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Check that valve clearance

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Posted by: Rich Rohrich---------------------

This is a YZ250F that we had in the shop over the weekend. Even though the Yamahas have a much better track record in terms of wearing out valves they still have to be checked regularly and the clearance maintained. General rule of thumb for the YZFs (250s & 4XX titanium valve models) is anytime your shims start dipping below 170 on the stock valves you are heading for trouble.

This bike had inadequate clearance and intake shims all less than 160 already, and it's amazing the valves didn't break the heads off. . One shim was ground by hand to 151. BTW, don't grind your shims to size no matter what anyone tells you. It's a BAD idea.

These pics are a comparison of this rider's intake valves with a new intake valve:
http://www.eric-gorr.com/250f_cam/P1010009.JPG
http://www.eric-gorr.com/250f_cam/P1010006.JPG

The following pics are the result of bearing damage. It could be from using a cheap torque wrench, or not replacing parts like the cam when they are worn past the service limits. It's tough to say for sure not knowing all the detaills of the engine's service life, but the cam is now undersize and the plain bearing in the head is way oversize. If you look close you can see the is a ridge of metal smeared over the cam bearing surface and into the lifter bore. This cam and and more than likely the head are history.


http://www.eric-gorr.com/250f_cam/P21010005.JPG
http://www.eric-gorr.com/250f_cam/P21010002.JPG
http://www.eric-gorr.com/250f_cam/P1010005.JPG
http://www.eric-gorr.com/250f_cam/P1010003.JPG


The YZF engines are probably the most reliable of all the late model MX engines but they still need to be maintained. If you own one of these bikes buy a good quality 1/4" torque wrench to use on the cam caps. As SFO (former Crew Chief for Doug Henry's YZF450 Supermoto team) once said "the cam bearings are the only plain bearings in the engine and they deserve respect . I couldn't have said it better myself.


Posted by: twoofeach---------------------

Thanks for keeping it real Rich. You also da man! What the heck is that? Melted metal fragments? What's that saying, "a picture is worth a thousand words" and "an ounce of prevention is worth $1500" or something like that. HaaHaa


Posted by: SFO---------------------

Maybe he thought that by adding clearance to the cam brg he could get more clearance for his valves?
Not to contradict you but I would usually get big shims from the full end of everyones shim kits and surface grind them to the half thousandth to get the clearances right where I wanted them instead of in between the sizes that were nominal.
I have seen some hand ground shims though that had appeared to have been ground with a belt sander or just a bench grinder, yikes.
It amazes me that people will spend a ton of dough on a fully prepped cylinder head only to get in a hurry and hand grind shims in the above mentioned way.
Shim under bucket does not mean they will stay there if the shim is not parallel. The shim will fall out of the pocket on the top of the retainer and ride between the bucket and the edge of the retainer causing automatic disassembly of the valve retainer juncture.
This is ugly.
It makes sense to me even more now that Rich and Eric will shim your top end as part of their tune ups.


Posted by: Rich Rohrich---------------------

Quote:
Originally Posted by SFO
Not to contradict you but I would usually get big shims from the full end of everyones shim kits and surface grind them to the half thousandth to get the clearances right where I wanted them instead of in between the sizes that were nominal.
I have seen some hand ground shims though that had appeared to have been ground with a belt sander or just a bench grinder, yikes.
It amazes me that people will spend a ton of dough on a fully prepped cylinder head only to get in a hurry and hand grind shims in the above mentioned way.


Bill - There is no doubt in my mind that someone with your skills can grind shims as good as the OEMs. As you pointed out the average guy in his garage has no business doing this given the dire consequences that come from doing it wrong, especially when the shims are so cheap as easy to get. So I'll revise my statement to say if you aren't some bad-ass machinst with access to a surface grinder, don't grind those shims.

The pictures I posted don't do this cylinder head justice, it's much worse than it looks.. I spent a ton of time on Saturday shaking my head trying to figure out how this thing ran as long as it did without tossing parts through the engine. I think this rider must have had some really good karma built up to make it as as he did with these parts.


Posted by: twoofeach---------------------

Quote:
Originally Posted by Rich Rohrich
I spent a ton of time on Saturday shaking my head trying to figure out how this thing ran as long as it did without tossing parts through the engine. I think this rider must have had some really good karma built up to make it as as he did with these parts.


OR..that damn Yamaha engine kicks arse! Even throwing metal into the engine will not make it explode! I imagine you've seen too many grenades to maintain an optomistic outlook.


Posted by: SFO---------------------

Lemme guess, it was still running?
It just came in for a basic service?


Posted by: Rich Rohrich---------------------

Quote:
Originally Posted by SFO
Lemme guess, it was still running?
It just came in for a basic service?



Exactly, it came in for the basic head service and inspection.


Posted by: VintageDirt---------------------

Quote:
Originally Posted by SFO
Lemme guess, it was still running?
It just came in for a basic service?

I had one that was running when it quit.


Posted by: showtime586---------------------

Rich,
Do you work for / with Eric?


Posted by: Rich Rohrich---------------------

Quote:
Originally Posted by showtime586
Rich,
Do you work for / with Eric?


YEP.


Posted by: BSWIFT---------------------

If that bike had been riden by Billy? at DW01, I don't think it would have lasted much past the start area at Casey!


Posted by: Treejumper---------------------

I'm scared to look at mine.


Posted by: Jaybird---------------------

Being the tightwad I am...I have been hesitant to plop down the $240 for a Precision Instruments 1/4" lighted dial.
The more I read, the more I think the price is justified. The wife doesn't buy my logic...but then she has zero bling tools.



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