Spark Plug Choice for 66 Big Block

[John L.]

For 10 + years and 5,000 miles have been running Autolite 403 plugs which I recently pulled to take a look at. There is a good amount of black soot on the outer ring of the plugs, and doing more research on the 403 plug, see that its a colder type plug with cross reference to AC 43,44. So with that said and after reading many threads in the Archives, decided to change plugs and recently purchased Champion RN12YC plugs to install which has cross reference to AC 45. The engine is an L72.

[Joe L.]

Re: Spark Plug Choice for 66 Big Block

For 10 + years and 5,000 miles have been running Autolite 403 plugs which I recently pulled to take a look at. There is a good amount of black soot on the outer ring of the plugs, and doing more research on the 403 plug, see that its a colder type plug with cross reference to AC 43,44. So with that said and after reading many threads in the Archives, decided to change plugs and recently purchased Champion RN12YC plugs to install which has cross reference to AC 45. The engine is an L72.

John-----


The Champion RN12YC are a good choice.

[Duke W.]

Re: Spark Plug Choice for 66 Big Block

AC R45XLS or equivalent geometry and heat range (AC 5) in another brand. Champion 12 might still be a bit on the cold side. Also since you are willing to run non-AC plugs consider a brand with a non-resistor offering equivalent to the 45XLS. I know that both Denso and NGK offer a non-resistor plugs equivalent to the long out-of-production AC 45. AFAIK AC no longer offers non-resistor plugs for automotive engine applications, so all begin with "R" which means a resistor type and most other brand resistor plugs also contain a "R" in the nomenclature to designate a resistor type plug.

Duke

[John L.]

Re: Spark Plug Choice for 66 Big Block

Thanks Joe and Duke. With respect to these Champion plugs, have noticed that the firing end (ground electrode) is more projected than the Autolite plugs they are replacing and some older AC 45 plugs I have in the garage. Assume this design is for better firing quality purposes and internal interference is not an issue?

[Joe L.]

Re: Spark Plug Choice for 66 Big Block

Thanks Joe and Duke. With respect to these Champion plugs, have noticed that the firing end (ground electrode) is more projected than the Autolite plugs they are replacing and some older AC 45 plugs I have in the garage. Assume this design is for better firing quality purposes and internal interference is not an issue?

John-----

Yes, the Champion RN12Y are an extended tip spark plug as are AC plugs with XLS suffix. I have found that extended tip spark plugs better resist fouling and, for me, seem to generally run better. There should be no problem with piston interference.