Not to stir the pot and say you don't need to- but on my '83 300d with 250k+ miles (broken odometer) and an entirely unknown maintenance history, i did not ream the holes. All of my glow plugs backed out with no issues, as with going in. i had two dead plugs- one that simply would not glow at all and one that the end melted off of and destroyed the prechamber and injector. I believe the prev owner said he used ether to start it so maybe that did it.