Triple X

iTrader: (1)

Thinking of getting a set of pistons to put in my STI 5 block. I can see that the compression is as standard but what is the piston head design like?
Is it a square dish type like the standard forged ones im replacing or something different ie with a dome in the middle and valve reccesses?


Ta, John

john banks

I had some Wisecos at 0.0035-0.0040" clearance, but others fared rather better with them than I, so maybe the pistons can't be blamed. Impreza GT TDi without the economy I'm afraid. Knocklink was pegged on big red on cruise

What about using the stock pistons with a generous bore clearance?

Triple X

iTrader: (1)

Bit confused really, ive heard that as standard STi pistons run less than 1 thou clearance, had the bores checked today and found there to be 0.5-1 thou wear on the thrust side of the bore. I noticed that when cold there was some slap so surely there going to be a lot of slap with say 2-3 thou clearance? And why can Subaru pistons run with so little clearance when compared to aftermarket pistons?

David_Wallis

I run wiseco pistons with 4thou clearances.. i have had the pistons machined to a round dish in order to lower my CR though.

David

Triple X

iTrader: (1)

Is that the clearance that Wiseco reccommend?

john banks

Yes. Subaru pistons do seem to be at risk of picking up though. I don't know if you run a Subaru forged piston at the widest bore clearance quoted in the service manual whether they would be quieter than an aftermarket forged item. However, the widest may still not be enough to stop them picking up?

RCMS seem to be able to run the Omegas at about 0.002" IIRC, and their engines sound fine.

Pavlo

The old rule of thumb of 4 thou per inch of bore is based on iron blocks.

With 100mm iron bore at 60ºC above initial, thermal expansion is .063mm or 2.5 thou
With 100mm Al 2618-t61 piston skirt (coolest point) at 70ºC above initial, thermal expansion is 0.1666mm or 6.6 thou.

With 4 thou clearance at 20ºC the running clearance would be effectively zero, if you assume block goes up another 10ºC the clearance becomes 0.32 thou.

If we assume a weighted average expansion for iron liner in Al block, you will get roughly 164% the expansion compared to an all iron block. This would increase the running clearance to 2.34 thou!!

Paul

Andy.F

Why does the iron liner not just expand at iron expansion rates then ? Does the alloy block actually pull it out further and will this vary depending on liner thickness ?
Serious question.

Andy

R19KET

Shouldn't that be 1 thou per inch ? 4 thou for a 100mm piston ?

Mark.

Pavlo

yes sorry, 1 thou per inch, or 4 thou for a 100mm bore

Andy.F

Mark/Paul

Views on the thermal expansion of the liners ?

Andy

Pavlo

It would depend on the state both the block and liner. Since the block is cast around the liner, one would expect the liner to be heated at the point the block is cast.

The other effect will be the change in temperature of the cylinder wall for a given water temp. The potentially higher running temp of the iron may offset any significant changes in the expansion of the 2

Andy.F

So do you consider that the ideal clearance would be subject to liner thickness and block construction ie closed/open deck ?
This gives us quite a number of different combinations throughout the years and sizes, all of which have a different ideal clearance, not to mention the different piston materials and constructions
Little wonder then that whilst some rattle loose, others sieze up !

Andy

Pavlo

I think we have to consider that possiblity. With the EJ257 liner only 1.5mm thick or so, it must surely lead to potential differences.

We certainly wouldn't expect a nikasil coating to prevent the expansion of an alumnium cylinder.

Paul

Triple X

iTrader: (1)

So do Omega pistons not expand as much when heated compared to the likes of Wiseco, since you can run with half the clearance of whats been suggested for the Wiseco's?

Pavlo

They should expand to a similar extent, as they are made from the same Aluminium alloy as the other forged pistons (2618 - T61)

R19KET

Wiseco would suggest 3 thou on a 92.5mm piston.

Mark.

Triple X

iTrader: (1)

Been looking on www.importperformanceparts.net at their "Rod + Piston Combos" and have to admit im tempted to go for the wiseco pistons with pauter rods for $1125.
I just need to understand the clearance issue a little more as i dont want a rattly engine or one that nips the bore!

Andy.F

Even Subaru have issues with that one !! with 97/98 cast pistons slapping around and later Sti's forged lumps picking up in the bore.

Regards the combo you mention, there is a cheaper option again using Eagle rods and CP pistons. Should be around $800 for 4 of each

Andy

Tone Loc

iTrader: (1)

Andy, do you know of anybody that has bought this combo and has had results. My memory is terrible but i think i remember a thread from way back that showed pics of the rods and pistons that a guy had imported. Price is very good

Tony.

Andy.F

I know the guy who bought them last year, same guy I bought my Type R from, he never used them though and sold them on to someone else. There is another car in Scotland running the same combo but he is not on the BBS anymore and I have not heard good or bad.
I did have a good look at them and the rods certainly looked very nice, pistons looked like Sti5/6 forged replicas, not sure they were CP though ?

Andy

Bats-Wrx

iTrader: (1)

Caz1562 i think thats his user name. Arias pistons and eagle rods. There was a link a while go with pics etc.

Andy.F

Thats the one

Tone Loc

iTrader: (1)

Yep.. thats the one . Just keeping my options open as i have a feeling my engine is on borrowed time.

Sorry to take the thread off in a tangent... Andy you said your knocklink picked up your ring land was knackered a while back. How did this show up, activity at cruise etc?????

Tony.

Andy.F

Yea, that was on my last Ra after a heavy dose of det due to a mismatched scoobyecu chip.
It would get a bit sparkly at very light load 1500 ish rpm, like 10mph in 2nd gear type load, then it would go quiet as I loaded it up a bit.
Also noticed more blowby when the oil filler cap was removed, compression dropped by approx 50psi in offending cylinder. If you catch it quick there's no further damage done.

Andy

Tone Loc

iTrader: (1)

Cheers Andy, many thanks for the info. Doesn't sound quite like mine. Hope it's something vibrating about lose in the engine bay.... hopefully .

Now back to Wiseco pistons

Tony.

David_Wallis

Which was nothing to do with me!!

David

Pavlo

remote mapping experiments from 500 miles away...

Triple X

iTrader: (1)

Andy, is that on the same site? I see it says "Custom Pistons any diameter and compression" but thought that was regarding the pistons in the above boxes ie Wiseco, Ross?

CustomScoobyIOM

Caz1562 (Derek) is on the SIDC and selling the car at the moment.

Jon.