Ive got a blown cylinder head on my 2009 klx 250s and found a cheap 2007 klx 300 cylinder head on the interwebs. I saw a thread on here stating that it may be possible, but no definitive answer. The part numbers seem to be identical. Anyone with any knowledge on this pls halp?

 

I'm not sure how one can "blow" a head. A head gasket maybe, but a head?

Personal opinion from a mechanical standpoint. Pull the proverbial blown head. Check to see if the mating surface is flat. One way would be to put Dychem bluing on the surface then put some lapping paste/grinding compound on a piece of glass laying on a flat counter top and slide the head around. If flat the Dychem will be removed pretty much uniformly across the surface. If good, do a valve job and seals on the head and use it.

Fact is it might even be reasonable to take the head to a good machine shop and have the head skimmed slightly. You'll gain a bit of compression and a flat surface. Then do the valve job and seals. Just don't go cutting a huge cut off the head. You might ask here to see what anyone else has done with their heads.

Odds are it won't cost much and you will essentially have what is a new rebuilt head, rather than take a chance on a used part having good seals, valves decently seated, and a flat mating surface.

 

The 300 head will install on the 250 with the exact same results as a 250 head...no problem. I too am curious as what your definition of "blown head" might be. Not questioning that you're necessarily wrong but would like to know more detail. Heads are "usually" damaged only by valves going astray in the head, warping by severe overheating, and/or cracking by severe overheating. If you have pics, it would be interesting.

 

camshafts seized due to low oil presumably, scoring on journals on the camshafts, the head and the camshaft cap aswell. tried to emory cloth it out but it was no beuno.

I've got the 300 cylinder head on the way, hopefully it doesn't suck.

 

Had the same problem with mine, somehow. It ran for thousands of km like that, I dont know how it happened, maybe the previous owner....
Anyway I was advised to buy a new head, cams,valves, but it was less than $300 here

 

If the engine was low enough on oil to damaged the head, you have more damage you haven't found yet. The same oil pump and oil that feeds the cam journals also feeds the crank journals. I bet you have more damage throughout the engine.

 

Oddly, not necessarily so. The bottom ends on these engines are not plain bearing insert design. Their crank and rod bearings need little pressure and seldom fail. Not impossible...just not as likely. The top ends on most of these bikes just have the cams riding in the machined surfaces of the heads. They go fast and go first when the oil pressure drops.

 

is there a way to find out whether the bottom end is damaged without stripping the engine down?

 

Not sure if it is the best way, but usually bearings going bad will make noise, whirring etc. If the cylinder comes off the rod can be checked to see if the rod bearing is loose.

As TNC said, the roller and ball bearings can work with very minimal oil - witness the same general crank/rod set up in a two stroke running premix.

I'm curious how low was the oil run? I know of a head being ruined running nearly 2 qt low in an engine that has maybe slightly over 2 qt capacity. I'm curious. More questions maybe after a response.

 

Same thing for me - top end cooked but bottom was fine.

It was a commuter bike (long freeway run) and burned enough oil that the cams seized in the head. Bottom end checked out just fine.

I ended up springing for a new head. Congrats on finding a cheaper way to go! Use all that money you saved to go 351.