Spark Plugs 427/435

[Brian Bates]

Does anyone have an AC spark plug guide to determine application options for a completely original 1969 427/435 L71?

[Clem Z.]

Re: Spark Plugs 427/435

for every day street driving use AC R45XLS gapped at.035

[Dave S.]

Re: Spark Plugs 427/435

You also could consider a straight 45N AC plug. No extended tip (S) to deal with.

[Clem Z.]

Re: Spark Plugs 427/435

extended tip plugs are no problem in BBC even with 12:1 pistons. the extended tip allows you to run a hotter plug to keep clean in town driving but at WOT the fuel wash helps cool the plug tip making it act like a colder plug.

[Daniel S.]

Re: Spark Plugs 427/435

Brian,

I have been using AC Rapid Fire #4's on my stock '68 L71.
5,000 street miles later on first set, no problems at all.

Dan

[Joe L.]

Re: Spark Plugs 427/435

Brian------

The 1969 GM specs for the L-71 spark plugs were as follows:

standard operation----AC R-43N

cold plug for highway driving----- AC R-42N

The above are not necessarily the best plugs to use, but they are what was recommended by GM in 1969 if that's what you're after.