What's difference between "commercial" and regular A/C spark plugs

[Wayne M.]

Thinking of mid-60's timeframe. On eBay, you often see C42N; or C43N or C44N (which I've seen described as commercial (= heavy duty) versions of the same plug number without the "C" prefix.

For non-judging (ie. a driver) would these perform the same and last the same (or longer) than the regular ones ?

[Terry D.]

Re: What's difference between "commercial" and regular A/C spark plugs

Wayne
Commercial plugs were usually a colder heat range and were designed for continious use motors. Don't think they would do very well in a car driven rarely.
Terry

[Clem Z.]

Re: What's difference between "commercial" and regular A/C spark plugs

the commercial AC plugs i remember from the 60s had a much larger diameter center electrode than the standard AC plugs

[Jeffrey S.]

Re: What's difference between "commercial" and regular A/C spark plugs

Wayne,
Commercial plugs have the exact same heat range and perform the same as regular plugs but have a thicker center and side electrode which gives longer life. Tomorrow when I go into the shop I will take some pictures of each and post.
Jeff

[John D.]

Re: What's difference between "commercial" and regular A/C spark plugs

As mentioned the comercial plugs are very heavy duty. Same heat range and should last a long time. You can get some good deals on them at the fleas as a lot of guys don't know much about them. No good for judging but probably the best for driving. Look forward to Jeff's pics. Thanx, JD

[Clem Z.]

Re: What's difference between "commercial" and regular A/C spark plugs

forgot to post gulf oil racing corvettes used them

[Duke W.]

Re: What's difference between "commercial" and regular A/C spark plugs

the commercial AC plugs i remember from the 60s had a much larger diameter center electrode than the standard AC plugs

That's the deal. They also had a larger ground electrode to match. I think they may only have been available in colder heat ranges - like 3 and 2.

They appear in the 1963 Owner's manual for "extended high output" type service. The suffix is COM. I'm not sure if later owner's manuals had such an extensive list of recommended plugs based on type of service, but you can check.

Back when I did track events with my SWC in the sixties I used the 43COM.

Duke

[Jack C.]

Re: What's difference between "commercial" and regular A/C spark plugs

According to the TIM&JG, spark plugs are not judged.

[John D.]

Re: What's difference between "commercial" and regular A/C spark plugs

Jack, The 63-64 class judges plugs. See page 103 of the manual. I don't have a 67 manual. My manuals are only 57 to 65. JD

[Jack C.]

Re: What's difference between "commercial" and regular A/C spark plugs

I stand corrected. I better stick to C3's.

[Wayne M.]

Re: What's difference between "commercial" and regular A/C spark plugs

......I'd better stick to C3's.

Speaking of C3's I should have checked Chevrolet tune-up charts first [also for Chevelle, Camaro, Nova]. For '68 shows AC C42N as the recommended cold plug for all h.p. versions of the 396 and the 427 (except aluminum heads which used AC 42XL)

For '69, shows AC-R C42N as cold plug for 350 and 375 versions of 396 cubes and 390, 400, 425 and 435 hp versions of the 427.

For 1970, the only C plug for cold service shows for 390 and 400 hp versions of the 427 (ie. other than the Corvette).

[Bill M.]

Re: What's difference between "commercial" and regular A/C spark plugs

I used C42-1s when running my '59 on a chassis dyno; still have the plugs.

[Jeffrey S.]

Re: What's difference between "commercial" and regular A/C spark plugs

Below is a photo of the difference between standard plugs and commercial plugs. The one on the left is a standard 44 and on the right is a C44. You can see the thicker side electrode and the beefier center electrode.
Jeff