Piston FAQ: Read if you are thinking about buying them!

[nedalrabadi]

what about the cosworth alloy they use (CosAL 010) ?
how good is it ?

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[RalliSpec]

A few comments....

1) Piston clearance is measured at the skirt. So thermal expansion data does not tell the whole story with regards to why you need to run certain piston clearances. You have to take into account thermal conductivity between the crown and the skirt as well as piston design to determine how much expansion will occur at the skirt relative to the thermal load the piston is experiencing.

2) Cosworth's alloy is essentially 4032. Alloy 4032 is more desireable for an engine being built as a daily driver or for an endurance application because of its increased wear resistance and other properties. It will maintain a better ring seal and reduced oil consumption over a longer period of time under normal driving conditions. I would only use 2618 in a racing application where high detonation resistance and thermal conductivity are necessitated but where regular overhauls are to be expected.

3) It is only assumed that USDM STI pistons are hypereutectic although their problems seem to bear this out. Older JDM STI models with forged pistons (Ver.7 and older) used 4032 alloy pistons. There are some newer cast alloys that have been developed with higher tensile strengths and improved ductiliy...its possible that they are being employed on JDM STI models rather than hypereutectic alloys. Who knows. But the change from forged to cast pistons was probably necessary to meet stricter emissions standards.

4) I would be a little wary of using piston dome coatings where the piston was not designed with it in mind. Anything that interferes with the thermal conductivity through the piston is going to alter the expansion characteristics of the piston. Some of the better pistons out there are machined such that they have their best fit when up to temperature. The design process involves analyzing how the piston shape changes as heat is conducted through the piston. The question would be whether the advantages at the crown offset the potential disruption of the expansion process.

[arf80]

Sorry if this is a stupid question. Is there a difference in the design of the 4g63/4b11 pistons vs the ej25 piston (hypereutectic casting process aside) that allows them to hold more power? I know of other cars as well that come with factory cast pistons capable of holding lots of power. Ring land thickness maybe? I'm curious why a beefed up aftermarket cast piston with a thick ring land couldn't be made for sub 400 whp daily driven modified cars.

[2slofouru]

Is there any reason some manufacturers use different or no wrist pin offset?

Also, is there a compiled list of piston weights somewhere on our site?

[turbo01gc8]

just took apart an ej251 from 02 legacy L. had 129K on it and everything looked perfect when i took apart. got sti pistons from 09 new and wondering where else to go with the build. noticed pistons looked the same in wrist pin area.... is there a reason i need oem sti rods to go with these or will any work? i've seen that oem sti rods are tapered and 251 rods weren't tapered whats up?


chris

[gc6sti]

awesome write up, will use this in the future!

[jockeygolfs]

Originally Posted by turbo01gc8

just took apart an ej251 from 02 legacy L. had 129K on it and everything looked perfect when i took apart. got sti pistons from 09 new and wondering where else to go with the build. noticed pistons looked the same in wrist pin area.... is there a reason i need oem sti rods to go with these or will any work? i've seen that oem sti rods are tapered and 251 rods weren't tapered whats up?

chris

Don't do oem pistons if you are putting in pistons. Get forged. Mine are done as of yesterday at 340whp 360tq.

[camaro630hp]

nice post lost of info

[RSTiKing]

Originally Posted by RalliSpec

A few comments....

1) Piston clearance is measured at the skirt. So thermal expansion data does not tell the whole story with regards to why you need to run certain piston clearances. You have to take into account thermal conductivity between the crown and the skirt as well as piston design to determine how much expansion will occur at the skirt relative to the thermal load the piston is experiencing.

2) Cosworth's alloy is essentially 4032. Alloy 4032 is more desireable for an engine being built as a daily driver or for an endurance application because of its increased wear resistance and other properties. It will maintain a better ring seal and reduced oil consumption over a longer period of time under normal driving conditions. I would only use 2618 in a racing application where high detonation resistance and thermal conductivity are necessitated but where regular overhauls are to be expected.

3) It is only assumed that USDM STI pistons are hypereutectic although their problems seem to bear this out. Older JDM STI models with forged pistons (Ver.7 and older) used 4032 alloy pistons. There are some newer cast alloys that have been developed with higher tensile strengths and improved ductiliy...its possible that they are being employed on JDM STI models rather than hypereutectic alloys. Who knows. But the change from forged to cast pistons was probably necessary to meet stricter emissions standards.

4) I would be a little wary of using piston dome coatings where the piston was not designed with it in mind. Anything that interferes with the thermal conductivity through the piston is going to alter the expansion characteristics of the piston. Some of the better pistons out there are machined such that they have their best fit when up to temperature. The design process involves analyzing how the piston shape changes as heat is conducted through the piston. The question would be whether the advantages at the crown offset the potential disruption of the expansion process.

I almost gave up looking for info on the JDM ej20!, now that I read this.. My question to you is what kind of power can I expect from these pistons safely.. Or from ANY of the stock internals actually... Should I be safe at around 350whp? Cuz I'm buying one to drop in an RS.. Thanks for your expertise

[gflatt01]

Most timely for me, I have a "06" EJ 257 with 150K on the engine stand that is showing head gasket problems. Looks like I may venture a little deeper if the
cylinders show any issues at all. I need another 150K out of her.

[redas]

Amazing info, thank you! Adrew your info was also very interesting

[db_amn]

Originally Posted by RSTiKing

I almost gave up looking for info on the JDM ej20!, now that I read this.. My question to you is what kind of power can I expect from these pistons safely.. Or from ANY of the stock internals actually... Should I be safe at around 350whp? Cuz I'm buying one to drop in an RS.. Thanks for your expertise

My friend has a ver 7 with a super 16g turbo that dyno'd around 350-360 whp and he drives it daily. He has never had any problems with the motor, only small insignificant things like spark plugs, clutch etc. he has been told the stock engine can handle a lot more as well. If you are wanting to stay at that power, you should have no problems at all driving it daily with the stock internals on the jdm ej207.

[WRXBeatsTheBest]

Great thread and very informative. Thanks mate

[whitecreekauto]

What is the factory clearance on the cast 2.5 turbo engines? Looking around the net I am having a hard time finding exact numbers but it seems in the .0004-.0012 range? How are you guys measuring these pistons with coated skirts? Every time I have dealt with them in the past on non-subaru's the piston company basically says bore it to .0xx and run it which Im not a fan of. Seems all the guys doing these engines with factory pistons or aftermarket are going off skirt measurements which is tricky with the coating.

Also curious where the factory wants you to check the piston size at? 90* from pin and parallel or lower/higher on skirt?

Thanks!
Chad

[timarbeau]

which pistons come with longer skirts? skirt length has not been listed where i see pistons for sale. i want to increase stability, etc.

[Warney]

This is awesome that you

[jaybo769]

Hi guy's I am a newbie and right now I'm doing all the mods to get to 325-350 WHP, that's my target anyway and I should have result's withing 2-weeks,,I hope. But I want to purchase a LB and here diferent things from everyone, Don't get the cosworth buy my motor cause I use these components, then the next guy says don't buy Cosworth buy mine for all the oppposite reasons of the last guy!! CRAZY!!

[jaybo769]

Wiesco is better cause???? JE is better cause??? Manley is better cause??? CP is better cause???

[hillsboro]

Did you try reading the first few posts? With that information you can decide which is better. All your going to get on here is opinions not facts. At least with the proper information you can decide if the opinions have any merit. You'll get respect if you do research, you'll get flamed if you expect people to spoon feed you the information. Talk to a local shop and see what they use and why. Good luck with your build.

[Big Filthy]

Awesome work on that! Thanks.

[AlaskanSubieFan]

My auto class teacher has a 1000hp camaro. I need to find out which brand pistons, rods, crank, cams would i need. I'm thinking to CNC the heads so that a large turbo won't hurt. if someone can help me out, it would be nice

[Kcwiro]

Good and very useful write up thank you to the Author!

[oregonhawkeye]

I have a fully built 2.5 after the two stock ones wouldnt hold up to the vf39 for some reason. I used CP Pistons and they are great! (with built bottom end) now I run a GT35R at 26psi making around 480 wheel.

[threadoflife]

Great information thanks for all of the dialouge

I'm thankfully an owner of a 2003 Subaru WRX Bugeye unfortunately roughly a month ago timing belt went at 73000 miles, I am not the original owner so am unaware of weather this had been done and had been informed that typically they go out to 100000 miles.

Of course this is all heresay as I'm past all that and on to 1 month out of the vehicle local mechanic shop has rebuilt the top end, Heads have bent sent everything is backing new timing assembly, etc.

In the mean time the mechanic recammended replacing the Clutch and Flywheel so I ordered 2002-2005 Excedy Organic Stage 1 Kit & lightweight flywheel from Exceedy as well. Upon instal the mechanics have been puzzled with the engagment of the clutch assemble, everything is suposedly installed properly, but they had to modify the arm that acuates because it wasn't long enough to disengage the cluthch with the stock one.

The clutch pedal engages close to the floor without much play, does not feel normal in comparisson to the stock one, in addition im getting a check in engine light and a boost lag or inlet lag I can push the gas pedal and hear spooling but almost like bleed air pressure not engaging enough to transfer to full engine power. The code reflects a malfunction with the Tumble Generator Sensor does anyone have any

Does anyone have experience with these clutches or have any advice?

[CoreyStapleton]

very helpful!