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YZ250F Big Bore Kit: Is It Worth Modifying A Stock Engine?

Trying to decide if a YZ250F big bore kit is worth the money or hassle? Is it the best money spent to get more power? This post will review all the aspects of installing a big bore cylinder kit on your 2001-2013 Yamaha YZ250F or WR250F and 2014+ models, including what it takes to install it and the jetting changes required.

Are you looking to get some more power out of your older YZ or WR250F so you can keep up with the newer faster bikes? A 290cc big bore kit (check price on Amazon) will increase torque and horsepower by simply swapping your cylinder and piston assembly.

What Is A Big Bore Kit?

A big bore kit comes with a larger-sized piston and a cylinder that is already machined to size. A bigger bore increases the engine displacement, thus giving you a “bigger engine”. A bigger engine means more power, right?

What Bikes Do The Athena and Cylinder Works Big Bore Kit Fit?

The Athena and Cylinder Works big bore kits will fit on a 2001-2007 Yamaha YZ250F or a 2001-2013 WR250F dirt bike. The YZ250F engine design was slightly changed in 2008, so that is why it won’t fit after ’07. The WR250F engine has been the same since the beginning until Yamaha went with the fuel-injected engine that is based on the YZ250F.

Is Your Old Carb’d Yamaha Slow?

Dirt Bike technology has advanced quite a bit in the past 20 years, and the result is faster and more powerful bikes. Have you been eyeing up the latest YZ250F or WR250F on the showroom floor but can’t afford to take out a loan for a new bike? Is your current 250 feeling slow and tired out?

Faster Lap Times

A big bore kit instantly adds power right where you need it most on a 250F. If you do nothing else other than install one of these kits and adjust the jetting accordingly, you will feel a nice increase in low-end torque and up through the mid-range. This will help you ride out of corners faster, get into the upper RPM’s quicker, and tire you out less because of the broad power band.

YZ250F 290cc Big Bore Kit
Having Fun On The Big Bore YZ250F

Simple Jetting Changes

If your YZ250F or WR250F is already jetting correctly, then you won’t have to alter it much after installing one of these kits. An engine is basically like a pump, and the bigger bore acts as a bigger pump than a stock bore, so it will pull in more fuel through the jets. This means that you should have to go down in size (leaner on the jetting) to compensate.

Why Do I Have To Go Leaner?

This will be the case if you are only adding a big bore kit and not changing the exhaust, cams, porting, intake, etc. If all else is the same except for the bore size, it’s going to pull in more gas, thus making it burn too rich.

Which Jets Do I Need To Replace?

The factory components are designed to work the way Yamaha engineered the intake, carb, engine, and exhaust to produce the power that it does. Everything works as a team, so if you change one thing, that will affect how the power is produced.

The intake and exhaust can only flow so much air and exhaust. If you make the engine bigger, it is still limited to the max potential that the intake and exhaust are capable of flowing. In other words; unless you have an intake and exhaust that flow better, going with a big bore piston will not increase top end horsepower much, if at all.

This is not necessarily a bad thing.

You Can’t Even Use All That Power

Most riders cannot use all the horsepower that a stock 250F has to its full potential. Peak HP numbers are cool to see, but actually riding it on the track or trails is extremely difficult in an efficient and effective way.

By increasing the low-end torque and broadening the power-band with a big bore kit, you will be able to ride longer and faster because you won’t get worn out as quickly.

Can I Buy Replacement Parts?

What if my big bore top-end needs to be rebuilt, can I buy spare parts for it? Yes, Athena and Cylinder Works produce and sell replacement piston kits, as well as gaskets sized for their big bore cylinders.

Cheaper Than Exhaust and Twice The Gains

What is the first mod that most dirt bike owners do, hoping they will gain performance? That’s right, an exhaust system seems like an easy way to add horsepower… But wait, does that new pipe and silencer that costs upwards of $1000+ really make that much difference?

Stock Exhaust Is Better Than You Think

A 4 stroke dirt bike aftermarket pipe may add more horsepower, but it may not be as much as you thought. It may also be in a different form than you want. Stock exhaust systems on dirt bikes are actually pretty good these days.

Going to an expensive pipe and muffler will probably add just tenths of a horsepower to most Motocross bikes. Some exhaust systems only move the power around, so you may gain some horsepower on the top-end, but you’ll lose that much needed torque down low.

Even the choked up WR250F exhaust can be un-corked by doing it yourself. This mod proves to gain power and throttle response without spending any money!

Best Money Spent – Big Bore Worth It?

Do you want to ride faster with less work? Some performance mods make your bike faster, but not all of them make your bike easier to ride. A big bore kit is the easiest way to broaden the power-band that actually makes a noticeable difference.

If you can replace the piston on your Yamaha 250 4 stroke then you are capable of installing a big bore kit.

Yamaha YZ250F top-end rebuild
YZ250F Top-End Rebuild

Athena vs Cylinder Works – Which is Better?

Now it comes down to which is the best YZ250F big bore kit. They are not the same kit, so there a couple of options to weigh out.

Do you want the bigger bore?

The first thing you should consider is the how big of an impact will this modification make? The Athena big bore kit will simply outperform the Cylinder Works kit because it has a larger diameter bore, thus giving it more displacement. Easy decision then, right?

Speed on the cheap

The Cylinder works big bore kit is the cheaper of the two options. It’s a little over 500 dollars (sometimes less when on sale) for a complete new cylinder and piston kit.

However, the kit upgrades your Yamaha YZ250F or WR250F to a 269cc engine. This is not a substantial difference, but it’s still an 8% increase in displacement, which will give you a nice boost in power.

More bang for a little more buck

The Athena big bore kit is the single best modification you can do to your Yamaha 250 4 stroke to gain more usable power. Bumping up to 290cc is a 16% gain in displacement. That’s twice the improvement of the Cylinder Works kit!

With corrected jetting your 290cc Yamaha will be like a new dirt bike… Less clutch work and easier to ride faster will be the most noticeable changes. The price? Well, it’s around 200 more than the Cylinder Works kit, but you get that added displacement and power.

Will I Lose Reliability?

A 269cc big bore will still have plenty of cylinder wall thickness to keep your engine reliable. A 290cc will obviously be thinner, but there’s still enough wall thickness to keep your bike running like a stock engine, but with a lot more performance.
As with any 4-stroke motocross bike, it’s never good to let it idle for long periods of time, or else it will overheat.

However, just like you should do with a stock high performance four stroke engine, take precaution and properly maintain it. Engine warm-up time is important, as well as regularly changing the oil and filters. Do these things, and your engine has the potential to last many hours.

A New Bike Without The Expense

A performance upgrade that completely changes your dirt bike (in a good way) for well under 1000 dollars is an easy step to making your riding days more fun and exciting. Why buy that ten thousand dollar dirt bike when you can modify your bike in the garage for a fraction of that? To Check the price of the 290cc Athena Big Bore Kit on Amazon for your Yamaha 250 click here.

Or if you want to save a few bucks, check the current price of the Cylinder Works 269cc kit here on Amazon.

2014 and newer YZ250F big bore kit

The YZ250F engine got a completely new design with fuel injection in 2014. With that, the older style big bore kits don’t fit. However, Cylinder Works has a 269cc big bore kit (check price on MotoSport) for the 2014+ style engine