Usually on aluminum blocks and heads if they are separated they should be machined flat. That introduces other issues with compression changes and volume that can require tuning.

If you can find a machinists straight edge and test it with feeler gauges the surfaces may be flat enough.

 

I am surprised how little information about the motor and head designations that Subaru proliferated. Too many variables in design and not enough info as to why. Anyway I am thinking not to separate the heads on the replacement motor but sure would like to know which heads might be better. Wouldnt be surprised if the combustion chambers/piston tops are also different. Would be a learning experience for me but not sure I want to waste my time.

Seems like I should do headgaskets but the JDM motor only has 60k

 

Exactly what information do you feel is missing here? There is a ton of information out there about head castings and hardware.

 

I cant find what the head numbers came on what motors and how they translate to any performance improvements. Its not that easy since Subaru seems to have many variations.

I have seen awesome tables related to transmission and rear axle ratios and exactly what came on what vehicle by year. Also summarized defects associted with those gearboxes.

Do you have access to something like that for motors, heads, cams, cranks, etc.

Thanks Mike

 

EJ257 received B25 (2004-2006), V25B (2007) and W25 (2008+)
EJ255 received B25 (2004 Forester XT and 2005 Legacy GT) and D25 (2009-2014 WRX and 2006+ Legacy GT and Forester XT)

the 2L EJ207 and EJ205 will be marked with S20 or S20V depending on the year and variation. Single AVCS heads will be marked similarly. To determine if they are big port, you will need to take measurements.

Later dual-AVCS variants were W20 to match the USDM W25 dual-AVCS heads.

 

what happens if i put a d25 ej255 in my 05lgt with b25 heads? will it run ok or will i have issues? ik the compression ratio is different.

 

What makes you think that the heads in your photos are JDM at all? They look like USDM non-AVCS 2L heads.

 

There are 2 sets of heads in the attachment and I agree the C008 heads do appear USDM but thats just from forum descriptions about splitter thickness. Otherwise I have no clear application data on those head numbers and what they mean. The new JDM motor I received has unusual markings B005 & B009 - very different intake runners.