2.5 SOHC short block compatibility

[Regno 1]

Does anyone know if the short block from an 03 Subaru Forester 2.5 SOHC with the dual cylinder exhaust ports uses the same short block as an 04 Subaru Outback 2.5 SOHC with the single cylinder exhaust port? I have the short block that came from the Outback and the cylinder heads that came off the Forester and was hoping I could combine the two to make a complete motor. If anyone could tell me whether they are compatible I would appreciate it.

[idosubaru 1910]

7 hours ago, Regno said:

Does anyone know if the short block from an 03 Subaru Forester 2.5 SOHC with the dual cylinder exhaust ports uses the same short block as an 04 Subaru Outback 2.5 SOHC with the single cylinder exhaust port? I have the short block that came from the Outback and the cylinder heads that came off the Forester and was hoping I could combine the two to make a complete motor. If anyone could tell me whether they are compatible I would appreciate it.

Something isn’t right here all 03 and 04 EJ25s are dual port exhaust.  So that’s not a stock engine if it’s single port. It’s not an EJ25 or something is wrong with the information we have been given  

Other than that problem with information/description/engine - all 03 and 04 EJ25 blocks swap all day long without question. 

Edited December 23, 2020 by idosubaru

[montana tom 206]

2004 California outback's had a different head with single port.

Yes you can.  Be sure to use Subaru #642 turbo head gaskets 

[Numbchux 673]

54 minutes ago, idosubaru said:

Something isn’t right here all 03 and 04 EJ25s are dual port exhaust.  So that’s not a stock engine if it’s single port. It’s not an EJ25 or something is wrong with the information we have been given  

Other than that problem with information/description/engine - all 03 and 04 EJ25 blocks swap all day long without question. 

You're forgetting the '04-05 EJ259. Single port exhaust similar to an EZ30.

AFAIK, the only thing unique about the 259 is intake and sensor related. The Block and probably heads are the same.

[tylertrend 38]

4 minutes ago, Numbchux said:

You're forgetting the '04-05 EJ259. Single port exhaust similar to an EZ30.

AFAIK, the only thing unique about the 259 is intake and sensor related. The Block and probably heads are the same.

The heads probably are the same, the block is only different in that the piston design is different from the 251/253. There are pics on google images, they're kinda unique. The manifold is different. I used to own a vehicle with this engine and it always ran smoother than any other SOHC Subaru

[Regno 1]

I was also under the impression that all the 2.5's SOHC had the dual exhaust cylinder head ports but upon further research I found out just like another member stated that if the engine was from a California car (2003-2005) they used a cylinder head with the single exhaust port to meet stricter California emissions. There isn't a ton of information out there on these engines that I could find that's why I was unsure if the short blocks were the same. I know the intake manifold, cylinder heads, exhaust, and general wiring is all different from my 03 Forester 2.5 SOHC but I was hoping that the short blocks were the same so I could bolt the intake from my forester onto the short block that had the California emissions heads. I have included two pictures, one is an 03-05 2.5 SOHC with California emissions (single port) and the other is a 99-05 2.5 SOHC (dual port) with federal emissions. If someone could tell me for sure whether the short blocks are the same I would really appreciate it.

[idosubaru 1910]

Yeah all EJ25 short blocks are interchangeable.

That 96-99 DOHC blocks but the pistons protrude above the block and I think I used a thicker gasket when using them in later engines. They’re all rusted away around here so I haven’t touched one in five years but still have one lying in my garage. 

[Numbchux 673]

Very interesting pictures. It looks as if the head casting might be the same, and just the ports machined out differently.

[ocei77 132]

The head and exhaust are  modified EZ30 units.

The big problem is that the  EJ259 had 3 cats and 5 O2 sensors!

O.

 

 

[Numbchux 673]

Well, the exhaust and port is similar to the EZ30, but beyond that, the head has nothing in common.

[Regno 1]

The biggest difference I have found in the two short blocks is the pistons, the EJ259 block (03-05 2.5 SOHC w/single port and California emissions) has a large dished out section in the piston and the EJ251 block (99-05 2.5 SOHC w/dual ports and federal emissions) has a more traditional and symmetric shape. I have heard from some people that EJ251 heads will work on a EJ259 block and I have also been told that they might not work. Im not sure if there would be issues with valve contact/clearance and compression ratios due to the odd shape of the EJ259 pistons. You can see in the pictures how different the tops of the pistons are.

[GeneralDisorder 1747]

Swap the pistons. 

GD

[Regno 1]

I am kinda leaning towards a piston swap like a few people have mentioned, I do have access to another EJ251 that has usable heads although I think a rebuild of them would be in order for reliability and peace of mind. I think the pistons are also in pretty good shape and would also be reusable with a new set of rings. Maybe someone could give me their input on whether reusing a set of good used pistons with new rings would be advisable seeing how I'm trying to keep down the cost wherever possible. I would also have to find out if the pistons from an EJ251 would be a direct swap into the EJ259 block as far as size and clearance goes. Anyone that has maybe done a swap like this or has more knowledge on this subject I would love to hear your advice or input, should I reuse the pistons? what kind of piston rings would best suit this swap? what kind of HG to use (turbo/non-turbo)? what parts would work best, OEM or aftermarket? Please give me your thoughts.

[GeneralDisorder 1747]

Used pistons are fine. Get new rings. Either NPR or Subaru. Pistons should swap without issue. 

Use the 770 turbo head gaskets.

GD

[Regno 1]

There is only one concern I have as far as the piston swap goes, I was told by 90% of the people I talked to that if I want use new rings on an old piston that I need to have the cylinders honed or de-glazed. I really didn't want to split the cases, and correct me if I'm wrong but don't you need to gut the block in order to properly hone the cylinders. I have read in a few instances where people removed the pistons and honed the cylinders with the crank still in place but that doesn't seem like the smartest move. Maybe you could get away with it but it doesn't seem like the best option, ( feel free to give me your opinion). I also was told that I would be better off using the old rings from the EJ259 California emissions engine (has about 120,000 miles and is the short block I will be using) and swapping them onto the EJ251 pistons if i dont hone the block because those rings are already seated to the cylinders in which they came from. Does that seem logical?

[idosubaru 1910]

7 hours ago, Regno said:

There is only one concern I have as far as the piston swap goes, I was told by 90% of the people I talked to that if I want use new rings on an old piston that I need to have the cylinders honed or de-glazed. I really didn't want to split the cases, and correct me if I'm wrong but don't you need to gut the block in order to properly hone the cylinders. I have read in a few instances where people removed the pistons and honed the cylinders with the crank still in place but that doesn't seem like the smartest move. Maybe you could get away with it but it doesn't seem like the best option, ( feel free to give me your opinion). I also was told that I would be better off using the old rings from the EJ259 California emissions engine (has about 120,000 miles and is the short block I will be using) and swapping them onto the EJ251 pistons if i dont hone the block because those rings are already seated to the cylinders in which they came from. Does that seem logical?

No. Do not hone a Subaru block, don’t split it or touch it. New rings on old pistons all day long. Zero issue for Subarus.

[Regno 1]

@idosubaru, thanks for the advice. When it comes to this California emissions EJ259 I have been told and heard so many conflicting statements on what I can do or should do its getting frustrating having to try and fact check everything I'm told just to see if there is any validity behind it. I have made similar posts on a few other subaru forums and a lot of people told me that if I use new rings on an old piston without honing the block the rings would never seat properly. I will admit that I'm not as well versed with Subaru engines as I am with GM LS motors, but I have put new rings on used pistons in a non honed cylinder on GM's a half dozen times without any issue of the rings not seating. Once again, I appreciate the advise, with a member status of elite master of the Subaru I tend to believe what your saying is based on knowledge and experience and not on personal opinion like the various other forums I have posted on.

[Rampage 228]

The thing about using standard rings is both the rings and cylinder wall will wear giving a good seat. Chrome rings, different story.

Check the ridge at the top of the cylinder. On other (non-Subaru) engines I have worked on I always used a ridge reamer to remove the ridge at the top of the cylinder to prevent cracking the new compression ring if it hits the ridge especially when using different pistons.

[idosubaru 1910]

9 hours ago, Regno said:

@idosubaru, thanks for the advice. When it comes to this California emissions EJ259 I have been told and heard so many conflicting statements on what I can do or should do its getting frustrating having to try and fact check everything I'm told just to see if there is any validity behind it. I have made similar posts on a few other subaru forums and a lot of people told me that if I use new rings on an old piston without honing the block the rings would never seat properly. I will admit that I'm not as well versed with Subaru engines as I am with GM LS motors, but I have put new rings on used pistons in a non honed cylinder on GM's a half dozen times without any issue of the rings not seating. Once again, I appreciate the advise, with a member status of elite master of the Subaru I tend to believe what your saying is based on knowledge and experience and not on personal opinion like the various other forums I have posted on.

I lean on GD for block recommendations and experience. He does this stuff all the time and has described in detail in other posts about why. Look those comments up if you need more background and fact checking. It’s enlightening and data and experience driven.

Im a member of a lot of Subaru forums since the 1990s, I know exactly what you mean!  Blocks are tough and I don’t think advice given is purposely misguided.  There’s just not much volume to go on. Few people are doing any volume of blocks over a long enough time to make more than anecdotal assumptions, there’s no good data. And almost no one has the volume or time to do it multiple ways and compare data.  

GD is about as close as you can get to that anywhere online when it comes to Subaru blocks. 

[GeneralDisorder 1747]

Honing hasn't been a good idea on any engine since before 1980 or so. If the bores are wrecked then you have to bore it oversized, and you finish with a plateau hone of about 600 grit (very fine). Subaru engines do not wear out their bores and if they need to be bored oversized you may as well just throw them in the trash as the machine work to do a main line hone (absolutely required), bore with a deckplate and re-finish the head mating surfaces is about the cost of a new set of case halves once you figure turn-around time and transportation costs. 

DO NOT split the block. If you do that you may as well just find a used block. The level of success an amateur is likely to achieve with a block split is very low. High probability it will fail in short order, and almost a certainty it would have a much shorter life than if it were just left alone. There are techniques required to properly reassemble them (or even one composed of all new parts), and very accurate measuring tools and procedures must be followed as the tolerances are much tighter than something like a GM small block due to the nature of the aluminum blocks. 

GD

[Regno 1]

@GeneralDisorder, what would your advice be on this particular piston swap. I have a fairly low mileage 04 EJ259 short block (w/single port exhaust and California emissions) and would like to swap the pistons, heads, intake, and exhaust from an 03 Forester EJ251 that I have. Am I wasting my time trying to swap the used EJ251 pistons into the EJ259 block? Assuming the donor pistons from the EJ251 are the right combination of "A" and "B" size pistons (or does it not really matter) should I reuse the original rings from the EJ259 block or buy brand new rings? Or am I better off with new pistons and rings? This is one of those deals where its a budget build and I don't have a lot of money to work with, I'm basically trying to take the best useable parts from several different EJ's and combine them to make one good motor. Based on your experience am I better off using oem or aftermarket parts (piston/rings) I would love to hear your opinion as your the "man" to ask from what Im told.

[GeneralDisorder 1747]

A, B, C, etc doesn't matter. Just stuff them and run it. It will probably have some level of piston slap when cold but really - it's a 20 year old car. Who cares? Aftermarket pistons aren't even offered in the A, B, C sizing like Subaru runs from the factory - they are 99.5mm..... and that's all you get. So just treat them like a set of aftermarket slugs and pay no attention to the factory sizing options. 

Get new rings. You need 251 rings for 251 pistons. Standard size shouldn't require any gap fitting but you should check the end gaps anyway. 

You are WAY overthinking it. The pistons and cylinder walls are of the least concern in building these engines. Don't touch them. Don't worry about minor scoring on walls or skirts - just run them and forget all that. It won't make any significant difference to your end results. 

1. Use pistons that match the heads - put whatever piston in whatever hole - doesn't matter. 

2. Use new rings. 

3. Turn key. 

GD

Edited January 8 by GeneralDisorder

[el_freddo 719]

@GeneralDisorder - same for an old phase 1 turbo engine, or is a little more finesse required with those? 

Asking for a friend   

Cheers 

Bennie

[GeneralDisorder 1747]

Yep. In the realm of turbocharging..... the old saying "loose is fast"..... absolutely true.  

GD

[el_freddo 719]

5 hours ago, GeneralDisorder said:

Yep. In the realm of turbocharging..... the old saying "loose is fast"..... absolutely true.  

GD

  

and does it end with  ? 

Cheers 

Bennie