My print head failed and I was unable to get it to work after following the HP instructions. I tried to find a replacement but couldn't find one.
It turns out that the print head is called a "whaler kit", from customer service representative Shana. I asked why it was called that but she couldn't tell me. But for a 130 bucks, I bought an 8710 for 10 bucks more. I will give my 8600 ink cartridge inventory to my sister who has an 8620.
The Officejet Pro 8600 was after my time at HP, but my guess is "Whaler" was the internal code name for the printhead project. The development teams typically have code names for projects long before part numbers are assigned. I have seen cases where these "stick" in the parts lists and sometimes even in the user manuals.
BTW, the printhead seems expensive, but it does include a complete set of setup cartridges that would cost over half the total at retail. That said, for a printer of that age getting the 8710 rather than risking something else being a problem may be a good choice. The Officejet Pro 8710 is comparable to the Officejet Pro 8600 Plus, it can scan and copy both sides of two sided documents automatically, a feature that has spoiled me.
Cartridges that were installed in your printer will not work in another printer if "cartridge protection" was not disabled before the cartridges were installed. Cartridge protection is set by default. There were also revisions to the 950/951 cartridges, if they are very old they may give an error in the 8620 about "previous generation cartridges".