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Old 07-05-2020, 07:52 PM   #1
Scooby Newbie
Member#: 458758
Join Date: Nov 2016
Default Subaru STI head deck machining?

My 2013 Subaru STi is currently at a dealership getting a new short block installed. During the process, the tech noted that the head deck was out of spec by around a thousandth of an inch and “recommends an OPTIONAL item to get the deck machined at a machining shop”

Have any of you guys done this when getting a new short block in? I wanted to get feedback on how “important/crucial” this item would be. It would be around $500 USD

Thanks in advance
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Old 07-05-2020, 08:11 PM   #2
Scooby Newbie
Member#: 326692
Join Date: Jul 2012
Location: Cda, id
2012 sti


I would definitely deck and wash heads before reusing however this should not be more then $200, why don’t you pick them and have local repeatable shop do it.
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Old 07-06-2020, 01:22 PM   #3
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Member#: 486182
Join Date: May 2018
Chapter/Region: MWSOC
Location: Cincinnati
2011 STI


You should, yes. $500 seems steep to me.
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Old 07-06-2020, 02:33 PM   #4
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Member#: 515308
Join Date: Jun 2020

Deck block, not heads he’s saying, I wonder if they disassemble the short block to shoot it, if so 500$ not so bad, if u took it apart urself I would think 60$ per side.
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Old 07-06-2020, 04:34 PM   #5
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Member#: 30804
Join Date: Dec 2002
Chapter/Region: SWIC
87 Yugo


I charge $100 to deck Subaru turbo heads. That's deck only. No disassembly, no reassembly, just cut the surface and make it flat. I absolutely HATE to do it that way though as there's quite a bit that can be missed.

Now, since I do the head work here on a LOT of Subaru motors I am more than happy to give you the full run down on the way I would go if you're looking to have this car for a while and take care of any issues that may arise in the heads in the future.

1) When it comes to Subarus the aluminum in the case and the head move. Even though they are short in length and have 6 bolts holding that head on they still move quite a bit. Even if you go the cheap route of just having them decked I still encourage you to do that, and do it properly. None of that whiz wheel on an angle grinder BS.

2) Even if I'm installing brand new out of the box head castings on a motor the head will get decked. I have seen time and again brand new Subaru heads being out of spec for flatness according to Subaru's accepted spec (No joke, and yes, that is pathetic).

3) A good machine shop, with good equipment, the proper cutters, AND with a machinist who gives a damn about their quality of work will put a better finish on the deck of the heads than you will ever see from the factory.

Now, if you came to me and said "Hey Clint. I need to do head gaskets on my Subaru (or put a new shortblock in) what needs to be done to the heads?" This is what I'm going to do for a normal refresh:

1) Remove heads, and initially inspect.
2) Pop the valve springs, pop the valves, clean them, inspect.
3) Pull the oil passage plugs
4) Inspect heads for dropped guides
5) Heads go into the big cabinet style washer for initial clean.
6) While the heads are in the washer, clean and face the valves. If any valves are bent or need facing that takes them out of spec it will show up at this time.
7) Pull heads from the big washer, and spray off with clean water.
8) Inspect heads for cracks in combustion chamber or cracked or dropped guides.
9) Allow head temp to normalize.
10) Measure bore on guides for size and straight.
11) If any of the guides have dropped then replace all of them of that side (for example if it's dropped an exhaust guide all the exhaust guides will be replaced).
12) If any of the guides are out of spec for bore or straight replace all the guides of that side (I also replace the guides if they are at the max allowable spec).
13) If I replaced any guides then the new ones are reamed to proper size.
14) Once guides are declared good either by passing inspection or replacement then the heads go through my ultrasonic cleaner for a while after which they are sprayed again with clean water, and allowed to temp normalize again.
15) Cut the valve seats on intake and exhaust and vacuum test to make sure they are sealing properly.
16) Check valve stem height to be within range for Subaru lifters given my target valve lash, tip and chamfer the valves.
17) Deck the heads taking the absolute minimum off to get a straight and clean deck surface.
18) Hand clean the heads, while hand cleaning run the valve springs, seats, and retainers through the ultrasonic.
19) Dry all components and hit the rust prone bits with some WD-40.
20) Install new valve seals.
21) Install the valves.
22) Cover the deck surface with some rather thick adhesive plastic film to keep it from getting scratched.
23) Install the valve spring seats, springs, and retainers using new keepers.
24) Remove plastic film.
25) Wipe deck surface clean with a lab grade, non-shredding paper towel and brake clean.
26) Install heads on shortblock (using head studs!)
27) Begin process to measure valve lash.

I hope that this clarifies any questions you may have.
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Old 07-07-2020, 09:38 AM   #6
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Member#: 468498
Join Date: May 2017
Chapter/Region: MWSOC
Location: Columbus, OH
2014 WRX Hatch


If I understand correctly, your're getting a fresh new shortblock, right? That means their either trying to get more $ out of you or the new shortblock came out of spec from factory. Seems to me a little bit ridiculous that a fresh shortblock would need the decks machined.
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Old 11-15-2020, 10:24 PM   #7
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Member#: 170038
Join Date: Jan 2008
Chapter/Region: MWSOC
Location: Fort Riley, KS
2006 WRX


Originally Posted by Jefe159 View Post
You should, yes. $500 seems steep to me.

I had mine decked last year and they ran right at $500
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Old 11-15-2020, 11:35 PM   #8
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Member#: 519102
Join Date: Oct 2020

I'd get a machinist straight edge and some feeler gauges and measure it my self. If it's out of spec, I'd tell him to get me a new deck.
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Old 11-16-2020, 12:14 PM   #9
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Member#: 452996
Join Date: Aug 2016

I had my EJ heads fully serviced like the poster above said, it was $500 well spent.

Decking was only one part, valve seals, adjust, hot tank, galley plugs removed...etc etc.
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