We have always known you cannot use 18MM rocker boxes on 20MM cylinder heads and vice versa. Some shops, we have been told, will change the cylinder heads and rocker boxes as assembly from an early engine to a late engine (18MM to 20MM) and vice versa using stepped dowels. Don’t know if it works or not.
To the best of our knowledge (then), all the 18MM (early pre-commonized) have 8 MM rocker box mounting screw with a .401” locating dowel. 20MM (late commonized) heads have 10 MM rocker box mounting bolts with .500” locating dowels so we never checked for that. We have since then on and never found another set. We sent out a pair of 613 castings (20MM), which had 8 MM rocker box mounting bolts with .401” locating dowels. The customer’s rocker box mounting bolts are 10 MM w/ .500” dowels.
Simple solution: enlarge the dowels to .500” and use the 8 MM mounting bolts. Problem was the bolt patters didn’t line up.
First suggestion from our local Ford dealer, which is Mullinax Ford: http://mullinaxfordfl.com/, in Apopka, Florida, (great people to do business with) was look at TSB 07-10-4. Here’s link if you want to read it: http://www.motorcraftservice.com/pubs/content/~WT/~MUS~LEN/3561/tsb07-10-04.pdf
It simply tells you how to identify the 18MM locating dowel early heads, “pre-commonized” (I have no idea what that means) and 20MM later heads, “commonized”. No help solving my issue, we have 20MM heads.
Then I called Mullinax back. It turns out in their catalog on rebuilt heads, there’s a note that explains this issue. Thomas couldn’t print from their electronic catalog so he sent me a photo:
As you can see, this covers 6.0 Ford diesel cylinder heads from 7/19/04 through 12/18/06. Sorry the end of the text is cut off, it says: “Rocker Carrier Bolts (and dowels) Have Been Eliminated, Head Bolts Are Sufficient To Retain (and align) Carrier Per Engineering.”
Here is another link on TSB 06-24-4 that echoes the same as above: http://www.underhoodservice.com/tech-tip-use-caution-when-replacing-ford-diesel-cylinder-heads/
After discussing all the above with our customer, he verifying everything Mullinax told us with his Ford dealer, we both decided to install our cylinder heads without rocker box mounting bolts or dowels.
I’m happy at report no issues on installation and the F550 has over 23,000 miles, running, according to the customer, “perfectly”.