This page lists big bore options for the KLX 250 and KLX 300. OEM parts, cloned OEM parts, and aftermarket big bore options are included.
If the valve cover gasket is worn, it is useful to replace when installing the big bore kit to prevent oil leaks. P/N
11060-1318 applies to all KLX 250 models and 1997-2007 KLX 300 models
For tuning, reference the appropriate tuning section from the previous page.
For 250 owners looking for an upgrade using Kawasaki parts, it is possible to use the cylinder, piston, and appropriate gaskets from the KLX 300. The chart below includes part numbers for the bits from the original (1997-2007) KLX 300R, updated part numbers for the original KLX 300, and part numbers for the 2021 KLX 300R and 300SM. These parts can also be used to refresh a KLX 300 if parts are worn.
|Part||Qty||300R (2007)||300R (2021)||300S/SM (2021)|
B&B Cycle Restoration (a.k.a. Bill Blue) makes a 351cc big bore kit that uses a custom sleeved cylinder.
Based on the posts on the KLX 250 forums, this is likely one of the more popular aftermarket options.
https://www.kawasakiforums.com/forum/klx-250s-71/bb351-lidless-cvk34-2009-klx250s-44765/ https://www.kawasakiforums.com/forum/klx-250s-71/bb351-bill-blue-update-circa-2020-a-46999/ https://www.kawasakiforums.com/forum/klx-250s-71/boilerplate-setup-cvk-351cc-45741/
Keng Engine out of Thailand lists a number of kits from 300cc all the way to 351cc in their FB shop. However, there are posts indicating that 381cc kits are also available. All kits come with the cylinder, piston, and needed gaskets but also appear to include a set of cams, clutch springs, and for those that need it a bigger injector and a custom ECU or an EJK.
The 300cc kit uses all OE parts except for a pre-programmed aftermarket ECU.
The KLX engine has been cloned, and is used in a number of dirt bikes and ATVs made by chinese manufacturerrs. Because of this, sellers on aliexpress and eBay are able to offer big bore kits for significantly cheaper than what OEM kawasaki parts cost. The 299cc AHL branded kits that are offered seem to work well, and appear to be compatible with the OEM cam chain tensioner. These cylinders have the letters “AHL” and “299” in the casting itself.
292cc kits also exist, but it is reported that there are differences in the cylinder that make it incompatible with the OEM cam chain tensioner and head gasket.
Reference kawasakiforums.com for more details:
Kustom-kraft makes a 331 cc big bore kit, but it uses a bored and plated cyliinder instead of one with an iron sleeve.