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Its really good that you did not rely on past postings such as those quoted in the above replies as they are both either wrong or misleading.
Measuring the thickness of a head is only an indication of which head it is
if the heads have not been milled. An F head that has been milled to increase the compression ratio could be the exact same thickness/height as an E head, and quite a few have been milled to this spec. But why go to the trouble of measuring the head when you can just look at them and tell the difference?
The key here is that the castings are different, both inside and outside, and not just in their height/thickness.
The differences on the outside indicate the differences inside. The exhaust ports are different and this may make as much difference in power as the different compression ratios.
Heads made with the early - through 71 at least in the US - exhaust port casting, whether carbed or FI, have the same round shape over both the left and right center exhaust ports. Those from 72 on with the later exhaust port shape, whether carbed or FI, have a flat above at least the right hand center exhaust port when looking at the port side of the head. 74 -75 heads made for K jet FI have flats above both of these exhaust ports. Some of these late B20F heads have hardened exhaust seats.
Now you can amaze other Volvo owners by glancing into a engine compartment and being able to tell not only which FI head is on an engine but also whether a carbed head is an early or late head.
To anticipate another question that has been covered before, although often incorrectly -
The early exhaust ports flow better in stock condition.
The later exhaust port is an "anti-smog" design the does not flow as
well as the earlier port casting, but can be modified to flow much better than
the earlier casting, so we use the later casting design for all of our modified heads.
John Parker
V-Performance.com
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