Tom is correct. I used Bosch plugs in the '60s and '70s but they don't seems the same now so I use the BP6HS NGK plugs due to a more forgiving heat range for your average 356 engine and less expensive than Bosch.
The threaded part is still 1/2" but the electrode of the NGK intrudes a bit further than the Bosch plugs but I have yet to have a 'street' 356 engine have an interference problem. Racing with high compression heads, yes, and you know that instantly when a plug closes when kissed by the piston under 'stretch' and that cylinder goes dead. There are racing plugs with recessed electrodes for those extreme situations.
If you are worried about clearance on any spark plug, a kit of "indexing washers" can be had from an auto supply and be used as shims rather than their intended purpose of getting a racing spark plug aligned for best spark propagation for the best burning of the air/fuel mixture when it explodes, again, in racing. Beyond clearance and burn pattern, if the change in c.c. matters when altered by the depth of a spark plug..... then you are indeed a serious racer!
-Bruce
The threaded part is still 1/2" but the electrode of the NGK intrudes a bit further than the Bosch plugs but I have yet to have a 'street' 356 engine have an interference problem. Racing with high compression heads, yes, and you know that instantly when a plug closes when kissed by the piston under 'stretch' and that cylinder goes dead. There are racing plugs with recessed electrodes for those extreme situations.
If you are worried about clearance on any spark plug, a kit of "indexing washers" can be had from an auto supply and be used as shims rather than their intended purpose of getting a racing spark plug aligned for best spark propagation for the best burning of the air/fuel mixture when it explodes, again, in racing. Beyond clearance and burn pattern, if the change in c.c. matters when altered by the depth of a spark plug..... then you are indeed a serious racer!
-Bruce
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