A big bore kit offers a significant upgrade opportunity for anyone looking for an immediate boost in performance while riding their dirt bike or ATV.

A big bore kit replaces the stock piston and cylinder bore with a larger version that provides extra displacement, increased combustion flow and lots of torque. On paper installing a big bore kit makes the size of the engine bigger. So for example, a bigger bore installed into a YZ250 gives you a 293cc bike which translates to an 18 percent bigger engine.

But that doesn't necessarily translate into 18 percent more power.

What a Big Bore Kit Accomplishes

Technically, once installed with all correct and necessary modifications and upgrades you will feel the larger bore and cylinder with one twist of the throttle. Thus, better starts come with the better throttle response you get from having a bigger bore. Adjusting to your "new bike" means challenging your riding skills especially when it comes to starts and corners. In fact, expect some adjustment time with a bigger bore if you don't have prior experience riding a larger engine.

However, just upgrading to a bigger bore without upgrading other related components could actually backfire and give you less power. An increase in displacement with the larger bore needs correct tuning and upgrades to the stock stroke and weight crank shaft. Therefore, large changes to the displacement size should be accompanied with not only an increased piston size but also with crank modifications. Otherwise, expect to spend money later replacing your quickly worn out engine from the increased stress thanks to the use of a larger piston size on stock crank components.

Also, to get the most out of power gains offered from a big bore kit and the upgraded crank modifications you need to incorporate the following adjustments:

  • Jetting
  • EFI controllers
  • Ignition mapping
  • Cam and other timing mods
  • Oversized valves
  • Head (4 stroke)/cylinder (2-stroke) modifications and porting
  • Larger or other aftermarket carburetors
  • Intake modifications
  • Larger stroke cranks

An aftermarket exhaust also helps a bigger bore realize its power, however, consider the above as required adjustments to prevent loss of power, potential engine damage and maddening frustration because you feel no change to your bike after installing the big bore kit.

Installing a Bigger Bore

A big bore kit includes the bore and cylinder as well as the head and applicable gaskets. On most dirt bikes and ATVs the process of installing a big bore kit is the same as a top end though a few models might require some engine case modifications which involve an engine builder. Installing requires lots of attention to detail, correct tools and more than basic mechanical experience. If none of this applies to you then find a professional for the install.

But remember, don't just slap in a big bore kit the next time you need a top end. You have to make all the other necessary adjustments and upgrades to prevent engine damage and/or diminished power.

Racing With a Bigger Bore

Some AMA sanctioned races or series with strict rules ban bigger bores so before heading into competition know the rules. However, most Motocross and off-road races allow you to run, for example, a 270cc engine alongside 250cc class engines. Open displacement classes like the 30+ and 40+ vet classes also allow the big bore set-up.

So, Should I Buy a Big Bore Kit?

If riding has grown a bit stale and you no longer feel challenged with your current set-up, then you might find an answer by installing a big bore kit the next time you need a top end. You will experience immediate results and find a new measure of success when it comes to testing your ability but only when making all the other changes.

Other times, riders have a "project bike" they build from the ground-up as time and money allows. Building a bike this way offers an ideal time to install a big bore kit because you can make all the required adjustments and necessary upgrades as you go along.

Regardless, if you have installed no upgrades and made no other modifications to the engine then don't bother with a big bore kit. Most riders tend to install the bigger bore AFTER installing an EFI controller, replacing the stock exhaust and making the other required engine modifications that can handle the bigger bore.

Written By: AndrewT