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Old 01-17-2005, 09:26 PM   #1
harvcharg
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Default Want to use 2.5 single over head pistions in a 2.5 dual over head cam motor.

Hi i have a 97 legacy outback with a dual overhead cams. You know after a while you get the pistion noise. well i have a set of 2.5 single overhead cam pistions. i was woundering if they will work in the dual over head cam moter without the pistions hitting the valves? i know the dual over head cam pistions have the square indentation on the top of the pistion. and the single over head came pistions have the small slots for the valves on top of the pistions. has anybody tried this swap or know if it will work. i am worried the the valves will hit the pistions. what do you guys think? I also know the compression will be higher. would it be so high that i would have to run 94 octain in it all the time? please let me know what you think. thanks Jim.

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Old 01-17-2005, 09:32 PM   #2
jamesfacts
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They will certainly work... but I think it will be more trouble wiring than its worth.
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Old 01-17-2005, 10:38 PM   #3
Quibron
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It wont work, the comp height is way different. However I just happen to have a brand new set of JE lo compression pistons for your dual overhead cam motor.
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Old 01-17-2005, 11:19 PM   #4
Scoobie Steve
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Quote:
Originally Posted by Quibron
It wont work, the comp height is way different. However I just happen to have a brand new set of JE lo compression pistons for your dual overhead cam motor.
I wouldnt say it wont work. The phase I pistons have a compression height of 1.190 and phase II 1.160. Thats a difference of .030. Now the phase I HG has a thickness of .060 and the phase II .023 difference of .037. If you put the Phase II pistons on Phase I rods the combo will have a neg deck height of -.030 (assuming stock deck height is 0.000) So by using a Phase 2 HG you will decrease the Quench height by .037 and actually increase the CR very slightly.
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Old 01-18-2005, 01:31 AM   #5
Storm
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But the phaseI pistons clear the deck @TDC. I don't remember by how much, but I can measure the amount when I get home from work. I was going to do the same thing, but decided to use a phaseII shortblock with my DOHC pistons and .027 headgaskets. The result is the need to run 94 octane...... I'm looking at north of 11.5:1 CR....

The safest way would be to install the pistons and put some clay on them prior to testfitting the heads to make sure you don't have any problems.

Americanyouth: None of my pistons have wiring....do yours?

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Old 01-18-2005, 09:11 PM   #6
harvcharg
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Default will 2.5 dual over head cam heads fit a 2.5 single over head cam block?

I have a 97 legacy with a blown block. the engine has dual overhead cams. I have a 2.5 single over head cam block. will the dual over head cam head fit the single overhead cam block? will i have any interfearence with the valves hitting the top of the pistions because the domes are different? pleas let me know if this swap has ben done before and is there anything i need to know about the swap if it works. thanks Jim

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Old 01-19-2005, 02:31 AM   #7
Storm
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This should be a seperate thread.
Quote:
Originally Posted by harvcharg
I have a 97 legacy with a blown block. the engine has dual overhead cams. I have a 2.5 single over head cam block. will the dual over head cam head fit the single overhead cam block? will i have any interfearence with the valves hitting the top of the pistions because the domes are different? pleas let me know if this swap has ben done before and is there anything i need to know about the swap if it works. thanks Jim

harvcharg@aol.com
It will work. A few of us are doing just this. The CR goes up above 10:1 because the chamber is smaller on the DOHC. It's been calculated at around 11.4:1CR. It goes up if the heads have been resurfaced....like mine have. I have to run 94 octane to keep the ECU happy, and I am thinking of raising the idle to 850-900 to keep the ECU happy. Look here for more info:
http://forums.nasioc.com/forums/showthread.php?t=689708
and here:
http://forums.nasioc.com/forums/showthread.php?t=386462


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Old 01-19-2005, 06:16 AM   #8
harvcharg
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Default gas octain

I do have to send the heads out. I think i would get a ping with the cheap gas correct? also if i look the heads over and decide i dont have to send the heads out will i still have to run high octain gas? this car is for my mother and you know how gas prices are. i am also trying to save some money. By the way how mutch are those pistions you have for sale?
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Old 01-19-2005, 02:26 PM   #9
jamesfacts
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Quote:
Originally Posted by Storm
Americanyouth: None of my pistons have wiring....do yours?

Jay Storm


No, but my heads do. I thought he was going to swap heads instead of the shortblock for some reason.


To the OP: yes, you will probably need at least mid-grade gas.
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