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Old 07-31-2010, 02:06 PM   #1 (permalink)
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Cool Original Head Depth 99 2.5L SOHC

Were there different Cylinder head Dimensions for 2.5 L SOHC engines ?

My book shows 3.870 inches for a 1999 Year car...the book for a 2003-04 Forester 2.5 list it at 3.839 inches. Mine measured 3.840 before I had them to the machine shop. The now are .010 to .011 shorter and have a .004 to .005 taper from one end to the other.

Trying to determine if the machine shop ruined my heads !!

Subaru spec says max mill spec .004 inches. Apparently some machine shops just have a .010 standard. I was referred there and am thinking they may have ruined my heads.

Help and comments appreciated.
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Old 07-31-2010, 04:50 PM   #2 (permalink)
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My shop manual says cylinder head for 2.5 SOHC is 3.870-inch height, surface warpage limit is 0.0020 (you are out of spec by this), and surface grinding limit of 0.0040.

I would return the head and insist on re-shaving to spec for surface warpage. Perhaps ask for a check of warpage while you watch. Are you sure of your measurements? I've tried to measure warpage w/o much success. Takes shop environment and precision straight-edge to get viable results.

good luck
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Old 07-31-2010, 05:01 PM   #3 (permalink)
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As long as the deck surface is FLAT they should work. The taper in the head thickness has to many variables unless you are comparing them to dimensions you took prior to getting them machined.

Going off the spec dimensions in the book and using those as your your baseline for checking after machining is worthless as those specs have a tolerance in the machining at the factory.

As for the ".010" minimum", it is pretty much industry standard. It can be difficult to get the required finish in some cases with smaller cuts, nature of the beast. I do also believe Subaru states they should be ground, but again milling is industry standard for aluminum heads. If the removal of the additional material is troublesome for you just add that amount back in head gasket thickness, the proper way to do it on a Subaru. Removing material from the block deck/heads effectively changes the cam timing. Look into Cometic head gaskets and special order your thickness.

I have taken .030" off the deck/head combined and not had any issues while running the stock .024 HG.

OEM SOHC (single layer) HG are ~.024 thick
OEM turbo/STI HG (MLS) are .040" IIRC
Cometic specs the same HG for both .040"

There shouldn't be any issue in running the heads. Even with the taper I don't think there would be any issues as .005" is minimal, actually the engine grows more then that from cold to operating temperature. The only issue I see would be the timing belt not tracking properly, I don't think it will be an issue though.
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Old 07-31-2010, 07:55 PM   #4 (permalink)
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Cool 99 heads

I did a mic measurement at each corner of each head before taking them to the machine shop to be "checked". My measurement was 3.840 at all 8 corners....so I would assume that to be factory. As already stated the 99 manual says 3.870 while the 03-04 says 3.839. Taper I referred to was only there after the machining process....and is front to back .005 inch. They seem to be flat but are tapered.

Some good info...thanks guys...didn't know about the over thick gaskets being available. If my above comments clear up any misunderstandings about my first post feel free to reply.

Thanks

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Old 08-04-2010, 04:54 AM   #5 (permalink)
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Any knowledge of the original overall head height from year to year ? My 99 to 2002 manual list 3.870 inches. The 03 to 04 manual lists 3.839 inches. I measured mine at 3.840 and believe them to be original as I bought this car with 50K miles on it and this is the first time I've had it apart.

Thoughts appreciated.
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Old 08-04-2010, 03:02 PM   #6 (permalink)
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In all honesty, Subaru has a history of playing mix and match from their lego bin of parts when these cars are being assembled. Which may or may not explain why your heads are short compared to the book spec. Would I assume the heads are original however, hell no. ALWAYS, ALWAYS error on the side of caution. A lot can happen in that 50K before you bought it, a lot that may not be in the service history.

For the side of caution I would assume that the previous owner had issues at some time and go with the assumption that the heads have already been milled before, maybe a lot OR maybe the heads were even replaced. In either case you can either do the math to see if the valves will hit the pistons witch may not be possible with out the proper measuring equipment.

Factory valve to piston clearance
- amount removed from head
+ amount cut from valve seats and valves
+/- any change in HG thickness
= you new theoretical valve to piston clearance(should be checked with clay(see below)

OR

(you need a degree wheel and test indicator + transfer clay)
The other way is to set the head/s up with a set of "checker springs", basically substitute valve springs with a real light tension that will allow the valve to get pushed back into the head if needed. In doing this method you assemble the engine as normal using a old (crushed) HG along with a piece of transfer clay(witness clay) in the valve reliefs in the pistons and no rocker arms/cams... Put a dial/depth indicator on the valve stems. Slowly and gently turn the engine over to the point where the valves would be closest to the pistons(not always max valve lift(degree wheel and cam specs will tell you this location)) once at that spot manually open the valve to the max valve lift by referencing the indicator. This will make a witness mark in the clay that can then be used to tell you how much piston to valve clearance you have at that point in the cycle or you can just move the valve from max valve lift until it contacts the piston, referencing the indicator will then tell you the clearance you have. Add to the HG thickness as needed to get your desired valve/piston clearance.

If there is no clearance, again do some math, max valve lift minus the lift you had at the point of interference + the amount you want/require for valve to piston clearance + your crushed HG thickness = your new CRUSHED HG thickness.

I would not consider the above to be a step by step of how to do it. It is only an example of how to get the answer, there may be spots left out. For a OEM street engine with OEM cams/valve springs I would want no less then .020" valve to piston clearance.

The simple thing to do would be just get a set of thicker HG to make up for the amount that was milled off. You know the heads ran on the engine before being milled so they should have no issues if that material is added back into the equation.

.010" removed from milling
.005" extra removed to create the taper
.024" OEM HG
= .039"(.040") replacement HG

That would put you right back where you started.

Again as long as the surface is flat there should not be any issues, only possible one I see is if the timing belt wants to severely track back towards the engine.

Regardless of what happened at the machine shop(their mistakes) you are in the end responsible for the fit and function of the engine as it was your hands that put it together and as such falls on you to verify the clearances.
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