I have a car that came originally equipped with the EJ251 (2000 Outback). My block was toasted by negligent mechanic work (totally different story). I have a shortblock from a 99 Outback (EJ25D) that I'd like to swap into the car.

 

Here are my questions:

- Is the 25D shortblock identical to my 251? Or are there piston differences that would affect CR, piston contacting the head, etc?

- Which head gasket do I use for this combination?

- Is there anything I'm missing or overlooking?

 

 

Thanks, guys and gals.

- T

25D pistons come "proud" out of the block.

 

Use the 25D gasket or you will have high compression and possible contact with head.

You MUST use 25D gaskets with 25D pistons or the pistons will contact the head surface.

 

Your compression will be reduced from 9.9 to 9.2 with this combo. It will run but you will lose some effeciency and power.

 

GD

Edited September 9, 201114 yr by GeneralDisorder

You MUST use 25D gaskets with 25D pistons or the pistons will contact the head surface.

 

Your compression will be reduced from 9.9 to 9.2 with this combo. It will run but you will lose some effeciency and power.

 

GD

 

GD, you are a gentleman and a scholar.

 

This news saddens me, though. I wish there were a way to get back some CR...

you could get pistons that would raise it back up. but that would cost a lot of money. you would be better off replacing the bottom end with the original one if you wanted the compression back.

i just did this exact same swap - same year 2000 vehicle even. i used the 610 headgasket (last 3 digits of the part number). guy at subaru wanted to give me the other, had to ask for help as he didn't know what they were.

 

having the heads machined to the minimum would bump the compression somewhat. the shop machined mine below the minimum spec's (not on purpose...long story like yours no doubt, thread with pictures on here). seemed like it wouldn't affect the gasket sealing so i used them as is. maybe i got *some* compression back.

 

either way - the car ran and drove fine, felt great to me.

 

seems weird - from one small inaccurate angle no doubt - that compression would be lowered so much. if the pistons are touching the heads it seems like the volume is about as small as you can get - if the other heads allow more clearance - they should allow more volume and less compression ratio? i know that's not accurate but hard to think around that sometimes....

Edited September 10, 201114 yr by grossgary

I know. it's a weird concept to grasp. I'm glad you have experience with this exact setup. I guess the trivial amount of power loss doesn't really bother me. Maybe I'll get another .5 mpg. ha ha