I am bolting together a new engine for a 2008 Outback, and did a core swap with a Subaru shop for rebuilt heads. Not until I put the cams back into the passenger side head did I notice that there are two "half moon" cutouts in the back side of the head that were not on the original heads. What are these cutouts for (see photos}, and is there a "plug" that fits in these? If not, I'll just spin a couple of plugs out of aluminum, cut them in half and glue them in.

Yes. That’s the rear of the camshaft right?  assemble and install the plastic caps that go there. Camshaft plugs they might be called. 

Yup dohc cam plugs

https://parts.subaru.com/p/Subaru_2004_STI-25L-TURBO-6MT-4WD-SEDAN/Engine-Camshaft-Plug/49223411/11051AA070.html

8 hours ago, idosubaru said:

Yes. That’s the rear of the camshaft right?  assemble and install the plastic caps that go there. Camshaft plugs they might be called. 

 

5 hours ago, nvu said:

Yup dohc cam plugs

https://parts.subaru.com/p/Subaru_2004_STI-25L-TURBO-6MT-4WD-SEDAN/Engine-Camshaft-Plug/49223411/11051AA070.html

Thanks for the responses. What are those cutouts intended for, if some D25's have them and others don't? 

1 hour ago, ebbsspeed said:

 

Thanks for the responses. What are those cutouts intended for, if some D25's have them and others don't? 

I think they used to be cast such that the left and right heads were identical parts. only need to cast one part for both heads.  Which just requires the cam support be cast into both ends, just plug the unused one as needed.

I worked in manufacturing engineering early in my career and this type of move was very common.  Having one type of cast product that could be finished as needed for multiple applications was more efficient than having two different casts, particularly for low volume products or initial development. So my guess would be the ones without plugs are later models. 

35 minutes ago, idosubaru said:

I think they used to be cast such that the left and right heads were identical parts. only need to cast one part for both heads.  Which just requires the cam support be cast into both ends, just plug the unused one as needed.

I worked in manufacturing engineering early in my career and this type of move was very common.  Having one type of cast product that could be finished as needed for multiple applications was more efficient than having two different casts, particularly for low volume products or initial development. So my guess would be the ones without plugs are later models. 

Thanks for that insight. I can see where that might be the case in many instances (like pretty much any American built V8), but given the lack of symmetry of the front and rear of these particular heads, they really do have to be cast specifically for the right and left hand sides.