Spark Plug Gap

[GTSpecial39]

I am going to put Motorcraft SP501 spark plugs in my 71 Mach 1 351 C M code and not sure what gap, I have seen .035-.044. Any thoughts? Thanks!!!

 

[Hemikiller]

Depends on your ignition system. If it's factory points type, .035".

IMO, precious metal plugs are a waste in our cars. They're designed for extended service intervals. Basic copper plugs are all you'll need.

 

[GTSpecial39]

Depends on your ignition system. If it's factory points type, .035".

IMO, precious metal plugs are a waster in our cars. They're designed for extended service intervals. Basic copper plugs are all you'll need.

I have a pertronix electtric ignition on it. If I go back to copper, what plug would you suggest? Thanks

 

[GTSpecial39]

I have a pertronix electtric ignition on it. If I go back to copper, what plug would you suggest? Thanks

And the gap..

 

[c9zx]

For that stock application I suggest Autolite Copper #24 at stock gap of .035. I agree with Hemi, the rare metal plugs are not well suited to this application. Chuck

 

[secluff]

Since you initially inquired about the Motorcraft plugs, I wanted to provide you with some additional info and do a shameless plug( ) for them. I'm sure you already know that the Motorcraft SP-501 is Ford part# ASF-32P which is a platinum plug, and the 32 is one step colder than the OE-installed plug.
Motorcraft is the only plug I have ever run in any of my cars, and I have never had a plug failure. The production-installed spark plug in your '71 351C is a Motorcraft ARF-42. They were service replaced by ASF-42. When Ford released the copper core plugs, they became ASF-42C, replacing the previous ASF-42. The current part number is ASF-42CX (SP-450X). When the platinum plugs were released as an alternative, ASF-42P was the Motorcraft number. The current part number is ASF-42PX (SP-502X). I agree that the copper core plugs are better suited for our '71-3 vehicles.
Many parts stores, such as O'Reilly, Auto Zone, Rock Auto, etc., sell Motorcraft. The "X" plugs are the latest in a series of number changes, so some parts stores and Ford dealers may have the plugs with or without the "X" suffice but still be the same part.
If the colder plug is what you want to run, the Motorcraft number for the copper core plug is ASF-32C (SP-415), the superseding number is ASF-32CX (SP-415X)

 

[GTSpecial39]

Since you initially inquired about the Motorcraft plugs, I wanted to provide you with some additional info and do a shameless plug( ) for them. I'm sure you already know that the Motorcraft SP-501 is Ford part# ASF-32P which is a platinum plug, and the 32 is one step colder than the OE-installed plug.
Motorcraft is the only plug I have ever run in any of my cars, and I have never had a plug failure. The production-installed spark plug in your '71 351C is a Motorcraft ARF-42. They were service replaced by ASF-42. When Ford released the copper core plugs, they became ASF-42C, replacing the previous ASF-42. The current part number is ASF-42CX (SP-450X). When the platinum plugs were released as an alternative, ASF-42P was the Motorcraft number. The current part number is ASF-42PX (SP-502X). I agree that the copper core plugs are better suited for our '71-3 vehicles.
Many parts stores, such as O'Reilly, Auto Zone, Rock Auto, etc., sell Motorcraft. The "X" plugs are the latest in a series of number changes, so some parts stores and Ford dealers may have the plugs with or without the "X" suffice but still be the same part.
If the colder plug is what you want to run, the Motorcraft number for the copper core plug is ASF-32C (SP-415), the superseding number is ASF-32CX (SP-415X)

I really appreciate the info, I do like learning.!! I posted a few weeks ago about popping in the exhaust when decelerating and knowing that my plugs have been in the motor for probably 5 yrs .Not sure what is going on with the popping but thought it couldn't hurt putting the new plugs in. I am probably going to stick them in and see what happens. I did have Autolite 25's in it.

 

[midlife]

Popping while decelerating usually indicates a timing issue. What's your initial timing and does your vacuum advance work correctly (you can suck on the hose and see if the breaker plate moves).

 

[delawarebill]

Autolite Copper #24 at stock gap of .035 is exactly what i used and gapped on my 351C. the only problem i had was install. can't see squat and do it by feel. took almost an hour. love small blocks.

 

[tony-muscle]

I gapped mine at 0.045 with a MSD CD ignition, NGK plugs and Duraspark style distributor. If stock then go with 0.035 as recommended.

 

[Stanglover]

I run my Autolite 24's at .045" with a Pertronix Ignitor II and matching coil.
Runs great at that.

 

[mrgmhale]

Depends on your ignition system. If it's factory points type, .035".

IMO, precious metal plugs are a waster in our cars. They're designed for extended service intervals. Basic copper plugs are all you'll need.

Once, about 22 years ago, I decided to try some platinum plugs in my 87 LSC. Imagine my disappointment when I got hardly 30 feet out the driveway when I started to feel significant misfire. Imagine how dejected I felt when I swap in some convention Motorcraft spark plugs and everything ran perfectly. The plug gap was about 0.035", so excessive gap was not the issue. I guess the spark was not fat enough to light off the A/F mixture. I was so bummed that I did not even pursue comparing the spark output between the two different plugs, visually.

Anyway, from that point on I decided to not use fancy platinum plugs, as conventional resistor plugs from Autolite or Motorcraft work perfectly well. That goes for our 69 GT500 as well as the two 73 Mustangs.My days of trying to get all fancy are over since that experience. None of them are driven competitively, I am not scared about performance at high RPMs. The stock stuff is perfectly fine.\\

 

[MaddMike]

I am going to replace my Autolite 24 spark Plugs (351C). They have only about 1,000 miles of use but the car sat in storage for 23 years but for occasional start-ups to keep the cobwebs out of it. I also purchased new plug wires (same reason) and a Pertronix Flame Thrower (40,000 volt) coil. The coil instructions talk about increasing plug gap to take advantage of the higher coil output. I don't want to burn a hole in a piston but I would like to take advantage of any potential increase performance. Can anyone suggest what to gap the plugs at with this info?

 

[Stanglover]

I am going to replace my Autolite 24 spark Plugs (351C). They have only about 1,000 miles of use but the car sat in storage for 23 years but for occasional start-ups to keep the cobwebs out of it. I also purchased new plug wires (same reason) and a Pertronix Flame Thrower (40,000 volt) coil. The coil instructions talk about increasing plug gap to take advantage of the higher coil output. I don't want to burn a hole in a piston but I would like to take advantage of any potential increase performance. Can anyone suggest what to gap the plugs at with this info?

See post #11 above. My plug wires are 8mm spiral wound copper.
Does your coil match your ignition source, i.e. Pertronix 1,2 or 3?

 

[basstrix]

I run mine at .040 on a 4v cc cleveland running the Duraspark I (red grommet) module and coil. I considered several factors including measuring the spark duration with an oscilloscope. The measures compared .035 to .050 gaps. Below is a table of plug gaps for different applications. I wish this listed compression ratio as that has some bearing on gap.

There's an episode of Engine Masters that looked at plug gap....I think they found diminishing returns as gap increased, but there was some benefit.

Don't worry about burning holes in pistons. An overly wide gap will misfire, rather than add too much heat to combustion chamber.

The plugs you use will have been made for a range of gaps...deviating from that range results in a ground strap that sits at an angle to the flat tip of electrode (assuming that's the type of electrode the plug has). The effect is to concentrate the discharges to the narrowest gap and wear those surfaces faster than if they were parallel. I doubt that's much of a deterrent, but worth mentioning.

 

[MaddMike]

See post #11 above. My plug wires are 8mm spiral wound copper.
Does your coil match your ignition source, i.e. Pertronix 1,2 or 3?

I had an old Jacobs Electronics unit I installed in 1999 - Remember, car was stored 23 years until it arrived here in Europe, last December - and it runs great BTW. Back then, it was a good unit. But obsolete so I am removing it. I have a Mallory Unilte and it performs well. The Jacobs unit has a very hot supercoil. So when I ditch it I will install the Pertronix Flame Thrower. New Plugs and new Taylor 8mm Spiro-Pro wires. The Unilte has barely 1,000 miles on it and runs great so I figure if it ain't broke. But the plugs and wires after 23 years - even in good storage conditions should be changed. And I just want to get rid of the Jacobs.

 

[Stanglover]

I had an old Jacobs Electronics unit I installed in 1999 - Remember, car was stored 23 years until it arrived here in Europe, last December - and it runs great BTW. Back then, it was a good unit. But obsolete so I am removing it. I have a Mallory Unilte and it performs well. The Jacobs unit has a very hot supercoil. So when I ditch it I will install the Pertronix Flame Thrower. New Plugs and new Taylor 8mm Spiro-Pro wires. The Unilte has barely 1,000 miles on it

Mike, I'm really no expert on this subject, I just went by what information I had from Pertronix. They say an increase of .005" above plug recommendation, but that also depends on the engine itself and what runs best. Mine seems to run best with .010" above or .045" gap. Plug color is good, no sign of burning or fouling.
I have no idea about the Jacobs unit, maybe other can shed light on this part of the question. As for matching the Mallory Unilite with the coil, that might need some research.
Other than that, I would think the rest should be all good.

 

[Hemikiller]

As you travel down the ignition rabbit hole, keep in mind that the Duraspark ignition systems of the 70s were designed to fire lean and EGR polluted mixtures. The large diameter cap was to prevent cross firing between adjacent terminals when used with large plug gaps.

If the car is running great, why get rid of the Jacobs? I had one of their Energy Team setups on my 70 Torino with a Duraspark distributor, it was a very good ignition.

With a Unilite, you're running a small diameter cap, so keep the gaps .045 or less. TBH, I gap all my plugs at .035", even the Mach 1 with the MSD 6AL / Blaster 2 / ProBillet setup. I'm not looking for every last 1/2 hp as a street driven car, and never found any noticeable differences when playing with different gaps.

 

[MaddMike]

Mike, I'm really no expert on this subject, I just went by what information I had from Pertronix. They say an increase of .005" above plug recommendation, but that also depends on the engine itself and what runs best. Mine seems to run best with .010" above or .045" gap. Plug color is good, no sign of burning or fouling.
I have no idea about the Jacobs unit, maybe other can shed light on this part of the question. As for matching the Mallory Unilite with the coil, that might need some research.
Other than that, I would think the rest should be all good.

Thanks Jeff, that is good info from Pertronix.

 

[MaddMike]

As you travel down the ignition rabbit hole, keep in mind that the Duraspark ignition systems of the 70s were designed to fire lean and EGR polluted mixtures. The large diameter cap was to prevent cross firing between adjacent terminals when used with large plug gaps.

If the car is running great, why get rid of the Jacobs? I had one of their Energy Team setups on my 70 Torino with a Duraspark distributor, it was a very good ignition.

With a Unilite, you're running a small diameter cap, so keep the gaps .045 or less. TBH, I gap all my plugs at .035", even the Mach 1 with the MSD 6AL / Blaster 2 / ProBillet setup. I'm not looking for every last 1/2 hp as a street driven car, and never found any noticeable differences when playing with different gaps.

Thanks for the info. As for the Jacobs unit - I never fully utilized at. Plus here in Czech Republic they are quite strict about Veteran Autos (Historical Vehicles) and they always mess with me about it. Actually, I am a headhunter / Recruiter and back in the mid-90s I did a search for Jacobs, in Texas. The VP of Sales gifted me a unit.