.025 bore in 400ex

I am going to put a .025 bore in my 2001 400ex. I was wondering if I am going to have to put new jets in it? How much better would it run with a K&N filter kit?Everything else is stock right now, but I want to add a new pipe-header combo in the future.Any suggestions that are not to expensive. Any other suggestions for more power without costing too much? Thanks for any help.

[Poncho]

GET:
your high comp. piston
FMF powerbomb header
white brothers e-series slip-on
xr400 cam
k&n air filter
3 angle valve job
harden rockers
maybe a fcr carb too

I already fried the stock piston so I am going to have to do at least a .010 bore job. It did not seize it just scratched up one side very bad and lost compression. Will I still be able to run pump gas if I get the .025 bore on it.

[wyndzer]

Who makes that size piston? The smallest overbore that I know of for a 400 is .040.

[Holty]

If I were you and money is an issue, this is what I would do:

Get a 11:1 compression .80 over pistion kit. $100 or less

Get what ever slip on silencer you like(white bros., fmf, etc...) $250 or less

Do not replace the factory header(it is a waste of money unless you have a fully built motor; the stock header flows very well)

Get the K&N power up kit(comes with filter & jet kit) $99

You will notice LARGE power gains over stock and this will keep everything simple, and basic.
The only thing you will need to do is have your piston installed/stock sleeve bored. the rest of the labor you can install yourself.

Who makes the 11:1 compression kit and will I still be able to run 92 octane gas with it. Will doing the piston that big thin the cylinder sleeve out too much? How much will it affect the reliability of the engine? What will this bring the displacement to 416,425? Will I have to re jet in the summer when it warms up? I already have the engine torn apart. What kind of oil should I run with the bigger piston. Also this should produce a lot more heat, how can I make it run cooler. Thanks for the help.

[Holty]

Answers:

Wiesco or JE makes the piston kit(both are good, but I think most people on here prefer the JE pistons because seem to be lighter.)

Yes, you will be able to run premium gas without causing any damage to the engine. Try to find Sunoco 94 octane, or run a 93 octane. I don't know of any place where I live that sells a 92 octane.

No, it will not be too much boring. You could bore the stock sleeve up to .120 without any problems. You probably could also do less boring if you choose. I chose to tell you .80 over as a good all around choice for power and reliability.

With any motor/performance upgrade you lessen the reliabilty of the motor. This is why you take precautions during the installation to ensure proper fit and compatiblity with the parts you install. Do a quality job with the proper tools and quality parts. (ie, gaskets, new seals, washers, etc).

I do not know exactly the true displacement of the motor after doing the .80 over bore. If you search some of the other posts on this topic you can find the formula to figure it out. It may equal out to be around 405cc, but i'm not sure.

Every bike is different. You may or may not have to rejet in the summertime. The only thing I can tell you on jetting is start reading previous posts and look at your spark plug. But most importantly, learn (from reading on this forum) how to tell if your bike is running good jetting or not. Just sit down and start reading the jetting posts. You'll get a good idea what to "feel" for when riding if your jetting is off.

Oil is also another good question that you will never get a exact answer on. Mostly it is personal preference and budget that determines your oil. Make sure you properly break in your new piston correctly and run oil that is made for dry sump/wet clutch/atv engines and you'll be fine. I personally run the Amsoil 0w-40 synthetic atv oil in the winter. When it gets really hot out, i'm probably gonna run a 20w-50.

Yes, I high compression piston will make more heat. To help reduce the heat, you may want to think about running race fuel. yes it is more expensive, and sometimes depending on your location, is hard to find. but it will help in cut down the heat. Also, depending on your finances, you could get some Air scoops?, larger oil tank?, or a larger oil cooler? Like I said you don't NEED to do any of these, but they will help with cooling. I personally will go with the oil tank and race fuel when it is time for my 416 kit.

Hope this helps.