I have a 98 OBW that's developed a rod knock. I got a ej251 long block at the pull it yourself yard. I could use 95 2.2 heads or the 25D heads. I'm leaning towards using the 2.5 heads as I already have them. I'm not sure which head gasket to use though. There are different part numbers for 251 and 25D. I have heard this combination will raise the Compression. Does anyone know how high? On that same topic what would the compression be elevated to if I used the 95 EJ22 heads? The 251 engine came out of a 2000 model legacy. Thanks in advance for any info.

the 251 block should work with the ej25d heads.

but tyou need to double check the gasket.

there is a head  / block combination where a thicker gasket is required.

 

but generally speaking,

the head gasket MUST match the block.

if the fire ring? on the gasket does not fit the block,

it will not seal.

HI,

The Ej22 heads/EJ251 block will be a 'franken engine" and raise comp to over 10:1. You'll want to run Premium.

 

I think you can run the STi 2.5 HG w/the Ej25D/EJ251 combo....read up on NASIOC.

 

GL,

TD

if you are using a 2.5 dohc block then you need to use the 2.5 dohc headgasket. The pistons come above deck height at TDC. if you use a thinner gasket it wont work.

^- He's using a SOHC EJ251....heads are the issue - not the block.