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Old 04-23-2008, 01:00 PM   #1
radrider
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Default Pistons for STI hybrid motor? -and- How to acheive correct bearing clearances??

I need to have an EJ257 shortblock rebuilt. The cylinders will be overbored for new pistons and the crank journals will be micro-polished if necessary.

There seems to be a lot of different opinions on bearing clearances. I am planning on sticking to the middle of the stock clearances unless someone provides good reasoning to go higher or lower. Can someone help me define the proper method to achieve the correct bearing clearances when rebuilding a shortblock??

As far as pistons go, I like the CP's since they have a lower compression than most of the other aftermarket pistons. I've heard good things about the Mahle pistons and they are light, but the compression would be raised on the already high compression hybrid. Any thoughts?

What pistons have you used on your WRX/STI hybrid and why?
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Old 04-23-2008, 02:39 PM   #2
jbwrx
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CPs are great pistons. I decided to go with wisecos for my wrx hybrid build mainly because of cost. I got a hell of a deal on them and they are very well proven. They are the pistons my tuner recommended also.
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Old 04-23-2008, 06:01 PM   #3
turboAZ2
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Quote:
Originally Posted by radrider View Post
I need to have an EJ257 shortblock rebuilt. The cylinders will be overbored for new pistons and the crank journals will be micro-polished if necessary.

There seems to be a lot of different opinions on bearing clearances. I am planning on sticking to the middle of the stock clearances unless someone provides good reasoning to go higher or lower. Can someone help me define the proper method to achieve the correct bearing clearances when rebuilding a shortblock??

As far as pistons go, I like the CP's since they have a lower compression than most of the other aftermarket pistons. I've heard good things about the Mahle pistons and they are light, but the compression would be raised on the already high compression hybrid. Any thoughts?

What pistons have you used on your WRX/STI hybrid and why?

I'm currently working with Probe to develop a piston for this specific application.
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Old 04-23-2008, 06:36 PM   #4
charliew
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There are plenty of good name brand pistons already available that work great. Anytime a new manufacturer comes along thats great. Personally I'm happy with the ones already in place but the more choices we have the better.

Last edited by charliew; 04-26-2008 at 12:12 AM.
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Old 04-23-2008, 11:07 PM   #5
jaxscuby
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plasti-guage..spelling?
looks like fishing line.
usually comes in a long paper sleeve
has width measurements on the outside

place a piece as long as rod bearing.
assemble rod with rod bearing onto crank torque to specs.
remove rod cap and use supplied plasti-guauge measurements
against the deform/flattened piece of plasti-guage.
this should be your connecting rod bearing clearance to crank shaft.

i have installed mahle, cp, je, wisco.
the machine shop that i use likes the cp
over the rest. he has more experience than
i ever would.

make sure you get the cylinders honed for pistions.
and install correct piston in its proper cylinder.

also pay attention to ring gaps. follow manufacture
recommendations.
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Old 04-24-2008, 01:03 AM   #6
TechWorksEngInc
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Quote:
Originally Posted by radrider View Post
I need to have an EJ257 shortblock rebuilt. The cylinders will be overbored for new pistons and the crank journals will be micro-polished if necessary.

There seems to be a lot of different opinions on bearing clearances. I am planning on sticking to the middle of the stock clearances unless someone provides good reasoning to go higher or lower. Can someone help me define the proper method to achieve the correct bearing clearances when rebuilding a shortblock??

As far as pistons go, I like the CP's since they have a lower compression than most of the other aftermarket pistons. I've heard good things about the Mahle pistons and they are light, but the compression would be raised on the already high compression hybrid. Any thoughts?

What pistons have you used on your WRX/STI hybrid and why?
The proper method of measuring bearing clearances is to measure the journal on the crank with an accurate micrometer, then with an equally accurate bore gauge ( at least 0.0001" or 1 micrometer 0.001mm) zero the bore gauge. Insert the bearing shells into the rods and into the engine cases and torque to spec. Measure each con rod and main journal bore with the bore gauge zeroed for the appropriate journal.
As for pistons, I may be rather biased, lol, but what you want is a piston that has proper thermal expansion characteristics (eg becomes nice and round when at operating temps) and has optimal quench designed crowns. On the Subarus, piston pin offset will reduce alot of the noise and skirt scuffing found on economical designs such as CP.
Take a look at the perimeter of your WRX combustion chamber, then look at the quench pads ( this is the flat area on the piston perimeter) on the piston shown.

You will notice that it is virtually a mirror image of each other. Proper quench pad area and clearance make an engine a LOT less detonation prone and requires less ignition timing due to faster combustion chamber characteristics. I may know where you can get a set of these.

OK, that was a shameless plug. lol
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Old 04-25-2008, 12:41 PM   #7
radrider
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So do I buy standard size bearings to start out with and then go back and get the proper sized bearing shells after everything has been measured? Will Subaru parts take back a bearing that was installed just to take measurements?
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Old 04-25-2008, 12:54 PM   #8
xilles
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cp stocks eagle stock cosworth bearings stock. building it right now as we speak. will post a dyno sheet
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Old 04-25-2008, 01:38 PM   #9
slowmike
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Does anyone have an off the shelf piston for hybrids that won't raise the compression higher that 8.2:1? I'm looking at having to call CP for a custom solution, Crawford doesn't do this anymore.
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Old 04-25-2008, 05:00 PM   #10
kellygnsd
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Wiseco will make you a piston with custon CR, all you have to do is call
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Old 04-25-2008, 05:08 PM   #11
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any engine builder should be willing to do a custom piston for you...as will most piston companies...you will see that the piston companies don't design in quench pad to their pistons. IMHO it would be worth having someone do the order for you. Not that I know anyone who can do that



I do agree 100% with techworks engineering's post!! We think too much alike.
I just like JE slugs more than wiseco's
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Old 04-26-2008, 10:44 AM   #12
slowmike
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Quote:
Originally Posted by TechWorksEngInc View Post
The proper method of measuring bearing clearances is to measure the journal on the crank with an accurate micrometer, then with an equally accurate bore gauge ( at least 0.0001" or 1 micrometer 0.001mm) zero the bore gauge. Insert the bearing shells into the rods and into the engine cases and torque to spec. Measure each con rod and main journal bore with the bore gauge zeroed for the appropriate journal.
As for pistons, I may be rather biased, lol, but what you want is a piston that has proper thermal expansion characteristics (eg becomes nice and round when at operating temps) and has optimal quench designed crowns. On the Subarus, piston pin offset will reduce alot of the noise and skirt scuffing found on economical designs such as CP.
Take a look at the perimeter of your WRX combustion chamber, then look at the quench pads ( this is the flat area on the piston perimeter) on the piston shown.

You will notice that it is virtually a mirror image of each other. Proper quench pad area and clearance make an engine a LOT less detonation prone and requires less ignition timing due to faster combustion chamber characteristics. I may know where you can get a set of these.

OK, that was a shameless plug. lol
Does the Wiseco piston have an offset pin?
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Old 04-26-2008, 11:04 AM   #13
kellygnsd
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Quote:
Originally Posted by slowmike View Post
Does the Wiseco piston have an offset pin?
Yes they do.
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Old 04-28-2008, 03:48 PM   #14
slowmike
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Quote:
Originally Posted by TechWorksEngInc View Post
The proper method of measuring bearing clearances is to measure the journal on the crank with an accurate micrometer, then with an equally accurate bore gauge ( at least 0.0001" or 1 micrometer 0.001mm) zero the bore gauge. Insert the bearing shells into the rods and into the engine cases and torque to spec. Measure each con rod and main journal bore with the bore gauge zeroed for the appropriate journal.
As for pistons, I may be rather biased, lol, but what you want is a piston that has proper thermal expansion characteristics (eg becomes nice and round when at operating temps) and has optimal quench designed crowns. On the Subarus, piston pin offset will reduce alot of the noise and skirt scuffing found on economical designs such as CP.
Take a look at the perimeter of your WRX combustion chamber, then look at the quench pads ( this is the flat area on the piston perimeter) on the piston shown.

You will notice that it is virtually a mirror image of each other. Proper quench pad area and clearance make an engine a LOT less detonation prone and requires less ignition timing due to faster combustion chamber characteristics. I may know where you can get a set of these.

OK, that was a shameless plug. lol
What will the static CR be using this piston with an STI head gasket?
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