I have a 1989 IROC-Z carbureted engine with aluminum heads. It has one of the original ZZZ 350 crate engines (bottom of page 6). The block is the same as the OEM engine. The internal parts (rods) crank up-beat which makes this car stout and fast.

It is time to replace the spark plugs. I understand iridium may be too hot for the aluminum heads.

I am considering the AC Delco 19307140 (aka the 41-808) and the 19301809 (aka the 41-905). I am leaning toward the 41-905 plug.

  • 41-808 gap = .047"
  • 41-808 reach = .75"

  • 41-905 gap = .035"

  • 41-905 reach = .43"

Would the AC Delco 41-905 sparkplug work well in my engine?

  • Which plugs are in there now?
    – tlhIngan
    Oct 29, 2016 at 19:38
  • How are iridium pugs to hot for aluminum heads?
    – vini_i
    Jan 1, 2017 at 18:20

1 Answer 1


Looks like the original plug for that engine is the FR5LS.
Source: http://www.corvetteforum.com/forums/c1-and-c2-corvettes/1059523-zz2-3-4-crate-engine-spark-plugs.html

FR5LS @ O'Reily Auto Parts: http://www.oreillyauto.com/site/c/detail/ACD0/FR5LS.oap?ck=Search_431_-1_-1&keyword=431&doInterchangeSearch=true

Gap (In): 0.035 Inch Thread Reach (In): 0.750 Inch

I would go for a plug with the same reach as your current plug.

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