Cret

Am about to do a rebuild which will be using a set of STi pistons a mate had spare in order to match the heads going in (it's a legacy motor that had rattly top end so V3/4 re-con STi heads are going on and Legacy pistons can't be used with them).

Now I had one set of STi pistons but I dropped one and chipped a bit of a skirt and was very reluctant to use it for obvious reasons, so I now have another set, but the thickness of the rings is much different to the first set. Is this just a variation from one year to the next?
The new ones have rings of 1.5mm/1.5mm/3mm thickness, and the other set were something like 1mm/1mm/2.5mm.

I've bought a set of new rings on ebay for £45 that are 1.5mm/1.5mm/3mm and specified as 92mm bore rings for EJ20 turbo so am hoping these are ok.
With regard to a hone though - is it definitely needed with the new rings? I'm assuming yes, even though the original cross hatching is clearly visible.
I've been told to avoid the 3 legged sprung hones, and I've found a flexhone/ballhone the right size and am wondering if this will do the job ok? I'm fairly handy with tools but is this something I should just leave to a mchine shop?

Also, as I'm not splitting the block and replacing bearings etc (I'm going for a 'light rebuild' due to budget) is there an easy way of preventing any stuff getting in the bottom end following a hone, or will I have to somehow rinse the block out afterwards to ensure no nasty stuff remains in it? If it's rinsed, how do I ensure everything is adequately lubed afterwards.

Appreciate any useful advice, although I'll be honest and say I'm not looking for answers that are questions such as 'why are you doing it this way?'/'why aren't you replacing the bearings?' etc.

Many thanks
Jim

merlin24

iTrader: (1)

Jim,
The OE Forged Pistons(HF) for the STi 3~4 will normally use the thick rings,the thinner rings are normally found on the Version 5 onwards.
IIRC the clearance between the ring and ring groove on the piston is approx. 0.0015 ~ 0.003" for the top and second rings.
You can use a ball/flex hone for "glaze busting" the bores provided they are in good condition ( no excessive wear/taper/ovality) but you will struggle to do the bottom of the bore due to the con-rod being in there.
There is no way you can be 100% sure of preventing any debris from getting inside the engine when honing a short motor,so i would personally split the crankcases to do it and it can be properly cleaned afterwards ( for the sake of a few block O- rings,rear main and some 3 bond sealant).

Mick

Cret

Thanks for the info Mick

I have worked out that even with the rods in place the hone will be able to get easily below the level that the rings get down to so it should be fine.

The bores visually at least are in very good condition. No marks other than the original crosshatching still being clearly visible. I'm assuming therefore that just glaze busting will be sufficient since there are obviously still grooves for the oil to get around.

I've seen a good few tales of getting minimal amount of debris down the bottom end, combined with magnetic sump plug and early oil change being fine. Another thing is that at some point in the future when I can afford a better bottom end I'll be doing it again so lasting til kingdom come is not the end of the world in that sense.

I doubt very much that I'll be splitting the bottom end in all honesty but out of interest and for future reference, what bits exactly must be replaced when this is done?

My understanding of the STi pistons btw is that contrary to popular belief they aren't actually forged but are just a better alloy or something than the normal cast ones? If nothing else they are a lot lighter as they are about 80 or 90grams lighter each than the cast ones from my legacy CDB.

Cheers
Jim

merlin24

iTrader: (1)

If you need to split the block in the future,you need to replace:
3 x Block O rings (black)
1 x Coolant Transfer Tube O ring (orange)
Rear Main Oil Seal.
You will also need a tube of Grey 3 Bond Sealant to seal the block halfs.
The Subaru STi HF(2/3/4) pistons have a smooth underside of the Crown and are Forged - non STi Pistons have a crosshatching on the underside of the Crown and are Cast.
If you are going to hone the short motor - it would be a very good idea to remove the sump,oil pickup pipe and windage tray so you can give the inside of the crankcase a good clean.

Mick

Cret

Thanks again for the details

I have a tube of 3bond 1215 sealant which I was recommended to get to add to the cam cover gaskets. Not sure what colour but I'm guessing it's the same stuff.
The pistons are the smooth STi ones you describe sure enough (holes for oil instead of a big slot too), and my old Legacy ones are cast. The shape of the skirt is also very different.

I'd always thought that these STi ones were forged as you say but I read somewhere recently that they aren't actually forged but are nonetheless much better than the cast ones. I can't say whether this is correct or not obviously and I forget where I saw it, but if I remember I'll post up a link.

Sump and pickup pipe are already off. Not sure what the windage tray is sorry, is it the baffle like bit?

Btw - what does HF stand for when you've referred to it?