Thinner??? As long as they are 14mm plugs they will be fine. BUT - if you have resistor leads AND resistor plugs you may have starting problems when cold...... I run N6Ys in my engine. Sometimes known to oil up when starting from cold..
From pictures of the Champion plug, I think you will find that they have a conical seat, which is not suitable for an A-Series head - they need the ordinary flat seat and compressible washer. They appear to be the same spec as an NGK BPR6EFS, which is broken down into the following characteristics (also has a conical seat):
B - 14mm dia thread, 20.8mm AF hex
P - Projected Insulator Type
R - Resistor Type
6 - Heat Range - Medium
E - 19mm thread reach
F - Conical seat
S - Copper core centre electrode
The plot thickens. Having examined them more closely, these RBN9Y plugs are much thinner than 14mm, but the thread is the same so they can be screwed into the head. This must've been what confused me in the first place.
Regarding the N9YC plugs I have, comparing them with a normal N9Y, the washers and seats are both flat, Richard, so I am curious as to which plugs were meant for what engine.
As long as they will fit I suppose it's just a matter of testing whilst I drive. Apart from the RBN9Y's of course.
The length maybe means a better ignition of the mixture - BUT - interestingly, Racing Plugs are NOT extended nose as these are. If they fit - they will be fine, but beware the double resistance of leads + plugs.....