View Full Version : Oversize Pistons

9th October 2000, 03:57
I had my engine rebuilt after it suffered major damage due to oil related problems. I have fitted 92.5 cosworth pistons (bought from JUN japan), these pistons were fitted using the clearence stated by the manufacturer. I have not had any pinging problems, or leaning problems, BUT i get this terrible piston slap noise, especially when the engine is cold, but even when it warms up the noise continues. I have been discussing this issue with another 22b member (Jan Shim), and he has also told me about this same problem in some Brunei WRX's, and some of them even suffered engine damage (AGAIN)....any out there uses Cosworth pistons, or has this problem....any solutions available?

Pete Croney
9th October 2000, 08:04
Many people had this problem with the early Cosworth pistons and they do not last long due to the rattling.

The latest design is much better.

Speak to Cosworth for more info.

9th October 2000, 11:41

Speak to Jan again.

I sent him an email yesterday about Cosworth Pistons.


9th October 2000, 16:16
I have already chatted with Jan, and he told me that a good solution maybe fitting new liners, has anybody tried this before?, also I really don't know what design of piston I have been sold, the only thing I can say is that I bought them in Feb. of this year.
And finally anybody know of a good source were I can get these linners?

Jon Hughes
9th October 2000, 16:22
What a good time for this topic to come up!

Iv'e just received some cosworth pistons in 92.5 oversize, however they came with two diffrent bore clearances....

Road use- 3.6405"/92.469mm/top limit
3.6400"/92.456mm/bottom limit

competition use- 3.6425"/92.520/top limit
3.6417"/92.500/bottom limit

I was assured that the road use clearance would be the best one to use?

Anyone care to shed some light..

Cheers Jon

[This message has been edited by Jon Hughes (edited 09 October 2000).]

9th October 2000, 17:15
John in other words, you were given the following advice:
For street use:
Bore size: 3.6405" @ suggested clearance of 0.013 mm
For racing use:
Bore size: 3.6425" @ suggested clearance of 6.02mm???? ( I think you got the top limit wrong can't be 98.52, when the piston is 92.5)....the suggested clearance should be of aprox. 0.02 mm

What happens when you use the racing specs. do you get a much greater wear on the bearings?

Jon Hughes
9th October 2000, 18:13
Sorry Carlos,iv'e put it right now DOH!

Those are the machning limits i.e to be between whats stated,not sure how much clearance between the piston and bore?.But as you can see one has 2 thou more than the other.

If you have already had yours rebored to the max then i would think the only way to get rid of the slap would be to put in new liners and rebore to a tighter clearance.


9th October 2000, 19:24

Ensure they are issue 3 pistons..or at least issue 2. Chances are they are Issue 2.. they came out about then.

Mic up each piston... and add the clearance onto that. Dont just go on the data on the box.

Rule of thumb used to be a thou for each inch of diameter.

So about 3.5 thou..

I run mine at 2.5'ish (allowing for tolerances...so basically 3). These are still knocky when cold.. but it soon goes away.


ps - Jon.. I offered... http://bbs.22b.com/ubb/frown.gif
pss - These above specs are my own.. and should be taken as such... dont blame me for any crap.. lol

10th October 2000, 00:17
Thanks Firefox,
but I don't understand were do you get the aprox 3.5" data from (3.5" is about 88.9 mm).
So at the end what should we do (the people that already are using these pistons), fit new linners and machine them to new specs?
I still don't understand why is it dangerous for the engine to use racing spec. bore size and clearance.....
BTW are you using the stock conrods Firefox?

10th October 2000, 00:20
Does anybody know if Subaru sells these linners?, if so what is the part no.?
Any special spec. for the linners.......

10th October 2000, 11:36
Hi there,

Too large clearance causes pistons slap (to be honest you always get piston slap - but its the magnitude thats the issue).

Piston slap is where the side skirt impacts into the bore..and allows the piston to twist.. this impact and twist causes excessive wear in the bores... and minimises the sealing effects of the rings...

eventually the pistons or liners will fail.

The life expectancy of an race engine can be as low as 1000 miles before a rebuild/new engine is required... so slap isnt so much a concern... but I think you would like you engine to last more than a few K miles ?

I have known of one compay (aria) who at one point suggest 10thou!! clearance for some of there pistons....

Yes Subaru sell new liners..

Check out issue 3 pistons. these are slightly larger than issue 2.. maybe they are large enough to minimise your clearances down to a reasonable level. If not, fit new liners, bored to a "better" clearance spec.

I said rule of thumb regarding stuff..and dont take any of my crap as gospel.

This will now be my last post on this issue.. thank you and good night.


11th October 2000, 04:26
Thanks J.

13th December 2000, 22:49
I know this is a past issue, but were can I get the "ISSUE 3 COSWORTH PISTONS" and for a reasonable price?
BTW do the cosworth pistons use the stock Subaru piston rings?


14th December 2000, 11:16

you should always use factory pison rings when installing pistons into factory cylinder liners. The ring material is chosen especially, to be compatible with the liner material... this ensures that there isn't excessive wear under normal use in either the ring or the liner.

If you do replace the liners, then get some rings which are compatible with the new liners... if they are Subaru liners, go with the normal rings, if they are other liners (for example, I believe that GGR do not use Subaru liners when re-linering blocks) then use rings compatible with that material (the liner will have been original equipment on something, so get the rings for that something....)

Hope this helps,


14th December 2000, 17:10
Anyone on the list has rebuilt their engines using new liners (either Subaru or other brand)....experiences??????

Still if anyone knows, who sells the "issue 3" cosworth pistons (the latest design), please let me know....

GGR..good service....but expensive.

Jan Shim
23rd December 2000, 20:40

I seem to be missing something here. I'm sure Carlos would want to protect his investment in the JUN 92.5mm pistons and I agree with your remarks about using Subaru or liner-compatible rings but do Subaru have rings for oversized pistons ?

24th December 2000, 16:04
Jan, Subaru already has 92.5mm pistons and rings............

3rd January 2001, 11:56

considering that .25 and .5mm oversize are "standard" subaru overboring sizes and are listed in the workshop manual, it would be reasonable to assume that they also have oversize pistons and therefore also rings, no? :)



[Edited coz Pat can't spell Jan, d'oh!]

[This message has been edited by pat (edited 03 January 2001).]

Jan Shim
4th January 2001, 23:12
Really ? Are the 92.5mm pistons the ones used on the 22B ? Does anyone have the Subaru part numbers ?

5th January 2001, 11:03

The 22B achieves the 2.2 litre capacity by boring alone, no stroke conversion, and increasing bore from 92.00mm to 92.50mm won't give 200cc :) The EJ22 engine uses 96.9mm pistons. The EJ25 uses 100mm pistons, but also a 79mm rather than 75mm crank, otherwise it would "only" be a 2.3 litre engine.

I do not have the part numbers for the pistons here, but maybe at home, however, they may well be different anyway; STi engines have dished pistons whereas "normal" engines seem to use flat top with valve recesses [according to "the bible" anyway :)]

I'm sure if you told a Subaru dealer that you need a .5 over piston for (say) a 1998 Impreza WRX STi Type R (Version 4) then they would be able to source one...



19th January 2001, 07:17
I think any piston with .013 (road ) and even more .02 mm (race ) will have piston slap even more so on Ej engine with it's short skirts , it's all about the silicon content and the claerance needed to allow for the expansion . you wouldn't wantany more than .005 mm before noise become to much . There is a guy in NZ that makes them for Possum Bourne
and doesn't use so much silicon so clearances can be kept down and keeped quiet .