The most cost-effective way to upgrade a KLX250 to KLX300 displacement is the AHL "299" big bore kit.
AHL "299" Kit
This kit is very similar to the OEM 292cc kit and includes everything needed:
- Cylinder
- Piston (78mm)
- Piston pin
- Piston rings
- Piston circlips
- Gaskets
Weight Comparison
| Piston | Weight (with pin and rings) |
|---|---|
| OEM 72mm (249cc) | 235.5g |
| AHL 78mm (292cc) | 272.7g |
The 37g weight increase is minimal and doesn't significantly affect engine feel.
Cost
In most countries: Under $200, sometimes under $150 for the entire kit.
This is far cheaper than:
- OEM Kawasaki 300cc cylinder kit
- Selling your 250 and buying a 300
Installation
The kit is designed for bolt-on installation with basic mechanic skills. You'll need:
- Torque wrench
- Cylinder hone (optional but recommended)
- Fresh oil and filter
- 2-3 hours of time
What to Expect
After installation:
- More torque throughout the range
- Better bottom-end for technical riding
- Slightly more top-end power
- Same reliability if installed correctly
Long-Term Reliability
After 7,000 km of hard trail use over 1.5 years:
Piston condition:
- No visible wear on skirt
- No scoring or damage
Cylinder condition:
- Honing marks still visible
- No scratches or scoring
- Plating in good condition
- No roundness issues
The kit addresses common skepticism about Chinese manufacturing. Most negative reviews come from people who haven't actually used it.
Tip: Look for "299" marking on the cylinder. This version has correct chain tensioner mounting points and is true bolt-on with zero modifications required.
Fuel Tuning Required
The increased displacement requires more fuel. Plan to add:
- 270cc Fuel Injector ($35) - verified 270±5 cc/min
- Or PCFC/EJK fuel controller (interim solution)
Quality Warning: Random eBay/AliExpress sellers don't verify flow rates. We've received injectors that flowed 220cc instead of 270cc. See the product page for details.
Coming Soon: Purpose-built fuel tuner for the KLX platform in development. Subscribe below for updates.
Related
- Engine Guide - Overview of all performance modifications
- ECU Tuning - Fuel management options
- Camshafts - Complement the displacement increase