Mercedes redesigned the head, AND head gasket, a total of five times - that's six different part numbers each. The original head was #14. There was a #15 but it's rare, I've heard of *one* in the USA. Next up is the #17, used on USA engines from 1990-95. After that there's a #20 head, followed by a #22. The revision between 20 and 22 is modification of the oil passage in front of the #1 cylinder, between it and the timing chain cavity, to help eliminate gasket failure in that area. The change between #15 and #17 is documented in the factory engine manual... "The reinforcements at the valve tappet guides and exhaust passages have been reinforced." I don't know the difference between the 14 and 15, or the 17 to 20. I also don't know the number of the missing "sixth" head, or if it exists - but the EPC shows six different part numbers (all superceding to the current #22 head.) The #17 heads are almost crack-proof, they require massive abuse (overheating) before they will crack. I've heard of one or two and both, IIRC, were driven for a while in the 120C+ range.
Oh - the #22 head is an OE design, you buy a new head from the dealer, it will be #22. There are no aftermarket heads.
The precombustion chambers were changed from 1990-up in USA engines, but for emissions reasons. The new style is inclined (angled 5 degrees) and uses shorter glow plugs as well. The old PC's may fit new heads, they may not. If not, you need to either machine the old PC's (like I did), or machine the bore in the head (much more difficult), to allow them to fully seat. Or, convert to the inclined injection setup at exorbitant cost (new PC's, lock rings, and injectors - $1500+ new.) According to the experts, the problem seems due to sloppy machining tolerances on the newer #22 heads - nobody knows why though.
Last comment: Although Mercedes did not import any 3.0L 603's (603.96x) into the USA except in 1987, they WERE shipped in the rest of the world from 1986 through about 1995. So there are tens of thousands of them running around Europe. Only the USA models got the dreaded trap oxidizer, however! There seems to be fewer failures in Europe, which lends credence to Marshall Booth's theory that the trap oxidizer's additional heat load may have been the cause of the rash of USA failures.
