Choosing the right 1968 Mustang 302 spark plugs means checking more than heat range. If your engine currently uses an NGK WR5 and you want a colder replacement, thread size, spark-plug reach, cylinder-head design, ignition timing, and fuel mixture all matter.
The wrong plug can cause misfires, fouling, or engine damage. Before switching to Autolite 24, Autolite 25, or NGK TR6, you need to confirm whether those plugs actually fit your Mustang’s cylinder heads.
Quick Answer
For a stock-headed 1968 Mustang 302, do not replace an NGK WR5 with Autolite 24, Autolite 25, or NGK TR6 without verifying fitment. WR5 uses 18 mm threads, while those alternatives use 14 mm threads. Factory tune-up references specify Autolite BF-32 plugs and a nominal 0.034-inch gap. Modified engines may require different plugs.
What Replaces a WR5 in a 1968 Mustang 302?
The correct replacement for an NGK WR5 depends on your Mustang’s cylinder heads and engine setup. For an original-style 1968 Ford 302, the replacement must match the required thread diameter, reach, seat design, and terminal connection before heat range is considered.
NGK lists the WR5, stock number 2438, with an 18 mm thread diameter and 10.9 mm thread length. Its published catalog gap is 1.0 mm, but that does not override the Mustang’s specified installed gap. See the NGK WR5 manufacturer’s specifications.
Ford’s original 1968 Mustang tune-up information identifies Autolite BF-32 as the factory-style plug for the 302 V8. That provides a better starting reference than choosing a modern part solely because it has a colder heat-range number.
Three replacement paths deserve consideration:
- Stock or mildly modified engine: Use a manufacturer-confirmed equivalent to the original Autolite BF-32 or the correctly fitted WR5 heat range.
- Modified engine requiring a colder plug: Investigate an appropriate colder 18 mm design. Ford’s period performance catalog lists the BF-22 as colder than the BF-32, but its availability and exact application must be verified.
- Engine with replacement cylinder heads: Use the cylinder-head manufacturer’s specified plug dimensions and heat range. Some aftermarket heads use 14 mm plugs instead of the original 18 mm style.
Warning: Never install a spark plug simply because its heat range appears suitable. Incorrect thread diameter, reach, or seat design can prevent proper installation, damage cylinder-head threads, or create serious engine problems.
A colder plug transfers heat away from its firing tip faster. It may reduce the risk of plug-induced pre-ignition, but it cannot correct excessive timing advance, lean fueling, low-octane fuel, or overheating.
Best Colder Spark Plugs for a 1968 Mustang 302
The best colder plug is one that matches the cylinder head and operates safely under the engine’s actual load. A stock street-driven 302 usually does not need a colder plug merely because it has a performance exhaust, upgraded carburetor, or aftermarket camshaft.
Higher compression, sustained high-rpm driving, forced induction, or other performance changes can increase the need for a colder heat range. Even then, fitment comes first.
Best Plug Heat Range
A spark plug’s heat range describes how quickly its firing tip releases heat through the plug body into the cylinder head. It does not describe how powerful or hot the electrical spark is.
On an NGK plug, a higher numerical heat rating generally means a colder plug. For example, an NGK heat range 6 is colder than an NGK heat range 5 when comparing appropriate plugs within the manufacturer’s rating system.
The NGK spark-plug technical guide explains that the normal firing-end operating range is approximately 500°C to 800°C (932°F to 1472°F).
A plug below its effective self-cleaning temperature may accumulate carbon. A plug that overheats can suffer electrode damage or contribute to pre-ignition.
Ford’s 1968 high-performance parts catalog places the Autolite BF-22 below the BF-32 on its heat-range chart. Both belong to the period 18 mm plug family.
That historical listing shows why BF-22 deserves investigation for certain performance builds. It does not establish that every modern 302 should run that plug.
302 Mustang Fitment
Original-style 1968 Mustang 302 cylinder heads use the older large-thread spark-plug arrangement. Later Ford small-block heads and many aftermarket replacements can use different dimensions.
For an engine that has been rebuilt or modified, the car’s model year alone cannot establish the correct plug.
Before ordering, verify these fitment details:
- Thread diameter: Confirm whether the installed heads require 18 mm or 14 mm plugs.
- Thread reach: The threaded section must extend the correct distance into the cylinder head.
- Seat design: Check whether the head uses a gasket-style seat or a tapered seat.
- Terminal connection: Ensure the spark-plug wires fit the plug terminals correctly.
- Heat range: Choose the required thermal rating only after mechanical compatibility is confirmed.
- Ignition system: Match the installed gap to the distributor, coil, ignition configuration, and engine requirements.
If the engine uses original heads, do not assume Autolite 24, Autolite 25, or NGK TR6 will fit merely because they are associated with Ford V8 applications.
How to Match Plug Heat Range to Your 302
Match heat range by starting with the correct factory-style plug and evaluating the engine’s operating conditions. A colder plug should solve an identified thermal requirement, not serve as a general upgrade.
Compression ratio, engine load, ignition timing, fuel quality, induction system, and driving conditions all influence the decision.
Matching Heat Range
The most reliable approach is to establish a known-good baseline before changing plug temperature. Use the current cylinder-head specifications and a verified plug application as your starting point.
- Identify the cylinder heads. Confirm their plug thread size, reach, and seat requirements.
- Document the current setup. Record the installed plug model, ignition system, compression changes, and relevant engine modifications.
- Check ignition timing. Verify initial advance and the distributor’s advance behavior before blaming the plugs.
- Check the fuel mixture. Correct rich or lean operating conditions and confirm suitable fuel octane.
- Inspect used plugs. Look for abnormal deposits, excessive electrode wear, glazing, or melted components.
- Select a verified heat range. Use manufacturer guidance or an experienced engine builder’s recommendation for the exact combination.
- Monitor the result. Compare starting, idle quality, loaded operation, and plug condition after the change.
One colder heat-range step may be appropriate for some modified engines. However, the original suggestion to automatically move two steps colder is not a universal rule for a 1968 Mustang 302.
A colder plug also does not significantly cool the entire combustion chamber. Its main function is to control the temperature of its own firing end.
Signs You Need Colder Plugs
A colder plug may be justified when the currently fitted plug repeatedly overheats despite correct ignition timing, fuel mixture, cooling, and fuel quality. Heavy-load operation is another reason to reassess the original heat range.
Possible warning signs include:
- Excessive erosion of the center or ground electrode.
- Blistering, glazing, or damage to the ceramic insulator.
- Burned or partially melted electrodes.
- Repeated signs of excessive firing-tip temperature during sustained load.
- A modified engine combination that the plug manufacturer specifies should use a colder range.
Engine knocking is a reason to stop and diagnose the tune, but it does not prove that the spark plug is too hot. Excessive ignition advance, inadequate fuel octane, high combustion temperatures, and lean mixtures can also cause knock.
Likewise, black soot, carbon deposits, and rough idle usually call for a fouling diagnosis rather than an immediate move to colder plugs.
Signs Your WR5 Is Too Hot?
An overheated WR5 may show blistered ceramic, excessive electrode wear, or damaged electrode tips. These symptoms deserve attention because they can indicate temperatures high enough to threaten engine durability.
However, not every abnormal plug appearance means the heat range is incorrect.
| Plug or engine symptom | Possible cause | What to check |
|---|---|---|
| Blistered or glazed insulator | Excessive operating temperature | Heat range, ignition timing, fuel mixture, cooling, and plug installation |
| Burned or melted electrodes | Severe overheating or pre-ignition | Stop operating the engine and investigate the cause |
| Dry black carbon deposits | Rich mixture, weak ignition, short trips, or plug too cold | Carburetor, ignition system, and operating conditions |
| Wet oily deposits | Oil entering the combustion chamber | Valve seals, rings, and overall engine condition |
| Light gray or tan deposits | May be consistent with normal operation | Compare with other plugs and check for additional damage |
| Knocking under load | Detonation from several possible causes | Timing, fuel octane, mixture, temperature, and combustion deposits |
According to NGK’s technical guidance, spark-plug firing ends that stay below approximately 450°C (842°F) may accumulate carbon because they cannot clean themselves effectively.
That temperature is associated with fouling, not overheating. The original advice suggesting a plug is too hot above 842°F confused the self-cleaning threshold with the upper thermal limit.
Note: If you see melted electrodes, severe blistering, or persistent knock, avoid continued high-load driving. An experienced mechanic or engine tuner should check for a combustion or ignition problem before you install different plugs.
WR5 vs TR6 vs Autolite 24/25: Which Fits?
NGK WR5, NGK TR6, and Autolite 24/25 are not interchangeable just because they are discussed as performance spark plugs. Their thread sizes, reach, seat designs, and heat-rating systems differ.
For a stock-headed 1968 Mustang 302, those physical differences are more important than their advertised heat ranges.
| Spark plug | Key dimensions | 1968 Mustang 302 fitment |
|---|---|---|
| NGK WR5 | 18 mm thread, 10.9 mm reach | Matches the large-thread style; confirm the complete application |
| NGK TR6 | 14 mm thread, approximately 17.5 mm reach, tapered seat | Not a direct replacement for original 18 mm heads |
| Autolite 24 | 14 mm thread, short reach, tapered seat | Not a direct replacement for original 18 mm heads |
| Autolite 25 | 14 mm thread, short reach, tapered seat | Not a direct replacement for original 18 mm heads |
| Autolite BF-32 | Period 18 mm design | Factory-style reference for the 1968 302 |
| Autolite BF-22 | Period 18 mm colder performance design | Historical option; verify exact application and availability |
The NGK TR6 product specifications confirm its 14 mm tapered-seat design. The Autolite 24 specifications and Autolite 25 specifications likewise identify 14 mm tapered-seat plugs.
Within that Autolite family, the 24 is colder than the 25. However, neither is a verified direct replacement for an original 18 mm BF-32 or WR5 installation.
An engine with compatible aftermarket heads is a different situation. In that case, the cylinder-head manufacturer’s plug recommendations determine which models belong on the shortlist.
Set the Spark Plug Gap for Strong Spark
The factory-style spark-plug gap for a 1968 Mustang 302 is approximately 0.034 inch, with period specifications commonly giving a range of 0.032 to 0.036 inch. The correct value depends on the actual engine and ignition configuration.
The original article’s repeated 0.0205-inch recommendation is not the correct factory spark-plug setting. It appears to confuse spark-plug gap with the much smaller distributor breaker-point gap.
The 1968 Mustang tune-up specifications list a 0.034-inch spark-plug gap for the 302 V8, along with separate breaker-point settings.
| Gap | Effect | Use |
|---|---|---|
| 0.034″ | Factory nominal setting | Original-style 302 ignition |
| 0.032″–0.036″ | Factory-style specification range | Stock configuration, subject to the correct service specification |
| Outside specification | May increase misfire risk or ignition demands | Only when justified by the particular ignition and engine setup |
A smaller gap generally requires less voltage for the spark to cross, but that does not automatically create a more powerful spark. An excessive gap may cause misfires if the ignition system cannot provide enough voltage under load.
Use a proper gap gauge to measure the distance between the center and ground electrodes. Adjust only plug designs that the manufacturer permits you to regap, and avoid damaging the center electrode or insulator.
For a specialized performance engine, follow the plug manufacturer’s or engine builder’s specified gap rather than automatically applying the factory value.
Pro Tip: Keep spark-plug gap and distributor-point gap recorded separately in your maintenance notes. They control different parts of the ignition system and should never be treated as interchangeable measurements.
Check Timing Before You Change Plugs
Check ignition timing before changing spark-plug heat range because incorrect timing can cause knocking, poor throttle response, overheating, or fouling. Replacing plugs without correcting the timing can leave the real problem untouched.
Period specifications for the 1968 Mustang 302 list 6 degrees before top dead center (BTDC) as the factory initial ignition timing reference. The original article’s 10–14° figure is not the factory baseline for that configuration.
Some modified engines operate with different initial timing, but the correct setting depends on the full distributor advance curve, combustion characteristics, and engine combination.
For an original-style ignition system:
- Warm the engine to normal operating temperature.
- Identify the timing marks and the correct initial timing specification.
- Connect a timing light according to its instructions.
- Follow the appropriate procedure for disconnecting and plugging the distributor’s vacuum-advance connection.
- Check timing at the specified idle conditions.
- Inspect the distributor’s advance operation if the reading is unstable or the engine knocks under load.
If the engine still uses breaker points, inspect their condition, adjustment, and dwell as part of the diagnosis. Changing the point gap can affect ignition timing.
Excessively advanced timing can increase the risk of detonation. Incorrectly retarded timing can reduce performance and contribute to abnormal running conditions.
Correct timing first, then evaluate whether a colder spark plug is justified.
Fix Rich Running That Fouls Plugs
A rich fuel mixture can leave black, dry carbon deposits on your Mustang’s spark plugs. When too much fuel enters the combustion chamber, plug-tip temperature may drop and deposits can build up.
That can cause rough idle, difficult starting, misfires, and weak throttle response. Installing an even colder plug may make the fouling worse.
Common causes worth checking include:
- Carburetor adjustment: An excessively rich idle mixture or incorrect fuel-metering setup.
- Choke operation: A choke that remains partially closed after the engine warms up.
- Fuel pressure: Excessive pressure or a carburetor needle-and-seat problem that allows too much fuel into the bowl.
- Air restriction: A dirty air filter or restricted intake airflow.
- Weak ignition: Worn breaker points, deteriorated ignition components, or poor spark delivery.
- Low-temperature operation: Repeated short trips and prolonged idling that prevent the plug from cleaning itself.
Oil consumption from worn rings or valve seals can also foul plugs, although oil-fouled deposits may appear wet or greasy instead of dry and sooty.
Autolite’s spark-plug fouling guide identifies excessive fuel, oil, poor ignition, and overly cold plugs among the causes of recurring fouling.
After addressing the likely cause, inspect the plugs again under comparable operating conditions. A consistent improvement in electrode condition and engine behavior is more useful than judging one isolated plug reading.
If the engine continues to foul correctly specified plugs, investigate compression, fuel delivery, and ignition performance before choosing a hotter heat range.
Frequently Asked Questions
Is It Better to Run a Colder Spark Plug?
A colder spark plug is better only when the engine needs it. Higher compression, forced induction, or sustained heavy-load operation may justify a colder heat range. For a stock street engine, an unnecessarily cold plug can accumulate carbon and cause misfires. Always confirm fitment and correct underlying tuning problems first.
What Spark Plug Is Hotter, 5 or 6?
In NGK’s heat-range system, 5 is hotter than 6. A higher NGK number identifies a colder plug within the appropriate product family. Other manufacturers may use different numbering systems, so do not compare heat-range numbers across brands without an official conversion reference.
Does a Hotter Spark Plug Burn Better?
A hotter spark plug does not automatically improve combustion or create a stronger spark. It retains more heat at the firing tip, which can help prevent carbon fouling when the original plug runs too cool. If the plug becomes excessively hot, it can increase the risk of pre-ignition and engine damage.
How to Make a Spark Plug Run Hotter?
Choose a mechanically compatible plug with a hotter heat rating if the engine manufacturer or tuner confirms that it is necessary. Hotter plug designs retain more heat at the insulator tip. Tightening the electrode gap, installing a stronger coil, or changing ignition timing does not reliably substitute for the proper heat range.
Will Autolite 24 or 25 Fit a Stock 1968 Mustang 302?
No, Autolite 24 and 25 are not direct replacements for the original 18 mm spark-plug heads. They use 14 mm threads and a different seat design. They may be appropriate for certain replacement heads, but only when the cylinder-head manufacturer specifies compatible dimensions and heat ranges.
What Spark Plug Gap Should a 1968 Mustang 302 Use?
The factory-style nominal gap is 0.034 inch, commonly within a 0.032 to 0.036-inch specification range. This applies to the original-style ignition setup. Modified ignition systems or specialized performance engines may require a different setting, so follow the specification for the actual engine and plug combination.
Conclusion
For a 1968 Mustang 302 with original cylinder heads, the correct spark-plug replacement must match the older 18 mm fitment before you consider a different heat range. NGK WR5 and factory-style Autolite BF-32 provide useful reference points, while a colder historical option such as BF-22 requires application verification.
Do not substitute Autolite 24, Autolite 25, or NGK TR6 without confirming cylinder-head compatibility. Start with the correct nominal 0.034-inch factory gap, verify the original 6° BTDC initial timing specification, and correct fuel or ignition problems first. Choose a colder plug only when the actual engine setup and plug condition justify it.
Sources
- NGK/NTK Part Finder: WR5 spark-plug thread diameter, reach, and published product specifications for a listed 302 application.
- Ford 1968 High-Performance Parts Catalog: Period 18 mm Autolite spark-plug family and relative BF-32/BF-22 heat ranges.
- Maine Mustang 1968 Tune-Up Specifications: Original 302 plug reference, nominal spark-plug gap, breaker-point settings, and factory ignition timing.
- NGK Spark Plug Basics: Heat-range numbering, firing-tip temperatures, fouling, overheating, and tuning considerations.
- NGK TR6 Product Specifications: TR6 thread size, reach, and tapered-seat design.
- Autolite 24 Product Specifications: Thread diameter, seat style, gap, and heat-range information.
- Autolite 25 Product Specifications: Thread diameter, seat style, gap, and heat-range information.
- Autolite: Signs of Bad and Fouled Spark Plugs: Common symptoms and causes of carbon and oil fouling.
