1. The 1438 twin cam head is lacking an additional water passage on the intake face, rear of the #4 intake port. All other heads have a water passage there. This makes other intake manifolds incompatible on the 1438 unless the water passage in the manifold is blocked. Conversely, the 1438 manifold cannot be used on the other heads unless the passage in the head is blocked.

2. The center-to-center combustion chamber spacing is the same on all heads. The combustion chamber sizes vary with the USA 2.0L amongst the largest, affording the lowest compression ratio.

3. The 1438 and 1608 heads can be distinguished from other twin cam by the size of the water passage on the head face in front of cylinder #1. The width of this passage on these heads is 14 mm (0.55"). The 1592, 1756 & 2.0L heads have a 9 mm (0.35") wide passage. Because of this, the 1438 & 1608 heads will not work on the 84 mm bore engines unless this water passage is welded. The 1592 & 1756 heads are simple bolt-ons to the 2.0L. This generally improves the compression ratio.

4. The exhaust port size is nearly identical on all DOHC heads. The intake port size varies. Generally, the smallest port size is found on the 1438. The 1608, 1756 & Volumex heads share approx the same,  large intake port size.