Name that Piston
08-15-2013, 07:31 PM
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Name that Piston Plan on doing a rebuild to my 99 Z28 due to major blow by. I've only owner this car for a few months and don't have any background info. I just pulled the head on one side and and my neighbor took one look and said that the pistons weren't stock? He says that the stock pistons didn't have valve reliefs? Thoughts?
I did a search for that manufacture and had 1100 hits! 
08-15-2013, 09:42 PM
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This motor was a previous boost set-up. I'm planning on doing a rebuild and returning to a NA motor. Will re ringing these pistons be a cost saving option? What I can find is that these pistons are -5cc which I think would increase compression over stock pistons.
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08-15-2013, 11:00 PM
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So to get back to stock compression using these pistons I could
A) mill the heads to reduce cc of combustion chamber ( 317's )
B) advance the timing of cam
C) go with a flat top piston with no valve reliefs
+ thinner head gaskets
This motor is a dog compared to my stock 99 a4 I don't want t throw a bunch of money at this setup if it would be cheaper to get a new short block.
A) mill the heads to reduce cc of combustion chamber ( 317's )
B) advance the timing of cam
C) go with a flat top piston with no valve reliefs
+ thinner head gaskets
This motor is a dog compared to my stock 99 a4 I don't want t throw a bunch of money at this setup if it would be cheaper to get a new short block.
08-16-2013, 02:05 AM
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Those are flat top pistons they are not a "low compression" piston. They are what some might call a street forging...they are not a race piston.
There are two different types of forged piston materials 2618 and 4032. Speed pro calls theirs VMS-75... Which is like a 4032. 11% silicon. These are meant for performance street applications because the don't expand as much, are more durable and use tighter clearances and are therefore quieter. Less cold start "slap". But are less desirable in a boosted or nitrous race applications because they are more brittle. A 2618 forging contains little or no silicon so the are more mailable and less likely to break, but require greater clearance because they expand more so are prone to slap. Which people don't like in a street car...therefore generally used only in race applications.
The moral to the story is you have a good set of street forged pistons (that run about $600 https://sdparts.com/details/sealed-power/l2621f ) and 5cc of relief is not going to make a hill of beans in terms of power loss (might even be beneficial if you want to go big on the cam) I'd throw some rings at them and let it eat.
There are two different types of forged piston materials 2618 and 4032. Speed pro calls theirs VMS-75... Which is like a 4032. 11% silicon. These are meant for performance street applications because the don't expand as much, are more durable and use tighter clearances and are therefore quieter. Less cold start "slap". But are less desirable in a boosted or nitrous race applications because they are more brittle. A 2618 forging contains little or no silicon so the are more mailable and less likely to break, but require greater clearance because they expand more so are prone to slap. Which people don't like in a street car...therefore generally used only in race applications.
The moral to the story is you have a good set of street forged pistons (that run about $600 https://sdparts.com/details/sealed-power/l2621f ) and 5cc of relief is not going to make a hill of beans in terms of power loss (might even be beneficial if you want to go big on the cam) I'd throw some rings at them and let it eat.
08-16-2013, 06:36 AM
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Great info.... Thanks guys! I'm going to pull these pistons and check for damage and most likely just re ring them. Is there calculator for ptv clearance or does this have to physically be measured?
08-18-2013, 04:05 AM
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The best and most acurate way is to physicaly measure PTV some people have a rediculous algebreic equation to guestimate at what the PTV is but since you didnt build the motor the last time you have no way of knowing actual deck height and if the heads are milled however many thousandth its just a must to physicaly measure PTV especialy in this situation. also with the compression issue the 2 valve relief technicaly lowers compression because it adds to chamber volume but once again not knowing what your actual specs are the builder may have done that for adequate PTV or to deshroud the valves no way of actually knowing. if the 317's havent been milled then you have a low static compression motor and would benefit greatly from a set of 243's to bump your compression you would notice good torque gains and better throttle response.
08-18-2013, 05:20 PM
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Baring any major scratches or heavy wear is it an otorion to just re ring these forged pistons? I was planning on getting an entire rotating assembly until I saw these after market postons when i pulled the heads! Would be awesome to have an extra 700-800 bucks to put toward new heads!
