Q&A / Tech

Ask Away with Jeff Smith: Choosing Spark Plugs for Chevy ZZ Crate Motors

What type of spark plugs do I use on my ZZ3 motor? I know that there are a lot of different spark plugs but what type do I use in my engine? I have some plugs that have a lot of threads and I have a set with short threads. Which ones do I use?

Jeff Smith: Each spark plug manufacturer uses a specific set of numbers and letters to designate individual spark plugs. You almost need a code book to decipher their meaning. We won’t get into that here because it could easily fill a book. I will break down what type of spark plug you need and then give you several part number recommendations.

Your ZZ3 engine uses the same aluminum heads as the mid-80’s Corvettes using the L98 engine. These engines used a stock AC plug with part number MR43LTS, but let’s get into an actual description of this spark plug so you can then choose from any number of different brands of plugs–in case you have a favorite.

This is the type of spark plug you will need for your ZZ small-block. Most stock small-block cylinder heads take this 0.704-inch thread reach plug with a tapered seat and a projected nose. This happens to be a Bosch copper center electrode plug.

This is the type of spark plug you will need for your ZZ small-block. Most stock small-block cylinder heads take this 0.704-inch thread reach plug with a tapered seat and a projected nose. This happens to be a Bosch copper center electrode plug.

Most modern spark plugs use a 14mm thread, which is what your heads use. As you have pointed out, the length of the threads is a critical part of the application. Your engine uses a spark plug with a reach or thread distance of 0.708-inch. This is the amount of thread that comes into contact with the cylinder head. At the top of the threads, this spark plug uses a tapered seat to seal the combustion pressure. Most aftermarket aluminum heads use a longer 0.750-inch thread with a gasket while most OE spark plugs seal with this tapered seat. Now let’s get to the business end of the spark plug.

The ZZ3 engine uses a projected nose design which means the plug’s active, center electrode protrudes past the end of the threaded portion of the plug and extends farther into the combustion chamber. This also requires the ground electrode to be longer to extend over the end of the center electrode. Projected nose plugs are useful for stock type engines as they place the spark closer to the center of the chamber for better combustion efficiency. The gap between the ground and center electrode will be about 0.035-inch, which is fine for your engine.

Now we can get into the spark plug’s heat range. Stock and mild performance engines use heat range plugs which tend to be on the hot side of the heat range chart. This is intentional so that carbon deposits will burn off rather than adhere to the spark plug. A hot spark plug can be identified visually because there is a deep gap from where the ceramic center of the spark plug contacts the steel body of the plug. If that depth is very shallow, that constitutes a cold heat range spark plug.

Besides the AC MR43LTS part number, I found several other plugs that are direct conversions including the Champion RS12YC, an Autolite 103, the Bosch +14 which is also called an HR8BC. This should be sufficient to find a good plug for your engine. If you are considering a colder race plug perhaps for nitrous, I’d suggest an Autolite AR103 that is about 3 steps colder.

You might also be considering some of the more exotic, “boutique” spark plugs with wild variations on plug gap , multiple ground straps, and the like. We’ve tried many of these plugs in performance automotive engines and never seen any advantage over a standard, single ground strap spark plug. There may be durability advantages to the “fine-wire” plugs that use platinum or other exotic metals. We’ll leave that choice up to you.

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  1. Chuck Zirkle says:

    I have installed a used ZZ# engine in my street rod.

    when I start it up it sounds like a rod knock.

    it goes away at this and then comes back.

    I’ve been told that the type of pistons in these engines produce a piston “slap”.

    is this true??

  2. John Fletcher says:

    You should NOT have used this picture of a conventional spark plug with SHORT threads when referring to the MR 43LTS (the “L” in LTS is for long threads = .75″ on 14mm plugs). The threads for many of the ZZ blocks (with Vortec heads) require a longer spark plug as the heads are taller.

  3. David Parker says:

    Considering changing plugs in my 84 C$ ZZ3 Stroker. Car runs rather hot and I am swapping out stock fan for twin high output fans. I recently upgraded to flame thrower module in the HEI, along with new cap, rotor and 50,000 volt coil. It ran pretty well, for about 30 miles and believe the module went out — all the above from Petronix…they are in process of reviewing possible module failure. Now, how about plugs??? Thoughts? Piston removed due to bent rod was a 14095478 — engine rebuilt about 4-5 years ago at a sloppy Chevy dealer. Probably not much beyond 10K miles on this engine

  4. Joseph Gomez says:

    Hello Mr. Smith,

    My name is Joseph and I have a question regarding the a spark plug gap.
    I have a ZZZ crate motor and the L98 heads.
    The Spec sheet for the motor calls for a FR5LS AC Delco plug.
    What should the gap be?

    Thank you,

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