Kx250f rebuild

Hey guys, im rebuilding my 04 kx250f top and bottom end. I went with a brand new 07 kx250f head cams buckets etc, steel valves, 270 big bore kit, and hot rods crank, bearings and seals, and oversized rads with silicone hoses. Im hopin all this will leave my 04 more reliable and give it a little more power.

I have a few questions about the rebuild. First being with a brand new head and valves do you have to have the valve seats cut for the valves, or can I just throw the new valves into the new head and go from there. Second is are the cam chain guides ok to reuse or should I go for new ones? And last question being how hard is a crank/ bearing, seal replacement? My local shop wants 430 bucks for the job and thats only the price provided I supply the crank, bearings, seals, gaskets. I would like to avoid paying that provided its not too much of a pain to do it my self. Thanks in advance for the help!

brand new valves and new seats should not have to be lapped or seated. In fact you cannot do this if they are titanium valves . they have a heat coating that gets removed if you abrasively lap the valves.

I would inspect the guides to make sure they aren't worn too badly. If not then ok to reuse them.


On splitting the cases to replace the crank seals and bearings, You need some special tools to do the work. I am not sure it's worth the money to get the case splitter and bearing pullers to save some money. I am sure it can be done if you could borrow the tools. P.S. carefully inspect the output shaft bearing and replace that. We have had a few bikes that those go bad on.

copy that. im still debating taking it to the local shop. i got few things more to ask bout settin my bike up. what size carb jets do you think i will need with a 270 bbk and the 07 head? also how do i go about finding the valve shim size needed when setting up a new head? just throw one in there and measure the clearance and adjust up or down from there? 


On the jetting, go bigger to feed the engine upon break in. I would go 190-200 on the main jet, 45 0n the pilot. 3rd clip from top on the needle. and go from there.

on the valve shimming , you can do this on the bench with the head / cams installed. Pick your thinnest shims and install them into the intake or exhaust cam side does not matter, make sure you torque the cam caps correctly ( 80-85 inch pounds ) then make sure the cam is rotated to the base circle of the cam, ( bottom of the lobe ) measure with a feeler gauge then do your math, Shim thickness should be written down some where and location you install it.  If you run into a valve that you cannot get a reading on, like the shims are too tight. Your going to have to buy a  thin one and go from there

so its

shim + gap with feeler gauge to get -0- clearance if feeler gauge is greater than suggested gap you need to get thicker shims and if tighter then thinner shims.

Alright thank you for the help! And ill do that. Ive done valve shim jobs before. Just never with a new head and no current clearance to go off of. Make sence now. I appreciate it.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now