I recently changed the engine in my 97 XK8 AJ-26 to a 2001 engine AJ-27

all the old ancillaries were use so in effect all that came off the new engine was the engine block and heads, cams, VVT, inlet manifold, throttle body etc were all from the AJ26 enine

It's running fine with the 97 spec plugs in but the spec for the AJ27 engine has a different part number for the plugs

I've just bought some genuine jag plugs part no NCA3850FA1 will these be the right ones as i'm a bit confused as to which ones I should be using now

Thanks

 

You would think you would use the plugs for the new block, but all of the electronics are from the older model. I would guess as you have that the old plugs would be the correct ones. I know this doesn't answer your questions but I agree with your logic. I'm sure a master tech will jump in here and comment.