Complete big bore kits for the Honda CRF110, Kawasaki KLX110, Yamaha TTR110, Honda Grom, and more. Double your horsepower. Drop-in installation. Built by TBparts since 1995.
A big bore kit (BBK) replaces your stock cylinder, piston, and top-end gaskets with larger-displacement components — increasing your engine's swept volume and producing more horsepower and torque. On mini-motos and pit bikes, a well-matched big bore kit is the single most effective performance modification available.
Why do riders install big bore kits? Stock mini-motos are tuned conservatively for beginners. As your skill level grows, the stock engine becomes the limiting factor — especially for adult riders, racers, or anyone who wants to keep up on the trail or at the track. A TBparts big bore kit closes that gap permanently without buying a new bike.
Every TBparts big bore kit is engineered specifically for its platform. Select your bike to see available displacement options, kit configurations, and pricing.
The Honda CRF110F runs a stock 109cc engine that leaves significant performance on the table for adult riders. TBparts offers 132cc and 147cc big bore options, with complete packages that include a performance cam and Mikuni VM26 carburetor for maximum gains. All kits are direct drop-in — no engine case modification required.
The KLX110 is TBparts' highest-volume platform for big bore kits — the most popular pit bike in flat-track and backyard racing. Stock displacement is 112cc. TBparts offers kits from 143cc up to the 155cc forged stroker, covering every performance level from weekend trail rider to competitive flat-track racer.
The TTR110E runs a stock 110cc single-cylinder engine. TBparts' 130cc big bore kit adds meaningful power while maintaining the smooth, tractable power delivery the TTR110 is known for. Available as a basic bore kit or with a matched air filter and jet kit for optimal tuning.
TBparts has been building big bore kits for vintage Honda mini-motos since 1995. Our kits for the Z50, CT70, ATC70, XR50/CRF50, and XR70/CRF70 are used for everything from restoring power to road-legal street bikes to building serious race machines. Some 95cc and 117cc kits using a 54mm cylinder require engine case boring.
Browse the full range of vintage Honda big bore kits — Z50, CT70, ATC70, XR50/CRF50, XR70/CRF70 and more.
Shop Z50 / XR50 / CRF50 KitsThe Honda Grom 125 and Kawasaki Z125 are fuel-injected street mini-motos with a growing race and track community. Big bore kits for these models require an aftermarket fuel controller for proper tuning. TBparts offers 186cc kits for the Grom and dedicated Z125 options, delivering real performance gains on the street and at the track.
Browse all Grom and Z125 big bore kits and engine performance parts.
Shop Grom KitsNot all big bore kits are created equal. The right kit depends on your bike model, intended use, budget, and whether you want to install additional supporting modifications.
| Bike Model | Stock CC | Kit Options | Case Boring? | Starting Price |
|---|---|---|---|---|
| Honda CRF110F | 109cc | 132cc, 147cc | No | $259 |
| Kawasaki KLX110 / 110L | 112cc | 143cc, 155cc stroker | No (143cc) | $205 |
| Kawasaki Z125 Pro | 125cc | 143cc | No | $205 |
| Yamaha TTR110E | 110cc | 130cc | No | $170 |
| Honda Z50 / XR50 / CRF50 | 49cc | 88cc, 95cc, 117cc | 95cc+ with 54mm | Contact us |
| Honda CT70 / ATC70 | 70cc | 88cc, 95cc | 95cc with 54mm | Contact us |
| Honda XR70 / CRF70 | 72cc | 88cc, larger | Depends on kit | Contact us |
| Honda Grom 125 | 125cc | 186cc | Depends on kit | Contact us |
A big bore kit increases airflow demands. Supporting modifications ensure your engine runs correctly and delivers maximum performance without running lean.
Yes — for most riders with basic mechanical skills. TBparts big bore kits install exactly like standard OEM top-end components. If you can replace a piston, you can install a big bore kit. The key differences are: the bore is larger, and the carburetor will need jetting adjustment.
If you're not familiar with engine work, a local motorcycle shop can install a basic bore kit in 1–2 hours. Ask for a quote on "new piston and cylinder installation with carb jetting." For kits that include a cam or carb kit, the additional installation time is minimal.
Note on case boring: A small number of TBparts kits require engine case boring to fit a larger cylinder skirt. These kits are clearly marked in the product listing. If case boring is required, complete engine disassembly is needed — recommend a professional motorcycle or machine shop for this procedure. TBparts kits that require case boring include: Honda 50/70 kits using a 54mm cylinder (95cc and 117cc), KLX110/Z125 kits over 143cc, and some Chinese engine kits.
Proper break-in protects your investment and ensures your new bore kit seats correctly. Follow these steps after any top-end rebuild or big bore installation.
Select your bike model above or browse the full TBparts catalog. Not sure which kit is right for your build? Contact us — we've been doing this since 1995.
Whether your a hobbyist rider or live and breath riding every single day, there's no doubt that TBparts.com offers many types of Big Bore Kits (BBK) for many different models of dirt bikes. This page is designed to help lead you into the right direction by providing you the most complete content.
Get some answers
Big bore kits
Typically include
A big bore kit typically consists of a new larger bore cylinder, piston kit, and top end gaskets. They replace your stock Original Equipment Manufacturer (OEM) parts and increase horsepower and torque by increasing the engine size (displacement). Some big bore kits will also include other items like bigger carburetors, performance camshafts, and big valve cylinder heads to deliver additional power and performance.
In the minibike/small bore engine world, the number one reason is to increase power and performance and therefore, fun! If you rode little Honda 50s or 70s as a kid and get back on one now as an adult, you will quickly find out they don’t pull like you remembered. Our big bore kits can double horsepower (or more depending on the kit) and will be a night and day difference over stock bringing huge grins on the first ride. If you are currently into riding and/or racing pit bikes like the popular Kawasaki KLX110, eventually you will want more power to level the playing field with the others who have already modified their engines. It’s no fun being last to the first corner off the start and a big bore kit can help fix that issue. Or, you may simply want some extra power and torque to get up the big hills on your local trails and a big bore kit will help with that as well.
Small bore street bikes
On small bore street bikes like the Honda Grom and Kawasaki Z125, a big bore kit definitely adds some fun over the underpowered stock engines at the track! Track racing these bikes also has become very popular and more power is almost certainly needed to compete these days. No matter how or where you ride your mini, the natural progression is to increase power and performance because it delivers more fun.
There are a few other reasons people install big bore kits – they are:
To replace a worn out or destroyed top end. In most cases, it’s cheaper to buy one of our big bore kits than an OEM piston kit, cylinder, and gasket kit.
Parents often buy them for their kids as their skill level goes up. Big bore kits can provide the extra power without having to spend money on a whole new bike.
For increasing speed on street legal vintage Honda 50s or 70s to keep up with traffic on local roads. Normally, increasing speed will also require sprocket changes.
tbparts quality bbk
Not all kits are created equal but here are some of the parts that you can typically find in a Big Bore Kit.
Let's break it down
Let's break it down
Big bore kits are the most affordable and effective way to increase your horsepower and torque when compared to other performance modifications. The actual cost will depend on your model, how big you want to go, or the type of kit. For many models we offer different size big bore kits and kits that include carburetor kits, performance camshafts and big valve cylinder heads. Usually, the best deal is to buy a complete big bore kit that includes a carburetor kit or one that includes those plus a big valve cylinder head. However, depending on your model and budget, you can also start gradually and build up your engine over time. For example, on a KLX110 you can start with a basic 143cc big bore kit, then later add a bigger carburetor kit, and finally add our V2 Race Cylinder Head if the most power is desired. If you are unsure how to proceed with your bike modifications, please contact us and we can provide some suggestions based on your particular situation.
Some other parts may be required or recommended when installing a big bore kit. Similar to above, this depends on your model and the type of big bore kit you want. In general, when installing a big bore kit it’s recommended to also use a performance exhaust and a bigger carburetor. A stock exhaust may restrict flow and cause your engine to run hotter while a bigger carburetor will provide the increased air/fuel needed to prevent a lean condition (which also would cause the engine to run hotter). In the case of fuel injected bikes, an aftermarket fuel controller may be required. Again, it all depends on the bike and kit. Here are some examples:
Definitely needs a bigger carburetor and performance exhaust
Can use the stock exhaust and carburetor (A performance exhaust and bigger carburetor would help performance though)
Can use the stock exhaust and carburetor (A performance exhaust and bigger carburetor would help performance though)
Definitely needs a bigger carburetor and performance exhaust
Can use the stock exhaust and carburetor (A performance exhaust and bigger carburetor would help performance though)
Definitely needs a performance exhaust and an aftermarket fuel controller
Note: If using the stock carburetor with a big bore kit, it most likely will need to be re-jetted but OEM jets are inexpensive and readily available. Additionally, if you go with a really large big bore kit or a bore kit and a big valve cylinder head, it’s possible you will also have to upgrade your clutch springs and plates to prevent clutch slippage due to the extra power.
If you aren’t sure what you may need for the big bore kit you are interested in, just email us and we can provide assistance.
see for yourself
Check out this video that shows the pull you can gain with installing a big bore kit on your mini bike.
Can I install the BBK kit myself? If not, what are the installation costs?
We recommend a professional technician install the big bore kit for you if you are not familiar with your engine and working on it. Having it installed professionally will bring peace of mind it was done properly while saving you a lot of time and headaches if you aren’t familiar with jetting a carburetor. The cost will vary greatly depending on the type of bike the big bore kit is being installed on and local shop labor rates. We recommend calling your local shops for quote. If you are doing just a big bore kit, all you have to ask them is for a quote on installing a new piston and cylinder and dialing in the carburetor. If you purchased a kit with a big carburetor kit and a cylinder head, it shouldn’t be much more expensive to install than just the bore kit (as they would have to re-install the stock OEM carburetor and cylinder head anyway).
Note: For fuel injected bikes, you will have to also ask for a quote on installing an aftermarket fuel controller and tuning it.
If you are familiar with working on engines (or interested in learning) and want to install a big bore kit yourself, it’s typically not a difficult or time consuming procedure. All the TBparts.com big bore kits are installed exactly like the stock OEM parts. So, if you are familiar with changing out a piston, installing a big bore kit will be the exact same procedure only you are putting on a larger bore cylinder and piston. Once installed, then you will have dial in your jetting to make sure it’s not running lean. If you are not familiar with replacing your piston and want to learn, all you have to do is pick up a Clymer or Haynes repair manual for your model bike. They will instruct novice mechanics on how to remove and replace your cylinder and piston, tune your engine/carburetor, as well as many other repairs and general maintenance procedures. They also provide many important specifications and are “must have” manuals if you plan to work on your bike. If Clymer or Haynes doesn’t offer a repair manual for your model, you can always purchase a shop manual from the manufacturer. They will do the same thing but are typically geared more towards the professional mechanic.
There is one important exception on big bore kits that makes installing them more complicated. Some kits will require engine case boring so the larger cylinder skirt will fit into the engine case. With these kits, complete engine disassembly is required and then the engine cases will need to be bored (recommend a professional motorcycle shop or machine shop for this procedure). If you have to disassemble your engine for other reasons, it’s a great time to install one of these types of big bore kits. One good reason would be the installation of a stroker crank since it also requires engine disassembly. Installing the crank and the bore kit at the same time will save a lot of time and expense. You also most likely will reach the maximum engine displacement for your model (and more power, of course). We only have a few of these kits that require engine case boring and they are:
Additionally, if the kit requires engine case boring, the product listing in our shop will mention that.
what are they?
After a bore kit or new piston is installed, the engine should go through a “break-in” procedure. There is a lot of debate on the proper way to do this and a simple Google search of “motorcycle bore kit break-in” will lead to many articles on this topic.
This is the engine break in procedure we recommend for our big bore kits:
Before starting your fresh engine, you should be sure to complete the following:
Supply the engine with break-in oil. Brad Penn or Royal Purple brands are recommended break-in oils.
Important Note: Never break-in a new engine with full synthetic oil.
Important Note: At this point, fuel injected bikes should have an aftermarket fuel controller installed and tuned per the manufacturer instructions
Next, you should start your fresh built engine and do its first heat cycle. Start your engine and while it’s on the stand, constantly blip the throttle and vary the RPM’s. It is important to note you do not want to rev the engine hard – just slight blips of the throttle while not letting the engine idle on its own is the goal. Continue to do this until the engine has reached its operating temperature (about 5 to 10 minutes – the bigger the bore kit, the quicker it will heat up). If able to measure the actual temperature, it should be approximately 190 to 200 degrees Fahrenheit.
Important Note: For carbureted bikes, while doing the first heat cycle, if the header pipe starts to turn red, shut off the engine immediately and increase the pilot jet size. If the engine is popping, check for air leaks or to see if it needs a pilot jet increase. If these situations occur on a fuel injected bike, shut off the engine and the fuel controller will need to be tuned again to fix the lean condition.
After the engine has reached operating temperature, shut off the engine and let it cool completely. When the engine has completely cooled, re-torque the head studs, manifold bolts, and exhaust flange bolts.
Repeat Step 2 and again, let the engine cool completely.
The next step is to take the bike for a ride. Locate an area that is very flat, such as a parking lot or field (this is important as you do not want an area that has grades which would induce a heavy load on the engine).
After finding a suitable flat area, start your engine and let it come up to operating temperature. Once there, take off and get the bike in 2nd or 3rd gear. You should rev the engine up to about ¾ throttle, then back off the throttle and let the engine brake itself to a lower rpm. It is important to note when letting the engine brake to a lower rpm, do not let it get so low that it causes lugging/bogging to the engine. Once the engine has dropped to a lower rpm, throttle back up to ¾ throttle and again back off the throttle and let engine brake to a lower rpm. Continue to repeat the rev up and engine brake down for 20 minutes.
The engine is now broken-in but there are still a few things to do before enjoying your bike and the added performance. After performing Step 5, let the engine cool down completely. Once the engine is cold:
Now you are ready to get out there and throw roost or hit the pavement! Once you have done the break-in on your big bore engine, below is a routine maintenance chart to follow:
| Item | Every Race | Every 3rd race/500 miles/10 hours | As required | Remarks |
| Engine oil | X | X | X | Racers change oil every race. Trail riders/street application every 500 miles or 10 hours |
| Valve lash | X | X | If valve lash starts to tighten quickly after each lash, valves need replacing | |
| Spark plug | X | X | ||
| Air filter | X | X | Every dirt ride should start with a fresh oiled air filter. Dry/dusty conditions filter should be changed between races | |
| Oil filter | X | X |