[W126 Coupe] question about spark plugs

[Gary]

i got the NGk Bp6ES.  when i started to put them on, i noticed the  part that connects to the boot is regular.  on the bosch plugs currently in the engine they are small threaded pencil width design and the boots pop on snuggly.  the ngk bp6es don't even fit into the boot.  Are there different ngk bp6es for different cars?   
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