later glow plugs are longer and have high probability of getting stuck in the aluminum. I believe they have less meat on them, or they alloy with the alumimum threads.
I spoke with the owner of a machine shop that does work for a dealer. He said the later diesels are a PITA and some mechanics are in a hurry, use no penetrating oil when removing GP's or else they use it but don't let it soak longer than a few minutes (it takes hours to overnight to really get into the threads) then the GP breaks clean off in the head. Then the head has to come off to go to the machine shop to have the GP removed (that takes an hour!) $$$$$
I always use a dap of anti-sieze on the 603 GP's. Some say no you shouldn't use it on a head but you know me, I'm Diesel DIEHARD! I use anti-sieze a lot of places and I'm sure the factory won't even bother with the extra effort. Moly (silver) anti-sieze on the head bolts, copper anti-sieze on exhaust bolts. SOme on suspension bolts, lug bolts YESSS! it's great stuff!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!