- 62mm Kit (175cc) – Machine cases to 65.15mm ± 0.075
- 63mm Kit (180cc) – Machine cases to 65.15mm ± 0.075
Update: We now offer pre-machined case kits. We’ll do the work, you bolt it together. Ready to accept 63mm.
“Can I use a dremel tool to machine the cases?”
You can, but it is really not recommended. We did something similar on our very first GY6 Big Bore project. It seemed to work well and the engine ran strong for a year, until the casting around one of the cylinder studs gave way leading to massive failure. The hole just wasn’t true and caused weakness in a critical area. The engine was totaled, written off to the local recycling facility.
You’ll be much happier contacting a machinist
If you’ve never worked around precision machining before, it’s best to get in contact with a local machinist (or a buddy with access). The actual machining process isn’t difficult or time intensive with the right equipment. Most of what you’ll pay for is setup time. Expect to pay around $35 to $50 for the job. Depending on which size Big Bore Kit you buy, give the machinist the appropriate information above.
More important information
Be sure that you have your your cases machined with the crankshaft removed and a new crankcase gasket installed between the case halves. The new gasket will keep the hole true to proper dimensions while machining.
Also, very important! Care must be taken while machining the cases not to hit the oil passages on either side. On some cases this is unavoidable, but there are easy ways to fix the oil passage. Reply in the comments if this happens to you!
Questions? Comments? Reply Below!
If you have a question or comment, just give us a shout in the reply box below.