Re: 1965 Plymouth Belvedere I A990 Super Stock 426 Hemi 4-Speed

David C Hurd

On Jun 2, 2026, at 6:52 AM, Michael <493mike@xxxxxxxxx> wrote:

David, if you are using anti-seize you may be over torquing your plugs as it acts as a lubricant. It is not necessary in iron heads anyway.
Mike

On 5/27/2026 5:09 PM, 'DAVID HURD' via The 1962 to 1965 Mopar Mail List Clubhouse wrote:

There are also interior modifications that are not period correct. No factory super stick came with a roll cage. I do not recall any back then.

Here is a question for any who have a 426 Max Wedge Engine.

I know this is a lot to soak in but here goes-

My 1962 Dodge has a 413 that has been bored out to 426. The entire engine has been upgraded. Crane roller cam, I purposely had the compression set at 10:1. To be able to run on lower octane premium.

A few years ago I lost the porcelain insulator on #1 plug. Replaced the plugs (Campion J12YC copper plus).

I hadn’t changed the plugs since as it runs perfectly with only a few hundred miles on them.

I hadn’t run the vehicle several months while helping my wife through cancer treatments.

I started it this Saturday and it started right up, few revelations and it’s running. Runs extremely well. I would take it anywhere.

It was running well, so I took it on the freeway and opened it up. Great power. After it decelerated. I had a familiar miss and it blew out another spark plug insulator. I replaced the plug and again it runs fine.

Here is my research on the problem and I think I’m good. It has had pre-ignition in the past and I run some JP 109 octane to reduce that.

I’m thinking the plugs being a few years old may have a weak point. Im going to put a new set of plugs in it. And thinking a different heat range may be needed.

Here are my engine Specs.

Motor date stamp is 19/12/62

Casting #1852029

Max Wedge 413 bored to 426

Cross Ram intake manifold

HP 563 @ 4600 rpm

Torque 545 @ 4600 rpm

Compression 10:1

Cam 284/296, 507/510 lift

Extreme high performance

Valves S.S. 214 intake, 1.81 exhaust with Crane Roller Rockers

The following is the research I have done:

Spark plug porcelain (insulator) blowing out or breaking on a 426 Max Wedge is a serious symptom, often tied to extreme cylinder pressures, detonation/pre-ignition, or installation issues in this high-performance big-block Mopar engine.

The 426 Max Wedge (1962–1964) was a factory race engine with high compression (often 11:1 to 13.5:1 depending on the stage), large ports, and aggressive tuning. It’s sensitive to setup, and problems like this point to combustion issues rather than just a bad plug.

Common Causes

• Detonation (knock) or pre-ignition: This is the top suspect in high-compression Wedge engines. Uncontrolled combustion creates shock waves and extreme pressure spikes that can crack or eject the porcelain insulator. Signs include black specks on plugs, melted electrodes, or aluminum particles.

• Causes: Advanced ignition timing, lean mixture, low-octane fuel, hot spots (carbon buildup, wrong plug heat range), overheating, or high ambient temps/load.

• Max Wedge engines from the era were often run on high-test leaded fuel; modern pump gas (especially without additives) makes this worse.

• Wrong spark plug heat range: Too hot a plug retains heat and promotes detonation/pre-ignition. Too cold can foul but is less likely to blow porcelain.

• Improper installation/torque: Under-torquing lets the plug loosen and vibrate (leading to thread damage or ejection). Over-torquing can crack the insulator. The Max Wedge uses 14mm plugs with 3/4” reach, gasket seat (typical for these heads). Torque is usually in the 15–25 ft-lbs range for cast-iron heads—use a torque wrench and anti-seize.

• Other factors: Carbon tracking, faulty ignition (weak spark, bad wires/distributor), mechanical issues (low compression in one cylinder allowing cross-fire or uneven combustion), or defective plugs.

Recommended Spark Plugs for 426 Max Wedge

Factory/recommended options (Champion numbers were common):

• Stock-ish: Champion J9Y, J10Y, J11Y, or J12Y (colder for more aggressive use).

• Modern equivalents: NGK equivalents in the 6–7 heat range (colder side for performance), or Champion C61CX/C59CX for higher compression builds.

• Gap: Typically .025”–.035” — check your setup.

Match to your compression, timing (often 25–35° total), fuel, and use (street vs. strip). Colder plugs (lower number) for race/high-load.

What to Do Next

1. Inspect all plugs — Look for signs of detonation (specks, pitting, melted tips) vs. just one failed plug. Check compression and leakdown on affected cylinders.

2. Address root causes:

• Verify total timing (initial + mechanical advance) — pull some if detonating.

• Ensure proper jetting on carbs (Max Wedge often ran dual AFBs or similar) — lean conditions are deadly.

• Use highest octane available + lead substitute if needed.

• Check cooling system, thermostat, and for hot spots.

3. If porcelain debris fell into the cylinder:

• Do not start the engine if possible.

• Remove with vacuum, compressed air (carefully), or borescope/magnet. Crank with plugs out (fuel disabled) to blow out pieces. Small bits may exit via exhaust, but large ones can score walls/pistons.

4. Prevention:

• Always torque plugs properly (hand-start to avoid cross-threading).

• Use quality plugs and fresh wires.

• Tune conservatively — these engines make big power but need respect.

• Consider modern upgrades like better ignition (MSD) or aluminum heads if it’s a frequent issue.

If this is a fresh build or modified (e.g., higher compression, cam, etc.), the tune is likely the culprit. Share details like compression ratio, timing, fuel, plug brand/heat range, and symptoms (misfire? Knock? When it happens) for more specific advice. A good Mopar specialist or dyno session would help dial it in safely. These are legendary engines—fix the root issue and it’ll run strong.

After sucking all this in. If any of you have experience or and recommendations, all are welcomed.

David C Hurd

On May 27, 2026, at 1:08 PM, Jimmy Peavy <peaver63@xxxxxxxxx> wrote:

those valve covers are not 1965

On Wed, May 27, 2026 at 12:18 PM William Jordan <wajordan56@xxxxxxxxx> wrote:

Spend some of your children’s inheritance!

https://www.hemmings.com/listing/1965-plymouth-belvedere-1-costa-mesa-ca-763351

Sent from my iPhone

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Re: 1965 Plymouth Belvedere I A990 Super Stock 426 Hemi 4-Speed - Hemmings.com