The above is very accurate. The 1997-99 CHASSIS YEARS, with 275-HP are the ones with poor design, too few plug threads. CHASSIS YEAR 2000 and later are the "PI" heads (305-HP) with a plug hole design having adequate threads. Unfortunately, some heads, and that of course means thousands, of heads were made on a machining line that didn't get the holes quite lined up with the seats. It took a few years of production to sort that out. It has to do with which plant did the machining. I believe it's possible to tell from the VIN where the engine and apparently the heads came from. When we bought ours nearly 10 years ago, I found somebody who asked for the VIN. I think he checked with an engine remanufacturer, came back and said I should be OK. We went ahead with the deal. With 30,000 original miles, prev owner hadn't changed plugs. At 40,000 now, I don't plan to do it any time soon. But I can say there've been no issues.
From 1997 to the early 2000's, the transmission was called 4R100, gearshift button marked "OD Off". It was replaced somewhere around 2004 with 5R110, gearshift button marked "Tow/Haul". 4R100 is a good unit, no problem there, but the 5R110 has some new tricks. I mention this because if the Class C has "Tow/Haul" it is also new enough to be past the years with some problems in some of the PI heads.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB