Clarkie GT

My 95 wrx wagon developed a lose of compression on cylinder 4 (rear right, by turbo) and oil in the breather system etc... so guessed that cyliner had ran lean as its at end of fuel rail, so pulled the engine out n took the heads off and was faced with this...





is this the result of running lean/pre-ignition? i was expecting the piston to be melted and out of shape, there's onnly a tiny hole that i can see, here's the piston next to it...



what do you guys think?

robimportwagon

if you pull no 4 piston (a) out i think that looking at it you will find that the gap at the bottom may actually be where the piston has melted and i do know that they also break between the rings. in my world we all call std scoob pistons chocolate pistons.

Clarkie GT

ha, cheers rob... ive herd that expresion a number of times, no worries then, ive brought a 2nd had set of rods and pistons from ebay out a 97 sti ra as these are alot stronger i beleive. so gonna replace the main/big ends while im at it

how can u tell the difference between standard wrx/turbo pistons and sti ones? would like to confirm they're sti ones before i fit them.

Splitpin

Clarkie, slight issue with your description/problem: The cylinder nearest the turbo is #3, not #4. If it's #3 that's gone, then that is the one at the end of the fuel rail so leaning is one possible explanation, but that is far from guaranteed. Probably the most important consideration prior to spending money fixing this is to determine as accurately as you can what caused the problem, or you'll just end up repeating it on an engine you've just spent a lot of money rebuilding. And, if you're going to fix it, especially if you're going to do the bottom end at the same time, it's not worth bodging it.

The pictures show damage that is fairly typical for cast pistons - you'll invariably find that the chip that you can see out of the bottom continues through the ring lands. Can you see any damage to the bore when the piston is at BDC by the way? Before going any further it'd be worth pulling the piston and checking - if you end up needing a rebore/rehone then you're going to need all four done, and new pistons/rings to match.

You face a number of issues trying to fit secondhand pistons and getting the car to run right - the first one being whether the set you've bought is the appropriate combination of size grades for your bores. As you can see from the pictures you've posted, the dead piston is an A and the other one is a B. These need to be replaced like for like - as do the two on the other side, otherwise you'll have clearance (too big/too small) issues. If you're lucky your "new" set will be the same combination of grades and orientation as the ones coming out, but the chances of that are something like 9-1 against.

In addition (although I'm far from an expert on the older engines) I'm not sure there was any fundamental difference between "normal" and STi pistons back in '97 anyway - so your question regarding whether or not your "new" pistons are STi or not may well be moot. Even if they are theoretically stronger, if they're not the right size there's no point fitting them.

You'll also probably need to at least run a glaze breaker down each bore you're replacing pistons/rings in order to ensure they bed in properly. In short, while I can see you're trying to fix this on a budget, I'm not sure trying to make these secondhand pistons work is the best way forward. If the three existing cylinders/pistons/rings are good and within usable tolerance, the preferable route would probably have been to clean them up, and replace the dead one with new.

Might be better to get a bit more advice before going ahead with this.

merlin24

iTrader: (1)

As Splitpin stated above - chances are you will have some piston debris from the det damage embedded on the bore,so pull the piston out and check the condition of the bore before buying anything.
You have Cast 'dog-bone' pistons fitted to your engine,so if you are thinking of upgrading to forged STi/RA items, then the picture below is of a Cast piston on the left from a '97/8 WRX and an STi Version 3~4 HF forged piston on the right.
The cast piston has cross hatching on the underside of the crown and the Forged piston has a smooth underside with HF/1/2/3 stamped on it.





Mick

Clarkie GT

Thanks lads, thats a big help, much appreiciated... think ill end up taking the block to an engine builder near me as it sounds like ive bitted off more than i can chew

and going by your pictures, looks like the pistons ive bought are not forged any way, good job they were cheap... http://cgi.ebay.co.uk/ws/eBayISAPI.d...=STRK:MEWNX:IT

tsl202

In addition (although I'm far from an expert on the older engines) I'm not sure there was any fundamental difference between "normal" and STi pistons back in '97 anyway - so your question regarding whether or not your "new" pistons are STi or not may well be moot. Even if they are theoretically stronger, if they're not the right size there's no point fitting them.
yes they are stronger as forged ones

scooby72

iTrader: (6)

I know this posting is late but am I missing something funny????

JohnD

iTrader: (5)

An unfortunate wrong smilie? Maybe should have been

JohnD