How To: Install A Big-Bore Kit

Increasing displacement is one of the most cost-effective ways available to boost the real-world power of your bike's engine. These days, you have more options in your method of growing bore size than ever, from having the stock cylinder sleeved with a cast-iron sleeve to installing completely new cast or billet cylinders with a plated aluminum bore-just like a stock cylinder.

Regardless of the method used to boost the bore, the job of installing the kit remains the same. We used a 2007 Kawasaki KX250F and an Athena 290cc kit that includes a complete new cylinder. The Athena kit comes with everything needed for the installation. Even though we used a Kawasaki, any four-stroke is essentially the same.

01 Any time you begin engine work, start by draining the coolant as well as the oil. The coolant is a necessity, but the oil is just a good plan to eliminate the chance of a mess. Make sure the bike is good and clean before you start; you may want to rewash it after the tank and seat are off.

02 Remove the valve cover and the appropriate timing plugs. Use a T-handle to turn the engine to top dead center. There are marks on the ignition that need to be lined up to ensure the piston is at the top of the stroke. The manual will specify the correct marks and their position.

03 On the KX-F the coolant hoses are in the way, and they need to be removed for best access to the cylinder area. The constant heating and cooling of the engine sort of glues the hoses to the fittings. A right-angle hook like this will slide under the hose. Pull it around and it will pop the hose loose.

04 We removed the water fitting from the case for better access. It is sealed by an O-ring, so we used the hook again to retrieve the O-ring without damaging it. The Athena kit does not include this O-ring, so purchase one if it looks damaged or deformed.

05 Before you can deal with the head, it is a good idea to check the valve clearance. If they're radically loose or tight, you'll need to have the head rebuilt to prevent wasting the other new parts you're installing. Write down the clearance so you have it for reassembly.

06 Remove the caps that hold the cams in place. These are high-precision parts with important clearances and torque values, so loosen the bolts a bit at a time in a crisscross pattern. Remove them carefully. Threading a bolt into the cam cover holes will help you lift them up gently.

07 There are usually parts under the caps that can fall into the engine, so lift the caps a bit at a time. Use a magnet to remove these steel clips that locate the cam bearings. You may want to remove the tappets and shims, but mark where they came from. Watch the dowels that locate the caps, too.

08 Before you can start with the cams, the cam-chain tensioner must be removed to allow enough slack in the cam chain to remove the cams. Start by taking out the center bolt and the spring that operates the tensioner. Check the aluminum washer under the center cap. If it is deformed, replace it.

09 Usually the tensioner can only go in one way, but we marked the top with a marker to make sure it would go back in quickly and correctly. If there is a gasket under the tensioner casting, it should be replaced whenever the tensioner is removed.

10 On this Kawasaki the head won't come out easily unless the engine is rocked forward. We removed the head stay, engine mount plates and the lower engine mount bolt. We didn't have to loosen the swingarm pivot bolt to rotate the engine forward.

11 With the engine rotated forward, the head has just enough room to clear this part of the frame where the head stay attaches. Unless the head can clear, it won't lift high enough to pass the cam-chain slides.

12 Now you can lift the cams out. Begin by lifting the end on the exhaust cam, then angling it to clear the chain. There is no need to label the cams. The exhaust cam has the auto-decompression system, so it looks nothing like the intake cam.

13 While the cams are out, inspect the parts of the valve train for wear. Defined swirly marks like these on the face of the tappets mean it is time to replace them. This means the tappets are rotating in the bore as they are supposed to.

14 The lobes of the cam shouldn't show any wear or serious discoloration. The bearing surface of the cam should be absolutely smooth and unmarked. The left side of the cam has a bearing, but the right side generally rides on aluminum protected by a high-pressure oil film. Any flaws mean the engine had a lubrication failure.

15 The same is true of the cam caps. The side that rides on the cam without a bearing should be perfectly smooth and show no wear. Otherwise, the oil film the cams are supposed to run on will not work correctly.