HOW TO: GY6 Big Bore Kit Prep

“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.

Pre-machined Short Case Kit (For Karts and ATVs)

Pre-machined Long Case Kit (For Scooters)

“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.

20 thoughts on “HOW TO: GY6 Big Bore Kit Prep

  1. Eric

    I damaged the oil passage while boring the case. How do you fix it?

  2. jeff

    I upgraded to a bigger bore on my gy6 150cc go kart (KD 150 GK2 ) motor. It came with .030 (A9) bigger cam installed it per camshaft installation guide and valve adjustment guide. The cam over compresses the valve spring and will not allow the motor to rotate 360 degrees ( too much lift ?) . Will I need to purchase a different cam and rocker arm holder?

    • Buggy Depot

      That’s correct, the new cam has a bit too much lift for your head and the valve springs are binding. Normally this isn’t a problem, but happens on heads from certain manufacturers with slightly spring seat height. There are a few different solutions, but by far the easiest way is to go with a cam with less lift.

      • jeff

        Can you make a suggestion on the size of new cam?

        • Buggy Depot

          The A12 is typically safe on heads with bind issues.

  3. Troy Beck

    Is the cut diameter the same for all 180cc big bore kits? I have a universal parts 63mm big bore. I’m about to have it machined to fit my jug. Just want to make sure I tell the machine shop the correct diameter.

    • Buggy Depot

      Yes it’s the same across the board, although some BBK manufacturers deviate from the usual spec. It’s a good idea to take the cylinder to the shop. They’ll be able to make sure it fits before releasing the case halves from the fixture.

  4. douglas parsley

    What is the valve clearance on a 63 mm bore kit intake and exhaust with a 2.5 stroker and what is a good cam for this set up

    • Buggy Depot

      With a 2.5mm stroker crank, you’ll get negative deck clearance. The piston will impact the head. There are some different ways around this, but the most reliable is a properly machined piston, available here:

      • douglas parsley

        Thank you i did install a spacer but i believe the piston is hitting the head would you have a 63 mm piston in stock. Im going to take the head off to see. What would be a good cam for this set up

        • Buggy Depot

          We don’t offer 63mm due to reliability issues with the thinner cylinder walls. I highly recommend downsizing to 62mm. The S220/S300 pistons have been tested for use with the A12 and A15 camshafts.

  5. Douglas parsley

    I bored the case to 63 mm if I use a 62 from you will that work in my engine case

    • Buggy Depot

      Yes, it will work. The outer diameter is the same from 62mm to 63mm (wall thickness/inner diameter is the difference). As long as you bored the case to the specs in the article above, the 62mm will fit.

  6. Douglas parsley

    Last question and I do thank you when I order this 62 mm kit with the piston used for my hoca 2.5 mm stroker do I need a spacer or will the super piston take care of that

    • Buggy Depot

      No spacer is needed. The S-series piston takes care of that hassle. Installation is the same from an assembly perspective as if you weren’t using a stroker; goes together without any tweaking or re-working. You’ll want the S300 version (not 220) for your 2.5mm stroker crank.

  7. Mathew

    I have a buggy carter brother 150cc gy6 talon gx i just bought a 61mm big bore kit ive heard u dont have to machine case on some gy6 motors is this true

    • Travis @ Buggy Depot

      The “No machining required” 61mm kits exist, but I highly recommend against them and I refuse to sell them at BD. The cylinder wall is far too thin for long term reliability. I’ve seen and personally experienced sleeve fractures at full speed with thin wall kits.

      Although some places offer shortcuts, going big bore something that you want to do the right way.

  8. Dreon

    If i just do the 63mm piston kit will that be ok leaving everything else stock

    • Travis @ Buggy Depot

      Yes. However, I strongly recommend 62mm or less. The cylinder wall of the 63mm is too thin (results in drastically reduced longevity) without expert-level experience tuning and adjusting riding habits.

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