W670 and r975 short spark plugs
 

Is there a theory why the spark plugs on a w670 and r975 engines are a lot shorter than the thread in the heads?
Most spark plugs I have come across use the full thread and protrude into the combustion chamber.
Have come across fairly heavy carbon build up on the threads more than once? On both the w670 and r975.

I don’t remember that being the case.

A Longer thread protruding into the cylinder, if clogged up with carbon, would be very difficult to remove.

Yes I can get to that but why not the same thread length ?

The NGK AS-6 shielded plugs I have removed from the w670 engine has a thread length of 8.46 mm to the crush washer but the threaded hole for the plug has a thread length of 14.41mm so the electrode of the plug is within the confines of the hole not protruding in to the combustion chamber ?
On the six w670 engines I have worked on all plugs have this shorter plug thread to head thread length!