I changed all 12 plugs this past sunday on the 00 E320. I used a piece of pipe on the ceramic to thread the plugs first. I also faced a strange problem like yours when I was using my proxxon plug socket which was actually contacting the recessed cavity too making it drag.

I then used the socket from husky and it was a perfect fit - I faced no problems at all with it. The old Bosch plugs were completely worn down - about twice the gap of original.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)