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Old 10-03-2001, 07:40 PM   #26
kaos200
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the "B" theory makes sense. Smaller Piston, less wall between the valve reliefs, and the edge of the piston. If you guys remember that the blown pistons that have been pictured were fragged at that very spot. Also just below the first ring, the thickness plays a role here too.

the cylinder four issue is probably because of better air flow towards that cylinder in the intake manifold. Although I say probably and not surely because it seems there are some that blow that arent running this manifold?
Kian
praying his block lives a long and healthy life!
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Old 10-04-2001, 03:03 AM   #27
Kcz
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I looked all over for the piston letters but I couldn't find it, can someone post a pic so I know where to look?
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Old 10-04-2001, 03:06 AM   #28
Eric SS
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I don;t have a pick but it is right on the top of the block towards the front of the block right smack in the middle near the seam.

Eric
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Old 10-04-2001, 04:08 AM   #29
8Complex

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On the block or on the piston?

On the block - http://mwic.org/8complex/destruction/20.jpg

It's the square raised section that you can see on the top of the center of the block, just under the bracket that is resting on top of the motor.

On the piston - http://mwic.org/8complex/destruction/25.jpg

You can very faintly see it right at the top of where the piston crown cracked off, kinda down and slightly left of it. It's hard to see in that one... think I may have melted it down a bit.
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Old 10-04-2001, 12:22 PM   #30
STiShawn
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Does the 99 RS suffer from this piston problem as well? Or just the higher CR 00+ motors?
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Old 10-04-2001, 12:42 PM   #31
Jewbaru
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I haven't popped a piston, but mine are AA AA. I'll snap a pic of where to look, you can see it under the plenum, but takes some moving around.
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Old 10-04-2001, 01:50 PM   #32
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I took a pic, but it's really easy to find the letters. Stand on the drivers side of the car, and under the wire coming from the top of the alternator (it has a red cap) are the letters.
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Old 10-05-2001, 12:44 PM   #33
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Hmm... so no one with an "A" cylinder blown in #4 yet... interesting. I wonder what Shiv has for pistons in his car.. I know they're stock... but I wonder whether they're "A's" or "B's."

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Last edited by Fitz; 10-05-2001 at 01:31 PM.
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Old 10-06-2001, 11:03 PM   #34
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Sorry I can't help here...my old block is gone...
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Old 10-07-2001, 03:24 PM   #35
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Question hmmm

Well, isn't this interesting, I looked at my block today, and it says AB AB ... so I have 2 B pistons. I have a 2001 2.5RS coupe, 5spd. Did they ad another piston to even out the pressure? I am a little confused.
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Old 10-08-2001, 12:47 PM   #36
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^
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Old 10-08-2001, 01:35 PM   #37
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I too have AB AB..99 RS, new block though. Wish I new what my other one was....I may not have blown a piston, but my motor threw a rod bearing.
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Old 10-08-2001, 01:45 PM   #38
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Thats strange, it seems as if they just randomly used A and B pistons throughout 99-01. My friend has a 99 RS with ONE B , and my 2001 has 2 B's. I wonder if there is ANY rhyme or reason behind it and if its safe for me to run 5-8 psi of boost.

A B
B A
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Old 10-08-2001, 02:10 PM   #39
8Complex

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bigbird - Look at my post on the first page of this thread. I explained that it is just variances in manufacturing that causes them to be A or B cylinder sizes, which can only be filled by A or B pistons.

For some reason, I hardly think that .05mm difference will cause one crown between the valve relief and edge to burn up -that- much easier then another.
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Old 10-08-2001, 03:52 PM   #40
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Who said it was the 0.05mm difference?? Maybe it was a casting flaw? How about the way they vented the mold? We really won't know until people write back and say what they blew in regards to the kind of piston.

Just saying "I don't think it was this" or " I don't think it was that" without any data to back it up is pretty pointless in my opinion. So... we go looking for data.. Peoples..??.. how about those piston letters??

Thanks,
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Old 10-08-2001, 06:26 PM   #41
8Complex

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Fitz - If it was a casting flaw, then they would have rejected the pistons to begin with.

The A and B designation is different sizes of pistons needed by the different cylinders. Not all cylinders can be machined the same from castings as they will almost always be flaws in the walls before after casting/before machining. Just depends on how much they take off to get the cylinders in good shape as to what pistons they install in there (A/B). If the cylinders or pistons are too far out of spec, then they will be rejected (at least if Quality Control is paying attention, they will).

The .05mm I am talking about is from what I observed as a difference of .1mm between A and B pistons - .05mm being half the difference, which would be how much closer the edge of the crown was to the valve relief, which is where most of the pistons break/crack/melt/etc.
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Old 10-08-2001, 08:36 PM   #42
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Nah.. 8.. you and I are of the same opinion that this isn't a weakness in the piston but simply the weaker area of the piston. Every system has a weak link. Both the A and B piston's are breaking in the same spots (sad to say I have mucho experience with this.. 5 pistons and counting).

If we at Teradyne did our job 100% of the time our external PPM would be absolutely beautiful... I won't comment on what it actually is.

I'm not convinced it is the valve relief either. My EJ22T pistons have dished reliefs with valve reliefs as well. Damn man.. we need a Suabru engineer to take a look-see at this one.

Can't we just call 1-800-subaru manufacturing engineer?

Fitz
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Old 10-09-2001, 12:55 PM   #43
stimpy
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To get a little more information here... of all those that lost the #4 piston, who was using the stock fuel rail setup?

I'm sick right now, so I can't tell what piston designations I have, but I did redo my fuel rails. Boost comes in about 1 month, so time will tell if I lose a piston or not.

-Jon
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Old 10-09-2001, 02:02 PM   #44
Fitz
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I had the stock fuel rails with 550cc injectors...

Fitz
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Old 10-09-2001, 02:11 PM   #45
bigbird
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I am getting the LUDE SPEED kit , stage I or II , not sure, but I wonder if I would blow the pistons since I have 2 B's
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Old 10-09-2001, 03:32 PM   #46
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I'm using the stock rails as well.

bigbird - Not if it was tuned correctly. I'm running my kit low (LOW) boost and it's running great.
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