I have misordered the pistons I need for my car (MY97 WRXRASTi GC8-058820 EJ20K>.

I have 4 x Grade A STI Forged Pistons but require 1 Grade B piston, just wondering if anyone knows where I can swap this or exchange it with anyone.

As stated my pistons are unused and brand new and I desperately need to swap only 1, for a Grade B, I am willing to pay a premium on top for the swap, as my supplier in Japan will not accept returns !!!

Just wondering if it would cause a problem if I used all Grade A pistons ??

 

the grade is just (very slightly) different sizes. You should be able to measure the piston clearances and see what the odd one is.

You might get away with it.

I've no idea on piston clearances for these engines, but plenty have rebuilt, I seem to remember about 2.5thou (0.1mm) for the cossie pistons, but it will differ from piston to piston.

Paul

 

I have been told by Subaru UK (indirectly) that the difference between the grades is 4 thou' of an inch, am getting the pistons measured to see if they will fit !!

I assume if you were to fit Cossie pistons you would fit 4 the same and they are not different sizes or grades ??

Cheers

 

4 thou difference is a lot!

SO I would expect one of the bores to be larger than the rest (piston pick up cleaned in bore?).

If you get a full set of B grade, it would be possible to bore and hone the other bores to suit

Paul

 

But Surely if you purchase 4 x Cosworth Pistons - will they all have be different sizes or do you need to bore them to fit ??

 

It is not 4 thou. It is I think, 0.01mm
I put second hand STI pistons into my legacy, and I did not get matching A's and B's. I just tried each one into the bore to see which fitted best, and all is well. No probs.
The difference will be so small, I wouldnt worry anyway. Not even sure why they bother making them different sizes, for all the difference there is.

 

That sounds like the answer I was looking for but it just suprising that if you were to replace 4 STI pistons with Cosworth pistons, surely they would be the same size hence not making sense why the original STI pistons are Grade A and B and slightly different sizes !!

 

when you replace with cosworth pistons, it is normally due to an engine failure which often causes pistons to pick up on bore,

this means you get the whole lot over bored and rehoned anyway, and buy 4 oversized cosworth pistons.

they do come in standard size, but for security you buy them with a 0.25 or 0.5mm overbore

 

Only original pistons are sized A and B. The cosworth pistons are all the same size. A friend bought a set of Cosworth pistons about a year ago, and at that time, only oversize were available, partly due to the A/B thing. I think they do now make them in std bore size, which must be one size fits all, so to speak.
There is a little more info on the Grahame Goode racing site.

 

Thanks for all your help - but I'm still left a little confused - should I fit all 4 Grade A Pistons or scrap 2 and buy two more Grade B ?

Just want to make sure that it doesn';t adversley effect the engine !!

 

I would sell the lot to someone who can use the lot.

and take a hit on the price, then buy 4 cosworth, omega, or je type pistons, in an oversize and have a rebore to fit them.

They are better pistons too!

 

It is stamped on the top of your block, towards the front, along the centreline of the block where it splits, where the A's and B's should be, and obviously how many.