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 waster 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.\\