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Old 08-08-2008, 01:44 PM   #1
rip_sti_ra
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Default Diagnosis on jdm ej207 heads pls?

Good day all,

A suspicious engine noise after a spirited run caused us to tear-down the engine.

On inspection of the shortblock, it was discovered that a piston had cracked.

At that point in time, we did not pay too much attention to the headers since we focused all our attention on obtaining parts for the shortblock which took a couple weeks and now brings us to present day.....

On cleaning and inspecting the heads, we saw the following on both the right and left side





Could anyone please tell me what could cause the heads to "crack" around the spark plug on both the left and right heads?

I have my suspicions but would prefer to hear from the wider community what may have caused the problem.

What would have been the tell-tale signs that cracks like these were present had the engine not been taken apart?

Thanks all for your time and looking foward to the diagnostics




OH......can these be re-built?
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Last edited by rip_sti_ra; 08-08-2008 at 05:43 PM.
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Old 08-08-2008, 04:19 PM   #2
gold rush
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I'm guessing serious heat. Did you do a leak-down before pulling the engine apart? Do the cracks go righ into the seats?
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Old 08-08-2008, 04:28 PM   #3
chanders
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Bump for more ideas!
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Old 08-08-2008, 05:42 PM   #4
rip_sti_ra
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Quote:
Originally Posted by gold rush View Post
I'm guessing serious heat. Did you do a leak-down before pulling the engine apart? Do the cracks go righ into the seats?

no...no leak down test done as yet...

even if cracks are superficial, wouldn't it progress to the buckets after a while?
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Old 08-08-2008, 07:29 PM   #5
Dom'sGurl
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I've seen this a lot lately.

One engine had small cracks and was driven "normally".

The other was beat to hell and had very large cracks.


My guess is that the large buildup of material around the spark plug head is retaining a lot of heat and concentrating stresses at that point. Combine that with the fact that the plugs don't go all the way down and you can see that the combustion forces would begin in that little chamber.

I have a new set of heads that we are going to put on a high HP engine and we're going to grind that part down a little to make it smoother and see what happens. My guess is that it'll prevent that from becoming a hot spot and it won't subject it to as much stress.
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Old 08-08-2008, 07:49 PM   #6
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The threads and machining there look like a stress riser heaven. I would bet my lunch Dom has the right idea.
Also, I wonder what those cracks do to the knock threshold of the engine. They seem like great hot spots.
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Old 08-08-2008, 09:56 PM   #7
BallPeenHamr
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I'd say it's a casting or design flaw if this case is no anomaly. The water jacket probably doesn't flow well around or come close enough to the spark plug area to cool it sufficiently. It might be why the newer subie heads take long thread spark plugs... it favors more volume for coolant within the casting in that area.

Your heads, my friend, are toast.
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Old 08-08-2008, 09:59 PM   #8
chanders
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Are the spark plugs supposed to go all the way down? If not, wouldnt this be a big flaw?
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Old 08-08-2008, 10:48 PM   #9
gold rush
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Were the heads on an ej207 or some kind of hybrid set up? If so what was the C.R.? 207 heads have a smaller combustion chamber.
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Old 08-08-2008, 10:54 PM   #10
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That is a chronic issue with the early WRX heads (before they went to the long reach plugs on the STi heads). It is probably due to heat, I have not seen a set of used heads that did not have at least one hair line crack in those locations. Given you have cracking on all four valve margins I would guess that set of heads was pushed pretty hard.

If I was building new heads I would probably get them thermal coated to help with that. There just does not seem to be enough material there to resist stresses in that area.

I suspect the long reach plugs and deeper spark plug boss on the newer heads is intended to address that by putting more material in that area of the head to control expansion and contraction stresses.

Larry

Last edited by hotrod; 08-10-2008 at 01:14 AM.
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Old 08-08-2008, 11:57 PM   #11
rip_sti_ra
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Quote:
Originally Posted by gold rush View Post
Were the heads on an ej207 or some kind of hybrid set up? If so what was the C.R.? 207 heads have a smaller combustion chamber.

heads were on a jdm sti v7 2.0 motor - avcs jdm sti heads

8:1 stock compression ratio

from what I've gathered so far from this thread is that this is a spot for high heat build up which eventually leads to cracking...



I'm not sure if that is the full reach of the plug, it may have been loosened during engine dis-assembly but if what you guys/gals say abt the longer reach spark plugs in the later models then it seems to be a manufacturing defect

Q: can a hairline fracture in the same positions as in the pictures cause hesitation on accelerating?

ALL...thanks for the suggestions.....please keep them coming...feel free to ask anymore questions as we hope it helps anyone else out there..

cheers

Quote:
Your heads, my friend, are toast.
^ from user BallPeenHamr

LOL......cry
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Old 08-09-2008, 12:17 PM   #12
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Quote:
Originally Posted by rip_sti_ra View Post
Q: can a hairline fracture in the same positions as in the pictures cause hesitation on accelerating?
Heat likes to cling to the sharp edges of a combustion chamber, ie cracks.. The heat will stay in those areas since it can't effective disperse. These hot spots can pre-ignite the fuel, especially under boost pressure. So what you essentially have in this situation is pre-ignition, or knock. This is why you typically see polished combustion chambers in performance applications.

Quote:
Originally Posted by rip_sti_ra View Post
^ from user BallPeenHamr

LOL......cry
Sorry dude..
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Old 08-09-2008, 07:18 PM   #13
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^ understood

is it possible to pick up the pre-ignition if the car was being logged?
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Old 08-10-2008, 01:13 AM   #14
hotrod
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Quote:
is it possible to pick up the pre-ignition if the car was being logged?
Not really. Pre-ignition under load melts pistons very quickly, but is difficult to detect. It is said by experience dyno operators that it makes a low rumble sound that is very difficult to hear. At low speed it will show up as run on after the engine is shut off on a carbureted engine since it gets fuel for a while, even if the ignition is off.

I know some folks are re-using heads with that sort of damage with no problems, provided the heads are not warped, and the valve seats are tight. If you want to do that you might, buff the combustion chambers to remove any sharp edges, and send the heads out to someone like swaintech and have the chamber thermal coated to close the cracks and help keep heat out of the head.
It won't repair the cracks but will close them off from the combustion process.

Also don't use so much ignition advance, or go with a water injection setup to keep combustion temps down under load.

Larry
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Old 08-10-2008, 03:19 PM   #15
rip_sti_ra
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^ great suggestion about the thermal coating. Thanks

We have sourced another pair of avcs jdm 2.0 v8 heads and are using it in the new re-build.
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Old 08-13-2008, 02:38 PM   #16
rip_sti_ra
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bump in the hope that some engine builders might see it and post something i may have overlooked....


thanks
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