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Forged pistons coding

WRX 2.0

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Got a quick question, don't know of you can answer it, it's about the legacy RS RA pistons. I'm trying to answer a question about whether or not they came factory as semi forged? I believe they did, but the old pistons that I took out are stamped ART L1. Do you know what that means? A guy told me all subaru semi forged pistons are stamped HF, which I thought maybe the case for the wrx engines, but perhaps the legacy had different stampings? There was a few years between them, a code change could be possible? What are your thoughts on this, and do you know what the stamp ART L1 means? Google yielded little results...


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Are they the old dog bone pistons, shaped like this?


Can you load a pic showing the underside of the piston and one showing the piston pins hole? A forged piston will generally be smooth and a cast will have cross hatched cast markings.

In this pic you can see the cross hatch cast markings on the underside:


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Haha! Sorry kamineko, amateur. Change a head gasket, swap a gearbox, upload a picture to the internet... look out! Yes, this guy is also saying about the hatching under the piston too, which is also no real sign, as yours don't have them, and a V5 STI set I sold didn't have them either! Confusing... I'm pretty much going off sales brochure, RS RA had semi forged from factory, these came from an RS RA, therfore these are semi forged. I could smash it to destruction under a pressure test, but seems a waste!

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whats that Mike? personally i have no idea what differentiates 'forged' from 'semi-forged' or even if its a real thing. i just know cast ≠ forged

care to link the sales brochure?

the cross hatch pattern is produced from the sand casting process... it indicates a cast part. youtube has some great production videos

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My knowledge extents are that cast pistons and semi-forged have the same initial manufacturing process, but then a semi forged is dipped in something providing a super strong surface on the top and sides of a piston, giving a forged like surface where wear and tear is prone. Forged is of course one big solid dense mass on top, sides, underneath, everywhere. A semi-forged provides faster expansion in bore than forged, lighter and quieter than fully forged, and yet they provide a stronger and more resilient piston than a traditional cast item. A very good middle ground and perfect for mass produced high performance road vehicles. Strength, lightweight, resilience, quietness, cheap...

As for hatching, yeah, who knows. Yours didn't have them, mine don't, yet some people are adamant that they do. Bit of an anomaly really. Maybe some factories buffed them out afterwards, who knows...

Edited by WRX 2.0
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Casting is where an alloy is heated to a liquid stage and poured into a mould where it cools and is then removed from the casting mould then machined. This is by far the cheapest process.

Forging is where an alloy solid billet is heated and then two dies are squeezed together at enormous pressure to form the initial shape which is then machined for final result

Semi forging is..

A billet is heated to its "Soft stage" then the soft material is injected into a cast die to form the initial shape which is then machined

Cast pistons are usually made of a "Cheaper" alloy (Lowest cost per unit)

Semi forged of a "Higher quality" alloy

Forged of a "Highest quality" alloy. (Highest cost per unit)

you will find especially in subarus pistons will be labeled or referred to as Hyperutectic.

This piston is die cast of a higher alloy when compared to cast pistons. But is of a lower quality material than "Forged" pistons.

so the term used frequently being "Semi forged" is incorrect. these pistons are "High temperature die cast"

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Thanks for the info. Semi forged is definitely a shorter term, but yes, doesn't fully label them correctly. So what would the code ART L1 specify, I have never seen a full run down list of piston codes like you would see with chassis codes. I guess it's purely a case of so many codes being used, it's hard to document every code used. Perhaps I should just label them on trademe as RS RA pistons and when someone asks if they're forged or what, I say no, they're "High temperature die cast". That should avoid confusion. They've lasted 25 years, they must be made of pretty good stuff.

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  • 2 years later...

something to factor in here is that the RS-RA is "hand crafted by sti" while the sti models up to ver2 were "tuned by sti". in regards to these early sti models the cars went from FHI factory to the sti factory then to the dealer, where as the ver3 onwards STI engine were built on the same line as the wrx engines etc


what this means is that the pistons are probably Rare A.F to find the actual real deal or info on what they were from factory.

even all the parts catalogues will show the wrong pistons without the correct FHI parts bulletin.


for comparison sake assume all pistons from said cars are from something else unless proof the engine is original rsra/ v1-2 sti and un opened etc etc

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11 hours ago, GC8E2DD said:

The other thing to factor in is that they are 30 year old thrashed to death pistons.

Agreed. They may be worth a bit if there is proof they are genuine RS-RA pistons and you find the right buyer, but otherwise they're fancy ashtrays.

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