semi forged pistons

[wildturkey]

I'm looking at using a set of these in my rebuild. I've heard they came out in the V3 sti. Can someone give me the low down on these? Are they also known as composite? I have been told that they don't require the same kind of tolerance as full forgies and can drop straight into the factory bore with a hone. Are they as strong as full forgies? what else should I know about them? cheers

[RSB]

Semi Forged are lighter than forged, the don't expand as fast but they do expand more.

Um, thats about as much as i know.

V3/4 and EJ20R had semi forged.

[BC5RA]

They are not as strong as Forged pistions ;D

It would depend on your bore, But after a hone they should drop in, and you won't be able to use your standard RS Heads (if you still have them)

[PhatRS]

 RSB said:

Semi Forged are lighter than forged, the don't expand as fast but they do expand more.

Um, thats about as much as i know.

V3/4 and EJ20R had semi forged.

Many a GTB/RSB has been pulled appart to reveal some cast jobbies so I wouldnt count on EJ20R pistons being forged everytime (subarus fickle build processes hard at work). As far as I know the "semi forged" STI pistons are forged but of a lower grade metal, hence not as strong.

[Treecrusher]

I don't know that there is such a thing as "semi forged" pistons as the term forged refers to the method used for creating the piston.

A forged piston is created from a piece of billet aluminium/silicon alloy where as a cast piston is created by pouring molten aluminium / silicon alloy into a cast.

The amount of expansion experienced during warm up has to do with the aluminium to silicon alloy ratio. More silicon adds strength but also makes it expand more during warm up. So a piston with a high silicon content will be rattly when cold. This can happen with a cast or forged piston.

The forging process makes a piston stronger because the grain deforms to follow the contour ensuring the grain is not broken throughout the piston.

EJ20K (v3/v4 sti), EJ20R ('96+ GTB) and EJ207 (v5+ sti) should have forged pistons from factory. EJ208's from BE abd BH legacys may also have forged pistons, but I am unsure.

[wildturkey]

ok I have 97 gtb heads to match them up to. So not as strong as fully forged pistons but not as rattly either?

[BC5RA]

Thats about right. This BC5 i have seen parked on kent terrace right?

[wildturkey]

u mean my bc5?......no i dont park there

[wildturkey]

The pistons I'm looking at using are out of a V3 sti............I have been told they are semi forged

[BC5RA]

Yeah yours, in the past not recently.

[wildturkey]

no i dont live in town and don't recall parking there ever

[BC5RA]

Ok, Have you bought this car recently?

[Treecrusher]

 wildturkey said:

The pistons I'm looking at using are out of a V3 sti............I have been told they are semi forged

v3 sti should have an EJ20K and should have forged pistons. If they are forged pistons they will look like this:

[wildturkey]

thanks tree........are they V3 forgies in ur pic? Can u post pics of the underside of them?

BC........nah haven't bought it recently had it about 18 months now.

[BC5RA]

Weird. I had lived in Wellington for 30 years, swear i used to see this car parked up near the Nissan dealership on Cambridge/Kent Tce. And then saw it advertised on Trademe awhile ago, had panel and paint done to it.

[wildturkey]

not mine BC .......come out to Lower Hutt tomorrow night if u want a closer look

[BC5RA]

I would but i live in auckland now :-[

Finding it hard up here.

[Treecrusher]

Yea they are v3 STI, but are the same as GTB '96/'97 with EJ20R

Picture of the bottom:

[wildturkey]

u see that pattern on the bottom?........I have been told that that means the pistons are cast........not forged? Maybe the person who told me that was all shite? He reckons forgies are smooth on the underside. Frakn confused now

How much would I expect to pay for a good second set of these V3 pistons?

[Treecrusher]

Not to expensive, easier to buy a couple of engines that have done a big end or cracked a piston. A couple of blown engines and you will probably have 6 good pistons to choose from and good heads as well.

Something to look out for to make your build a bit easier. Subaru had 2 different piston sizes "A" and "B". B size pistons are slightly larger than A size and therefore will allow more room for hone/bore. Best bet is to find a closed deck block with 4 A size bores and then find 4 B size pistons.

You tell the bore sizes from the outside of the engine by looking at a machined flat surface where the block halves meet under where the alternator would normally sit. Stamped into the flat surface will be four letters arranged like the cylinders. Each of the letters will be either an A or a B.

Heres a photo:

[Leighos]

Thats very interesting about the bore sizes. I'm in the middle of a rebuild on an V2 STi engine. I noticed the A and B stamps and wondered what it meant, the pistons are also marked L & R meaning they are handed also?

http://i635.photobucket.com/albums/uu74/leighosative/DSCF0007.jpg

[Guest]

Correct, offest gudgeon pins from memory on those L and R/H slugs

[Guest]

 Treecrusher said:

Yea they are v3 STI, but are the same as GTB '96/'97 with EJ20R

Picture of the bottom:

They are CAST, you can see the moulds they came out of !!

[Leighos]

So would it be normal to have different bore sizes from the factory? My #1 was B, #2 A, #3 A and #4 B.

[Guest]

Perfectly normal

JUST LIKE ME