Spark Plugs

[Ralph P.]

I know we recommend AC R45XLS plugs for 427's...... The R45S appears to have the same configuration except for a shorter thread reach. Is there any difference ...ie, reason to prefer one over the other ?

[Jack H.]

Re: Spark Plugs

You might have changed stroke or used piston with different dome geometry and the XLS tip extends too far into the combustion chamber for safe clearance. If this isn't the case, in general, extended tipped plugs tend to ignite the air/fuel mixture a bit more efficently due to their longer reach (more centered in the combustion chamber)...

[Dave S.]

Re: Spark Plugs

I know we recommend AC R45XLS plugs for 427's...... The R45S appears to have the same configuration except for a shorter thread reach. Is there any difference ...ie, reason to prefer one over the other ?

Ralph,
An R45S will not work in your 427. You need an "N" plug like a 43N, 44N, 45N, R44N, R45N etc. All Big Blocks require a plug with an "N" designation. "S" is for extended tip. I'd have to check my AC Plug book to see what X and L designate.

[Ralph P.]

Re: Spark Plugs

Dave, the R45XLS has been recommended many times on this forum as appropriate for any vintage 427 used in "normal" highway use....

[Jeffrey S.]

Re: Spark Plugs

Ralph:
The XL designation is for a fully threaded, 3/4" reach plug. The "N" plugs are also 3/4" reach but only 1/2 threaded. The XL's were specified for aluminum heads like L-88, L89. They will thread into a steel head just the same. The "S" as was said is for extended tip which effectively adds a 1/2 heat range to the plug so a R45XLS would have a 5 1/2 heat range. The R45S has only a 3/8" reach and will not go into the combustion chamber of a big block.
Jeff

[Ralph P.]

Re: Spark Plugs

OK... that's my answer ! The shorter reach plugs won't fit a BB Thanks !

[John H.]

Re: Spark Plugs

OK... that's my answer ! The shorter reach plugs won't fit a BB Thanks !

Correct...but the R45S plugs work great in small-blocks.