Originally Posted by homeslice

whats up, new guy here. Recently a valve broke off on my 02 YZ250F and destroyed my head and piston (need a replated cylinder too) Is this very common?

Originally Posted by VintageDirt

I've felt your pain.

Originally Posted by zcookie49

I just want to know, is it inevitable for the titantium valve to break off, or if you keep up on the maintenace and check the valve clearance ( i guess for me, every season), will it not happen?..

Originally Posted by zcookie49

Or since you said a 10% drop in shim, should I keep in mind that next time I check the valves, instead of shimming if it is close to that amount, replace all of the valves?

Originally Posted by Rich Rohrich

...Waiting too long after that usually results in valve heads breaking off and doing some serious damage.

Originally Posted by rv6junkie

Is there a service interval for the valves? To avoid expensive repairs, why not change them after XX number of hours?

Originally Posted by Rich Rohrich

Lots of people fall into the trap of assuming that because it's a four-stroke you just have to change the oil and keep the air filter clean and never do any other work, just like their old XR75. Clueless magazine gits and numbskulls spouting giggerish on random websites hasn't helped this much.

Originally Posted by KelvinKDX

Actually this has been an informative thread for me as i am thinking about picking up an late model used WR250f and this has me thinking baout the valve issue.

Originally Posted by Rich Rohrich

The bulk of the 250F engines we've had to rebuild after a broken valve did it's thing were second owners. It usually goes something like this. First owner rides it hard and maybe does a top end and shims the valves during it's life span. He sells it to a new guy telling him the top end is fresh and the bike has been a model of reliability, which is all true. The problem is the new owner has no idea how many hours are really on the bike so he won't really know if the cam chain is worn and tearing up the end of the crank, or if the valves are worn to the point of breaking. Chances are good he'll never find out how much the clearance has changed on the valves unless the original owner kept great records and is totally honest (a fairly rare combination) . So the new owner rides his bike and loves it to death until somthing breaks, or if he is lucky the valve clearance closes up tight enough to keep it from starting and he asks someone about it. Let me tell you that's an INTERESTING conversation to have with a customer.

That's why when I rebuilt Okie's 250F I changed the piston, crank, cam chain, main bearings, output shaft bearings, valves, and valves springs. If the bike is a couple of years old and has been ridden regularly you'll want to see receipts for similar work. Otherwise you have to treat them like you are buying a used 125cc two-stroke race bike and assume it will need some real work.

Originally Posted by KelvinKDX

Or maybe one from someone who never rides ... i won't mention any names.

Originally Posted by VintageDirt

And anyway mine's not for sale, at least not at a reasonable price.

Originally Posted by Rich Rohrich

Otherwise you have to treat them like you are buying a used 125cc two-stroke race bike and assume it will need some real work.

Originally Posted by bclapham

ive also seen plenty of honda 450s smoking like my suzuki on start up- that cant be good.

Originally Posted by homeslice

Rick,

Should I be worried about my crank? I got out all the shavings I could and drained oil through it. Ive talked to numerous guys who have tons of miles on their stock bottom end.

Originally Posted by bclapham

BTW Rich, a lot of the Honda450s we have seen fail, have had problems blowing oil from the breather into the airbox- whats causing this? too much engine case pressure from bad seals????

Originally Posted by Rich Rohrich

The single compression ring wears out and blow by increases. When blow by goes up so does crankcase pressure and oil gets blown through the breather tube, which is routed into the airbox on the Honda.

Simple solution is to change the rings at the appropriate intervals.
Rerouting the breather tube into a small external filter is a good idea as well.

Originally Posted by mxer842

Rich, when would you reccomend a bottom end job to be done? My '03 is apart now for a top end. It has a little under a year of racing on it by a fast B rider, and then 30 hours on it by me.

How much are just the seals?

Originally Posted by homeslice

Rich, another question for ya.
Have you ever split the case on one of these? I understand you need a clutch puller from yamaha to do it properly?

Originally Posted by Rich Rohrich

I've split more of these than I can count at this point.

Originally Posted by homeslice

Now i have to take the head and cylinder off again to split the case. Can I reuse the head and base gaskets? They just sat overnight and were torqued to spec, plus the engine has not been ran.

Originally Posted by homeslice

The crank is 2 thousandths out of spec, is it ok to run it like that?

Originally Posted by ben.handasyde

Rich, you mntioned not needing and special tools what about getting the magneto off and splitting the actual cases themselves?

Originally Posted by NO HAND

I wonder why the valve springs have to be changed also.

Originally Posted by homeslice

You dont need to pull the magneto off to split the cases on a 250F.

Originally Posted by homeslice

I put a dial indicator on the crank halves (counterbalance weights) with the crank still in the case and it was .004" and .002" was listed as in spec.