I have an 02 400EX, I have to tear down my entire engine due to my piston being destroyed. I am going to bored it out, but not sure how big I should go. I am putting a stage 2 hotcam, stage 3 cdi box and coil on it during the rebuild. I'd like to have some room later as well if it decides to destroy itself again. So I am thinking 1 or 2 over. Tell me the upsides and down sides of each bore size.

 

My engine machinist hates boring anything less than 0.5mm at a time. If the piston broke, you may have to go this much, or more, just to get the marks out. You do get a midge more power on max bore size, but there is no more room for further re-bores. Some aftermarket piston suppliers go bigger than Honda, but the cylinder liner is thinner, so weaker. The combination of a tuned engine and weakened cylinder is not ideal.

 

Just got 1 over if you can that way you'll have room to go over again if you need it.

 

You will really need to get the jug to your machinist to see what options you have. The thing is, being an air cooled engine, they don't like a huge over bore hence why the 440 kits can be problematic. Of you are just going up a size or two from stock should be no issues. In any case, a new jug isn't that expensive and probably less hassle if you do a kit and are doing the rest of the work yourself. At least it is easier for me to replace parts since I don't have the talent or machinery to do the cylinder bore and hone myself.