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Old 09-28-2010, 09:10 PM   #10
Junior2JZ
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Member#: 64908
Join Date: Jun 2004
Chapter/Region: Tri-State
Location: Tuning 8/9sec Subies @ P&L
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Best thread title ever!!!

Jay read below, this was the stuff I was telling you about. See ya when you drop off the motor!!


Quote:
Originally Posted by DJIMPREZA View Post
Guys

A friend of mine, running his 10 sec sti on a cosworth large valve head/cam kit have lost one of his cylinder number 2 intake valve after 8K miles of street use and some runs at the drag strip, here it is the answer from cosworth, wich makes sense, so you guys running larger valves, next time make sure you use the subaru procedure for installment, and turn the intake and ex LH head cam sprockets in the right direction, and turn them with the less angle possible and do not make them move, wich is difficult, but can be done..

so here it is

"The main issue is that it's difficult to keep the cam gears in the proper position when installing the belt and that you don't open intake and exhaust valves at the same time causing contact. As long as you follow the factory service manual instructions exactly, you will be fine. I've attached a PDF copy of the timing belt procedure. On page 5, they explain the importance of the steps and rotation directions. With +1mm valves and higher lift cams, the danger of valve to valve contact during timing belt installation is even greater.
once the valve is slightly bent, the higher spring rate and constant contact of the valve to the valve seat tries to straighten the valve. Plastic deformation sets in, like bending a paperclip back and forth, and then the valve head falls off causing failure. We inspected a number of these valves and the material showed evidence to support our diagnosis.
It's always number #2 that fails, so we're confident this is the problem. Some people argue about piston to valve contact with the AVCS full advanced or retarded, but we've bolted on a cutaway head and manually clocked the avcs cam to each extreme and never came close to piston to valve contact. If the cam timing is phased properly you don't have to worry about the AVCS.
Hopefully the customer in Venezuela can elaborate.
Thanks"

So be aware with the timing belt on high lift/large valve heads

Cheers

DJ

Quote:
Originally Posted by BallPeenHamr View Post
Thanks DJ. I'd like to add some additional insight on this particular type of failure if you don't mind?

The following pictures are of intake valve stems from two different failed engines with cosworth cylinder heads... and guess from which cylinder they're from? Yep, cylinder #2!



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Notice, first of all, the valve stem is still straight after the failure- which means the failure wasn't a result of piston contact, or foreign debris in the cylinder. If that were the case the valve stems would have bent. Instead what we have here is a clean break, like the valve head said, "later man, I'm outta here", and took a vacation.

Secondly, look at the beach marks on the broken surface of the stem. See how the half-moon lines on the face begin on one side of the stem and get bigger? This is a sign of fatigue, and in this case, plastic deformation. It happens when a slightly bent valve tries to straighten itself out over many miles of operation. And at the weakest point of the stem (in this case where the stem is necked down, just before the radius into valve head begins) a microscopic crack appears then gets bigger and bigger over time. Eventually, POP goes the valve head.

Note this will always happen to the intake valves and not the exhaust due to the thinner (weaker) stem, and the larger head diameter- they bend easier.

Also, if an engine failure such as this happens at a low rpm and the driver shuts the engine off quickly enough (like the examples above), it is likely you will see only one, not both intake valves in cylinder #2 that will look like this because both valve heads will never decide to pop off at the same time, and it is always that first popped valve head that takes out the other intake valve (and everything else for that matter) before it has a chance to do it on it's own. However, a said failure at higher rpms will just obliterate everything, including the chance you'll find a relatively clean valve stem break to look at such as the ones you see above.

When it comes to installing a timing belt, I imagine the common logic of some engine builders/mechanics is to think the cam sprockets should only be rotated in the direction of engine operation, and that you always begin with the intake cam gear first... because after all, the mechanical cycle of engine operation is dictated by the intake valves opening before the exhaust valves do, right? So when it comes to lining up your marks to install the belt, it would make sense to phase the cam gears in the same manner, right?? WRONG WRONG WRONG!! Especially if you are running +1mm over-sized valves, be warned that you WILL bend the intake valves when they come in contact with the exhaust valves, in cylinder #2, on an EJ engine if you adapt this logic.

This is why it is very important to refer to the factory service manual when phasing the cam gears before you install the timing belt. If you don't, you run the risk catastrophic engine failure, and not to mention a very expensive repair.
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