AC Delco Spark Plugs
Thread Starter
Instructor
Joined: Jan 2019
Posts: 167
Likes: 33
From: Connecticut, USA
AC Delco Spark Plugs
I know this has been beat to Hell, but I just have to vent.
Most will recommend AC Delco Rapidfire #5. If they are the right plug why aren't they pregapped at .035", and, is what is in the factory manual correct? FR5LS .035"?
Most of these spark plugs are not as they were back in 1989.
Is the factory manual wrong with a .035" gap?
Should we just forget all this and use a modern iridium plug with the original pregap?
Let's see how many answers we can get
)))
Most will recommend AC Delco Rapidfire #5. If they are the right plug why aren't they pregapped at .035", and, is what is in the factory manual correct? FR5LS .035"?
Most of these spark plugs are not as they were back in 1989.
Is the factory manual wrong with a .035" gap?
Should we just forget all this and use a modern iridium plug with the original pregap?
Let's see how many answers we can get
)))
Drifting
Joined: Jan 2021
Posts: 1,312
Likes: 384
From: Northern California
In my recent plug replacement, '89, I went with the RapidFire #5 and .045 gap. Works perfectly.
I have no idea as to why they're recommended but come with a different factory gap. I always assume I'll have to check and gap plugs when they arrive, just because I want to know they're gapped perfectly by me upon installation.
I have no idea as to why they're recommended but come with a different factory gap. I always assume I'll have to check and gap plugs when they arrive, just because I want to know they're gapped perfectly by me upon installation.
Thread Starter
Instructor
Joined: Jan 2019
Posts: 167
Likes: 33
From: Connecticut, USA
That's the thing. The authors of the manual know we have HEI. Why do they still say .035"?
NGK Iridium's have about.045" Why are we not using those (the ones with crush washers)?
There are so many inconsistencies and no corrections.
I have a separate addendum with all the corrections to the shop manual. I'll go over it and see if there are changes.
NGK Iridium's have about.045" Why are we not using those (the ones with crush washers)?
There are so many inconsistencies and no corrections.
I have a separate addendum with all the corrections to the shop manual. I'll go over it and see if there are changes.
Burning Brakes
Joined: May 2005
Posts: 1,096
Likes: 36
From: Coldville
If they say Pre Gap I leave them alone.
I think they test that In the Engineering and use what works.
The Factory plugs gap may be different because of the Materials used like Conductivity of the Metals used and Structural make up of the plugs.
You can look up the Company and find a Tech or Customer service number and call and ask them why.
I think they test that In the Engineering and use what works.
The Factory plugs gap may be different because of the Materials used like Conductivity of the Metals used and Structural make up of the plugs.
You can look up the Company and find a Tech or Customer service number and call and ask them why.
Thread Starter
Instructor
Joined: Jan 2019
Posts: 167
Likes: 33
From: Connecticut, USA
I'm baffled by the fact when you try to look this up there's so much contradictory info.
One web site I went to was recommending using tapered plugs in aluminum heads...geesh!
Melting Slicks
Joined: Nov 2016
Posts: 2,256
Likes: 3,082
From: So Cal
2023 C4 of the Year Finalist - Unmodified
Over many decades, I have been pleased with the results of copper core plugs in my engines.
I like using plug part numbers that I understand the heat range. I have many years experience with Delco, Champion, and NGK heat range numbers. I have no idea what the heat range of a merchandising number is, and finding credible heat range cross-reference info online for the merchandising numbers can be challenging.
I'm old school. I use what I've had good results and experience with. Others' mileage may vary. Cheers.
Last edited by IHBD; Nov 23, 2024 at 10:28 AM.
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Lt1 & Lt4 are factory tapered seat in Aluminum heads. Just have to carefully adhere to torque specs! Mostly you need to use whatever seat style you're heads were designed for. Not really interchangeable. What engine/ heads are you running?