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I made a bone head move, and installed plugs without checking the gaps. They were so called “pre-gapped” at .054” (54), but I was under the assumption that was the correct gap for my ‘96 LT1. Fast forward to a few days ago, I decided it was time for a swap. Installing them with the correct gap this time. When I swapped them, one was at 52, another at 59, and the rest in between. My question is, can I get an opinion on these plugs? I have a vague idea, but I’m seeking clarity. Thanks!
Last edited by TheKodiak; Sep 3, 2025 at 10:57 AM.
Don Terrill… who has built qualifying engines (SBC’s) for NASCAR back in the day, and also a lot of drag race engines before that, says he would never gap a spark plug. Adjusting the ground strap will develop a fatigue zone that will make it more likely to break it during operation and it will chew up the combustion chamber, pistons, valves etc. If you have a lot invested in the engine, and need to make it to the end of the day, it’s proper risk management to get extra plugs and install the ones that have correct gap, and throw the rest away.
I can't believe that bending the ground strap 3 or 4 thousands is going to cause enough stress for it to break. Dan
I never had one break but I believe Don. He’s that smart. Asked ChatGPT and here’s the answer it provided:
Exactly — Don Terrell (Performance Ignition Systems, etc.) was spot-on there. The ground strap isn’t just a little piece of bent metal — it’s a precision welded, heat-cycled, stress-loaded component.
Here’s why his advice matters:
1. What Happens When You Bend the Strap
The ground strap is welded to the shell. That weld is designed to hold up under tens of millions of heat cycles.
When you bend or “re-gap” it, you’re creating work hardening and micro-cracks right at the bend radius. That area is already the hottest part of the plug.
Once fatigued, the strap is more likely to crack off under combustion load. In a mild street motor it might live — in a high-compression or boosted engine, it can be the kiss of death.
2. Why Plug Makers Still Talk About Gapping
NGK, Champion, Autolite all still say you can adjust gaps — because 95% of daily drivers are running tame engines where the risk is low.
In racing or severe service, they usually hedge: order the correct part number with the gap you need rather than tweaking. NGK and Denso both offer “race” plugs in specific heat ranges and pre-set gaps for this reason.
3. Terrell’s Point
His line is basically: “Don’t cheap out and bend straps. Select the plug with the right projected tip, heat range, and gap from the start.”
Especially in engines that run on the edge (nitrous, turbo, high-CR NA), a fatigued strap can let go and trash a piston faster than any other ignition failure
4. Real-World Scenarios
Drag/boost guys: Often run cut-back or side-fire ground straps (shorter, stronger, less heat load). If you start bending those, you just ruined the geometry that kept it safe.
Endurance racers: They stock exact part numbers in the right gap. Swapping plugs mid-event is safer than bending gaps.
Street builds: Yeah, you can still gap to spec. But if you’re anywhere near the edge, Terrell’s rule is the safe play.
Bottom Line
He’s right:
Adjusting = introduces fatigue risk.
Selecting the right plug = zero risk.
In mild engines, re-gapping is “good enough.” But in performance builds — especially where you’ve already seen strap failures — it’s smart to treat ground straps as no-bend, no-compromise parts.
I never had one break but I believe Don. He’s that smart. Asked ChatGPT and here’s the answer it provided:
Exactly — Don Terrell (Performance Ignition Systems, etc.) was spot-on there. The ground strap isn’t just a little piece of bent metal — it’s a precision welded, heat-cycled, stress-loaded component.
Here’s why his advice matters:
1. What Happens When You Bend the Strap
The ground strap is welded to the shell. That weld is designed to hold up under tens of millions of heat cycles.
When you bend or “re-gap” it, you’re creating work hardening and micro-cracks right at the bend radius. That area is already the hottest part of the plug.
Once fatigued, the strap is more likely to crack off under combustion load. In a mild street motor it might live — in a high-compression or boosted engine, it can be the kiss of death.
2. Why Plug Makers Still Talk About Gapping
NGK, Champion, Autolite all still say you can adjust gaps — because 95% of daily drivers are running tame engines where the risk is low.
In racing or severe service, they usually hedge: order the correct part number with the gap you need rather than tweaking. NGK and Denso both offer “race” plugs in specific heat ranges and pre-set gaps for this reason.
3. Terrell’s Point
His line is basically: “Don’t cheap out and bend straps. Select the plug with the right projected tip, heat range, and gap from the start.”
Especially in engines that run on the edge (nitrous, turbo, high-CR NA), a fatigued strap can let go and trash a piston faster than any other ignition failure
4. Real-World Scenarios
Drag/boost guys: Often run cut-back or side-fire ground straps (shorter, stronger, less heat load). If you start bending those, you just ruined the geometry that kept it safe.
Endurance racers: They stock exact part numbers in the right gap. Swapping plugs mid-event is safer than bending gaps.
Street builds: Yeah, you can still gap to spec. But if you’re anywhere near the edge, Terrell’s rule is the safe play.
Bottom Line
He’s right:
Adjusting = introduces fatigue risk.
Selecting the right plug = zero risk.
In mild engines, re-gapping is “good enough.” But in performance builds — especially where you’ve already seen strap failures — it’s smart to treat ground straps as no-bend, no-compromise parts.
Very nice write up! Thank you for taking the time. Dan
Very good information to grow on… Aside from that, the top plug, it’s gapped at 59, which is the most fouled. While the bottom one is at 52, least fouled and closer to specs. Can the spark gap difference cause the plugs to burn out faster? The motor is bone stock, minus the 1.6 rockers, and no tune. Is there a possible internal leak? Should I be concerned, and what would be the best gap to go with? Thanks again
I’d like to also add, I recently installed the egr valve back in. From my understanding, without the proper tune, the engine will run hotter and leaner. So I figured I’d put it back on. Keep the comments rolling, very go stuff!
They look pretty decent to me. Just a little carbon build up. The porcelain looks good. No extra oil making it in there. Plugs are cheap so like 30K miles on copper plugs is what I would run.
in my experience of over nearly 6 decades of engine fettling shows that gapping a new plug that needs it might take a try or two to get it perfect. this is not representative of cyclic loading, where stresses are induced over time by repeated bending leading to surface micro-cracking. there is no "metal fatigue" or "work hardening" accomplished when gapping a plug.
that being said, I simply check the gap these days and I find that gaps are usually spot-on.
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Spark Plugs
