When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Will I have a cross-fire problem with the way my spark plug wires are arranged?
At the distributor I have installed and arranged my car’s S/P wires as shown in the 67 AIM (pictures 1 and 2). Edit >>> I forgot to mention (if it matters) that my car is a 67 427 L36 with a radio. Picture 1 - taken from the 67 AIM Picture 2
.
.
While the NCRS may have some evidence that supports the S/P wires being arranged in the rubber grommet as shown in picture 3, I have chosen to arrange my S/P wires as shown in pictures 4 and 5.
Picture 3 - NCRS arrangement Picture 4 - My preferred arrangement Picture 5 - my preferred arrangement
.
.
I was recently made aware of a cross-fire issue running S/P wires #5 and #7 in contact with each other but I admit that I do not know if this issue happens BEFORE or AFTER those S/P wires exit the rubber grommet. Before I fire up the engine again, I want to do what I can to ensure I won’t have this cross-fire problem with my S/P wires arranged as they are.
.
Here is how I currently have my S/P wires arranged in the rubber grommet. Could this arrangement result in that cross-fire problem?
Last edited by CADbrian; Apr 17, 2026 at 12:01 PM.
Reason: clarification added
With reasonably good spark plug wires (and yours appear to be), crossfiring is very rare. It is far more probable that carbon tracing occurs in the distributor cap or rotor. Look inside the cap for traces of what appears to be **** or smoke and do the same for the rotor. In your case I would be more concerned with #7 and #1 which both are close to the primary coil connections --- especially if it's possible for the primary terminals to nick the wires or boots --- so any clearance you can add there will be good. It might be wise to check the resistance of each wire and if, for example, one wire shows very high resistance relative to the others then that can easily lead to arcing within the cap as that path might be lower in resistance than through the bad wire --- and that becomes the very definition of crossfiring. Similarly (though rare) a spark plug with very high resistance can do the same thing.
Last edited by acstephenson; Apr 17, 2026 at 11:34 AM.
It's helpful to understand that the spark energy will follow the path of least resistance. This would normally be through the core of the wire, through the resistor in the plug (presuming resistor-type plugs) and across the gap in the spark plug.
Insulation on the wire, if it's of good quality and in good condition, will prevent the spark from instead breaking through the insulation to anything adjacent.
If a leaky insulator does permit arc-through, odds are it will occur within the more lengthy portion of the wire that is within the braided shield (which is a dead short to ground) than in the relatively short portion that is unshielded.
That energy is far less likely to break through the insulation on both the wire in question and an adjacent wire, and then pass through the series (additive) resistance of the adjacent wire's core, the resistor in that spark plug and the gap across that spark plug, than it is to go where it's supposed to or anywhere else.
If your wires and plugs are of high quality and in good shape, and the plugs are properly gapped, the insulation on the wires should be able to contain the spark energy with no trouble.
Hey Brian,
Your NCRS 4-square placement is from GM (in the AIM). GM also put the #5 wire, in the 4-across valve cover holder, in the forward most fingers and #7 in the rear most fingers. I don't know how spark plug wires of 1967 compare to what's available today, so maybe it's a moot point and a rare occurrence.
It's helpful to understand that the spark energy will follow the path of least resistance. This would normally be through the core of the wire, through the resistor in the plug (presuming resistor-type plugs) and across the gap in the spark plug.
Insulation on the wire, if it's of good quality and in good condition, will prevent the spark from instead breaking through the insulation to anything adjacent.
If a leaky insulator does permit arc-through, odds are it will occur within the more lengthy portion of the wire that is within the braided shield (which is a dead short to ground) than in the relatively short portion that is unshielded.
That energy is far less likely to break through the insulation on both the wire in question and an adjacent wire, and then pass through the series (additive) resistance of the adjacent wire's core, the resistor in that spark plug and the gap across that spark plug, than it is to go where it's supposed to or anywhere else.
If your wires and plugs are of high quality and in good shape, and the plugs are properly gapped, the insulation on the wires should be able to contain the spark energy with no trouble.
Live well,
SJW
I agree that insulation breakdown seems extremenly unlikely if the cables are in good physical condition. The solid insulation on a single wire is usually sufficient to prevent breakdown. For one cable to break down to an adjacent cable would require the solid insulation on both wires to break down in the same location, despite the fact that the adjacent wire is not grounded. This is a very unlikely scenario.
As an electrical engineer, I have always assumed that crossfire between two adjacent ignition cables was due to electromagnetic induction rather than insulation breakdown.
If two wires are run in parallel for a long distance, an impulse current in one wire will induce a corresponding voltage in the adjacent wire (this how transformers work, coupling energy from one wire to a separate wire from which it is fully insulated). Good induction coupling requires that the two wires run in parallel, very close to each other, for a long distance. Cables that cross at right angles have almost no induction coupling at all, and cables that are far apart (say, one or more inches) have very little induction coupling.
I suspect that GM's routing scheme for plug wires was to minimize induction coupling by avoiding long parallel cable runs for cylinders that were close in the firing order. I've never looked closely enough at GM's routing to verify this, but I suspect that is what I would find.
On my 1966 Chevelle, the #5 and #7 wires are routed next to each other, but the valve cover loom has a large separation of those two (#5 & #7), and ONLY those two…
On my 1966 Chevelle, the #5 and #7 wires are routed next to each other, but the valve cover loom has a large separation of those two (#5 & #7), and ONLY those two…
perhaps GM separated them for a reason…
I looked up the firing order and it is 1-8-4-3-6-5-7-2. I think the biggest risk would be for a cylinder that is firing to inductively couple to the cylinder that is next-in-line to fire. The next-in-line cylinder has a fresh fuel charge and the piston is starting to compress the charge. A premature spark would cause pre-ignition.
A premature spark on a cylinder that is on its exhaust stroke would not cause a problem.
If we look at just the pairs of cylinders that are directly in sequence, we have 1-8, 8-4, 4-3, 3-6, 6-5, 5-7, 7-2, and 2-1. Most of these pairs are on opposite sides of the engine, but 8-4 are both on the passenger side and 5-7 are both on the driver side. So, these are the pairs in the firing sequence that run the longest distance in parallel.
On the passenger side, cylinders 8 and 4 have cylinder 6 between them, which may help to keep them separated. On the driver side, cylinders 5 and 7 are immediately adjacent. So, in terms of potential inductive crossfire, it appears that cylinder 5 coupling into cylinder 7 presents the highest risk.
That may be why GM made an effort to provide extra separation between those two wires.
This is all conjecture on my part, but my conjecture is based on the physics of insulation breakdown (unlikely) and electromagnetic induction (plausible). Either of these two mechanisms could lead to crossfire, but if the cable insulation is in good physical condition, insulation breakdown is very unlikely.
None of the AIMs that I have ('67, '69, & '72) clearly show all aspects of wire routing; each shows something the others don't. Maybe the '66, '68, or '70, '71 AIM show these better. Most of this is in the U69 section in each book.
From the '67 AIM, this shows the shielded wire pairs under the valve cover bolts. You can see the crisscross of #5 & #7 in the square rubber holder, but the straight-4 holder has them next to each other.
The '69 AIM best shows the placement of what wire number goes in which hole of the rubber squares - this is for both the L-36 and L-71.
From the '72 AIM, you have to trace the black, striped, white wires to see placement in the straight-4 holder. This is the LS-5, which was quite similar to the L-36.
None of the AIMs that I have ('67, '69, & '72) clearly show all aspects of wire routing; each shows something the others don't. Maybe the '66, '68, or '70, '71 AIM show these better. Most of this is in the U69 section in each book.
From the '67 AIM, this shows the shielded wire pairs under the valve cover bolts. You can see the crisscross of #5 & #7 in the square rubber holder, but the straight-4 holder has them next to each other.
The '69 AIM best shows the placement of what wire number goes in which hole of the rubber squares - this is for both the L-36 and L-71.
From the '72 AIM, you have to trace the black, striped, white wires to see placement in the straight-4 holder. This is the LS-5, which was quite similar to the L-36.
.
Owen, for days I studied magnified illustrations from the 67 AIM and traced all the S/P wires back to the distributor through the 4 hole rubber grommets, for days... Eventually, I threw my hands in the air and said "I'm gonna do the best I can and go with it." It won't be much longer before I can fire up my car again and I can report back to the forum HOW I have my S/P wires and if I experienced any problems such as a cross-fire.
If you've ever seen how the old flathead Fords were wired, you wouldn't give this a second thought. The wires were stuffed into a tube which ran along each cylinder head extending to the distributor. It would be difficult to get longer completely parallel wire runs using unshielded cables with much poorer insulating quality and typically copper cores for plenty on inductance coupling and they never had a problem. The only crossfire situations I've ever seen originated within the distributor cap. I would say run your wires as close to the AIM as you can get for NCRS purposes and declare victory.
The question posted was in reference to a BBC, with braided shields over the plug wires. Any leakage would most likely end up arcing to the shields rather than to the core of an adjacent wire, and the shields would prevent inductive coupling.
A SBC with the hockey stick shields presents a different scenario, but I've never heard of it causing a problem, and GM obviously wasn't too concerned about it.
The question posted was in reference to a BBC, with braided shields over the plug wires. Any leakage would most likely end up arcing to the shields rather than to the core of an adjacent wire, and the shields would prevent inductive coupling.
A SBC with the hockey stick shields presents a different scenario, but I've never heard of it causing a problem, and GM obviously wasn't too concerned about it.
Live well,
SJW
Yes, I overlooked this detail in my earlier response. It would be virtually "impossible" for one shielded wire to arc over to an adjacent shielded wire, through two layers of shielding, if the shields are grounded. So, for the case of insulation breakdown, which was already unlikely even without shielding, the shielding makes this scenario even less likely.
I think the primary risk for crossfire among the cables is induction. That risk can be lowered by carefully minimizing long, close proximity, parallel cable runs for cylinders that are adjacent in the firing order. For the case where these long parallel cables are shielded, the shielding will likely reduce the inductive coupling, but the analysis of that scenario is complicated.
Will I have a cross-fire problem with the way my spark plug wires are arranged?
