Ok, need some help here, hoping someone can help out (@GeneralDisorder?)

First GT forester series 2 (2002) head is off. Piston is noticeably different - GT on the left, EJ20G/whatever it is on the right. This EJ20G/x piston is what came with the heads I’ll be running with.

I’m now worried the valves might hit the pistons. I’m hoping I can put one head on (not fully torqued down and thinking without the gasket too), refit the timing gear and rotate the engine to find out if the valves will kiss the piston. Also to note, the GT had thin MLS head gaskets, the HG units I got ages ago in prep for this build are thicker composite gaskets (OEM). 

I really don’t want to swap pistons on this thing! 

Here’s hoping! 

Some background info on the build: this will be going into my Gen1 RS turbo. The heads match the intake bolt pattern and the intake equipment matches the engine management system, hence the head swap onto a known good block. 

And no, I don’t want to run later model management or run aftermarket management, please leave comments like that to yourself thanks!

Cheers 

Bennie

I should add that I have hydraulic buckets, I’m thinking that the total lift of the valves may not be achieved with just turning over by had as I won’t have full oil pressure. 

Thoughts? 

I don’t have any plasticine or the like to squish and see how close things get. 

Cheers 

Bennie

Ok, I’m now at the point of “bugger it” swap the pistons. Put fresh rings in too. 

I have an A B A B block and have a set of pistons that are the same and correctly orientated - a left side A & B and a right side A & B piston sets as per the block markings. 

Will speak with the engine shop on Tuesday (most likely) to see what they reckon. I might even get them to put the pistons in if the price is good. 

Cheers 

Bennie

PS - glad I go all reactive then think about it, you can see this process in this thread where I basically talk to myself. Still happy to hear suggestions/experience with this/other ideas on the topic. 

Edited August 15, 20205 yr by el_freddo

The dome shape of the right piston in unfamiliar to me. 

Watch what you mix and match because the compression height, and connecting rod length is not the same on all EJ engines. You can really screw yourself if you don't take all factors into account. You need proper compression and proper quench or you will end up with something that has no power either due to low compression or compression so high it can't handle any timing on pump gas. Ideal compression on high octane pump gas is about 10 to 10.5 depending on cam profile, etc. 8.5 for turbo charging. Compare that to E85, where ideal NA compression is about 21:1

GD 

4 hours ago, GeneralDisorder said:

The dome shape of the right piston in unfamiliar to me. 

It’s the first wrx version of the EJ20 turbo, or EJ20G. The Gen1 Liberty/Legacy had a different piston again and DOHC heads that were more inline with the NA EJ22E - they even shared the same hydraulic lash adjusters and cam followers which is why many had the heads and pistons swapped under warranty due to lifter tick - I believe the exhaust lifters bleed out as things cooled down as they’re upside down in orientation. Or poor oil change routine was the issue. 

Now I’m hoping the rod length is the same. This game keeps getting better and better    The learning curve is steep and unanticipated! 

Thanks GD! I’m also guessing the phase two DOHC has a different cam belt length too. Time to go count some teeth... 

Cheers 

Bennie

Edited August 15, 20205 yr by el_freddo

Don't forget the compression height. That's the distance from the wrist pin to the piston face. That changed along with rod length. You generally want to keep pistons and rods together as a set since Subaru selected the rod length and compression height to match the chamber volume and HG thickness to achieve a specific compression ratio and quench. 

GD

Thanks mate. Seems I’ve opened a real can of worms on this one. 

Seems I need to pull one piston from the GT block (EJ205?) and take some measurements from centre of the gudgeon/wrist pin to the top of the piston and compare to what I have - unless you happen to have one of these pistons out already and can grab a measurement for me when you have five mins to scratch yourself? 

Thanks for all your help and insight on this one. 

Cheers 

Bennie

Well this slows things down  

Not happy Jan... 

Bennie.