Hi guys. For Christmas I’m getting a full gasket kit for my 03 400ex. What a joy. But the head gasket, valve cover gasket, oil filter cover have all been slowly leaking almost a year now. While it’s apart im going to file my clutch basket I have a suspicion it’s notched, replace the clutch and throttle cable, and paint the frame. I have been debating getting a new piston as well, but I am entirely unfamiliar with piston stuff. I replaced the timing chain with a crf450 timing chain but I’ve never opened the bottom end. That was my first and only time in the top and I stripped out like 4 bolts. Anyways I don’t want to send it to a machine shop, and from my reading ands and my brothers slow 400, I don’t think there is much gain from a higher comp with the stock bore. So will any increase in piston size require machining or is there just a thinner sleeve to use? I’m also trying to stay cheap, I’ve seen some after market piston for like 100-150 but it’s just a piston is that all I need?

Hi guys. For Christmas I’m getting a full gasket kit for my 03 400ex. What a joy. But the head gasket, valve cover gasket, oil filter cover have all been slowly leaking almost a year now. While it’s apart im going to file my clutch basket I have a suspicion it’s notched, replace the clutch and throttle cable, and paint the frame. I have been debating getting a new piston as well, but I am entirely unfamiliar with piston stuff. I replaced the timing chain with a crf450 timing chain but I’ve never opened the bottom end. That was my first and only time in the top and I stripped out like 4 bolts. Anyways I don’t want to send it to a machine shop, and from my reading ands and my brothers slow 400, I don’t think there is much gain from a higher comp with the stock bore. So will any increase in piston size require machining or is there just a thinner sleeve to use? I’m also trying to stay cheap, I’ve seen some after market piston for like 100-150 but it’s just a piston is that all I need?