EJ20G v2 STI RA - PTW Clearance

[Leighos]

I\'ve got a copy of the factory service manual for the early EJ20G and it states a standard piston to wall clearance of 0.0004 - 0.0012 with a 0.0024 limit.

Assuming this data is correct for the typical cast pistons, does anyone know what (if any) difference there would be for the factory forged pistons that are used in the STi RA EJ20G?

[Rosssub]

 Leighos said:

I\'ve got a copy of the factory service manual for the early EJ20G and it states a standard piston to wall clearance of 0.0004 - 0.0012 with a 0.0024 limit.

Assuming this data is correct for the typical cast pistons, does anyone know what (if any) difference there would be for the factory forged pistons that are used in the STi RA EJ20G?

Those Imperial Figures look about identical to my EJ208 specs. PTW Limit 0.05mm @ 20c.

I would guess most clearance Specs stay the same through most models???

Are you looking at Upgrading Pistons? Check things like:

Piston Dish Size/Pot Size - Compression Ratio/HG Thickness

Piston Overall Width - Will it fit/Bore Size/PTW/Do I need a Rebore

Clearance To Deck - How tall the Pistons are/Clearance at TDC

Also wouldn\'t the STI Engine be EJ20r or EJ20k?

[Timmah]

 Rosssub']

[quote name='Leighos said:

I\'ve got a copy of the factory service manual for the early EJ20G and it states a standard piston to wall clearance of 0.0004 - 0.0012 with a 0.0024 limit.

Assuming this data is correct for the typical cast pistons, does anyone know what (if any) difference there would be for the factory forged pistons that are used in the STi RA EJ20G?

/quote]

Those Imperial Figures look about identical to my EJ208 specs. PTW Limit 0.05mm @ 20c.

I would guess most clearance Specs stay the same through most models???

Are you looking at Upgrading Pistons? Check things like:

Piston Dish Size/Pot Size - Compression Ratio/HG Thickness

Piston Overall Width - Will it fit/Bore Size/PTW/Do I need a Rebore

Clearance To Deck - How tall the Pistons are/Clearance at TDC

Also wouldn\'t the STI Engine be EJ20r or EJ20k?

20K = V3/4 STi or V3/4 non-STi Sedan.

20r= early TT or Legacy turbo engine?

[Leighos]

 Timmah']

[quote name='Leighos said:

I\'ve got a copy of the factory service manual for the early EJ20G and it states a standard piston to wall clearance of 0.0004 - 0.0012 with a 0.0024 limit.

Assuming this data is correct for the typical cast pistons, does anyone know what (if any) difference there would be for the factory forged pistons that are used in the STi RA EJ20G?

/quote]

Those Imperial Figures look about identical to my EJ208 specs. PTW Limit 0.05mm @ 20c.

I would guess most clearance Specs stay the same through most models???

Are you looking at Upgrading Pistons? Check things like:

Piston Dish Size/Pot Size - Compression Ratio/HG Thickness

Piston Overall Width - Will it fit/Bore Size/PTW/Do I need a Rebore

Clearance To Deck - How tall the Pistons are/Clearance at TDC

Also wouldn\'t the STI Engine be EJ20r or EJ20k?

20K = V3/4 STi or V3/4 non-STi Sedan.

20r= early TT or Legacy turbo engine?

Yeah early EJ20G (this one a 1996) pulled from an original GC8 v2 STi RA, the one with TD05 and 5th injector from factory. No CDB though must have been just on the change over on the production line.

Not looking at upgrading pistons, just a factory spec overhaul.

Bores are in within spec limits, its just that pistons are WELL under size compared to spec in the FSM, which lead me to think the values stated in the manual are for cast pistons running a much tighter PTW than forged pistons do.

And of course they\'re not excessively worn, the machine tool marks are still clear and present on the skirts.

I\'m getting between .0045" and .0055" PTW with these pistons (sorry for the imp, measured at an aircraft engine overhaul shop) that\'s about .110mm to .140mm

So my problem is I cant find any values for service limits on these factory fitted forged pistons.

What I\'ve got here for PTW wouldn\'t be uncommon for an aftermarket forged piston so I\'m lead to believe looking at specs on the popular brands CP, Wiseco etc... but these are an unknown alloy (to me anyway ) so can\'t really compare and will certainly have a different expansion rate.

[Rosssub]

Sweet.

Being that most Non Forged PTW clearances seem to be the same. Maybe check your Huge PTW clearances against a known Forged PTW Spec?

Maybe V5/6 STI? Or EG20R?

[kamineko]

 Leighos said:

..

Yeah early EJ20G (this one a 1996) pulled from an original GC8 v2 STi RA, the one with TD05 and 5th injector from factory. No CDB though must have been just on the change over on the production line.

..

interesting. do your pistons look like this? do you have a chassis number from the car?

[Leighos]

That\'s them, HF2 embossed under the crown.

Car is long gone, but I may have written it down somewhere - I\'ll take a look through my old notes.

[Leighos]

 Rosssub said:

Sweet.

Being that most Non Forged PTW clearances seem to be the same. Maybe check your Huge PTW clearances against a known Forged PTW Spec?

Maybe V5/6 STI? Or EG20R?

Good call, I\'ll have a hunt around for something on the EJ20K as that would be closer - pretty sure the 20R are cast, quick thread search seems to have thrashed that debate to death, although you never can be to sure what gets used up on a mass production line

[Rosssub]

 kamineko']

[quote name='Leighos said:

..

Yeah early EJ20G (this one a 1996) pulled from an original GC8 v2 STi RA, the one with TD05 and 5th injector from factory. No CDB though must have been just on the change over on the production line.

..

/quote]

interesting. do your pistons look like this? do you have a chassis number from the car?

Those are Forged Alloy aren\'t they?

This is my Cast Alloy, covered in Mold/Cast Marks and rough inside:

[Rosssub]

 Leighos']

[quote name='Rosssub said:

Sweet.

Being that most Non Forged PTW clearances seem to be the same. Maybe check your Huge PTW clearances against a known Forged PTW Spec?

Maybe V5/6 STI? Or EG20R?

/quote]

Good call, I\'ll have a hunt around for something on the EJ20K as that would be closer - pretty sure the 20R are cast, quick thread search seems to have thrashed that debate to death, although you never can be to sure what gets used up on a mass production line

Also are you checking your PTW (piston to wall) clearances:

Top of Piston Skirt?

Bottom of Piston Skirt?

Top of Bore?

Bottom of Bore?

My Piston Head is 91.61 mm wide at the Very Top. But 92.04 at the Lower Skirt? Giving very large PTW Gap at Upper Piston Head.

[Leighos]

 Rosssub']

[quote name='Rosssub said:

Sweet.

Being that most Non Forged PTW clearances seem to be the same. Maybe check your Huge PTW clearances against a known Forged PTW Spec?

Maybe V5/6 STI? Or EG20R?

/quote]

Good call, I\'ll have a hunt around for something on the EJ20K as that would be closer - pretty sure the 20R are cast, quick thread search seems to have thrashed that debate to death, although you never can be to sure what gets used up on a mass production line

Also are you checking your PTW (piston to wall) clearances:

Top of Piston Skirt?

Bottom of Piston Skirt?

Top of Bore?

Bottom of Bore?

My Piston Head is 91.61 mm wide at the Very Top. But 92.04 at the Lower Skirt? Giving very large PTW Gap at Upper Piston Head.

Measuring bottom of piston skirt (approx. 12mm up from the bottom edge and 90° to the gudgeon) with micrometer, this is where the standard measurement is taken according to the (aero) engine shop I frequent.

Like this...

[Rosssub]

Spot on.

So you must have heavily worn Bores as well? How do the Bore Dimensions look compared to Spec Limits?

What about Ring Gap/Ring Width?

[Leighos]

 Rosssub said:

Spot on.

So you must have heavily worn Bores as well? How do the Bore Dimensions look compared to Spec Limits?

What about Ring Gap/Ring Width?

Bores are surprisingly good, measure an average of 92.010 so still within factory spec.

[Rosssub]

 Leighos']

[quote name='Rosssub said:

Spot on.

So you must have heavily worn Bores as well? How do the Bore Dimensions look compared to Spec Limits?

What about Ring Gap/Ring Width?

/quote]

Bores are surprisingly good, measure an average of 92.010 so still within factory spec.

Bore Spec, Wear Limit, is 92.015mm.

Your at 92.01, so only 0.005 away from being over.

With PTW (piston to wall) Limits at 0.05. Your at 0.140mm on your worst Cylinder.

Pistons and Rings, come Standard Size/0.025mm Oversized/0.05mm Oversized. With standard Piston Part Numbers for each size, for ordering.

My new EJ207 is currently being Re-Bored 0.025mm Oversized. So will need new 0.025mm Over Sized Pistons and Rings to suit. So I know I can Re-Bore this Block once more, using OEM Pistons. Providing Bore Wall Thicknesses, don\'t go Beyond Spec Minimum.

AFAIK OEM Cast Alloy Pistons are about $65 each, Forged Alloy about $95 each. New through Subaru, but I haven\'t ordered any yet to confirm it.

So you may as well get your Block/Cylinders Bored Out, to 0.025mm Over Normal Factory Spec. Then Order 0.025mm Oversized Forged Alloy Pistons and Rings to suit.