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Spark plug tubes, choices, and replacement on the slant six engine

The slant six engine went into production in 1959 for the 1960 model year, and was finally dropped in 1991, last used in cars in 1983 and in U.S. trucks in 1987. During that time, some changes were made with serious repercussions for spark plug installation.

Spark plug washers: only to be used on slant sixes from before 1963, after 1974

old spark plug tube  gasketMost notably (and thanks to Ben Deutschman of the Slant Six Club of New York/New Jersey for pointing it out), 1963-74 engine owners should not put on spark plugs with the "crush washer" (the loose metal washer at the end of the plug). When buying plugs wtih preinstalled washers, they should take off the washer first, if they have a 1963-1974 engine. Owners of engines made during these model years should also replace the plug tube seals when changing the plugs.

Before 1963, the crush washers were needed. Then, in 1963, the slant six heads were redesigned, with a new relief at the top of the spark plug hole. According to Daniel Stern, the spark plug tube acted as a seal with this design; using the crush washer moved the electrodes so they're incorrectly placed in the chamber, while interfering with heat transfer from the plug to the head.

After 1974 (starting in 1975), the head was redesigned, the plug tubes were eliminated, and taper seat plugs were used. As part of the 1975 change, if you plan to replace lifters, you must do it while the heads are off.

Spark plug choices

In 1997, Dan Stern recommended Autolite 56 (nonresistor), 66 (resistor), or AP66 spark plugs for the pre-1975 slant six. Since that first recommendation in 1997 or so, he wrote:

The Autolite 925 works much better, I've learned in the meantime. It usually has to be specially ordered, but its extra-long electrodes put the spark closer to the center of the combustion chamber and away from a quenchout area. These plugs were originally designed for engines with difficult combustion conditions (stratified charges, extra-lean mixtures, etc.) such as the emission-controlled AMC 232 and 258, and Chrysler's own 1981-83 318 TBI engine in the Imperial. [Again, this is for 1960-74 heads.]

More recently, in April 2010, Dan wrote:

Champion plugs were original equipment, but their quality has declined (never increased) in fits and starts as the line has changed owners again and again. Their present corporate parent is cheapening all their parts rapidly and dramatically.

The NGK ZFR5N spark plug is a better quality plug than Champion makes, and has a special electrode configuration that improves driveability and economy. The electrodes are extra-long, so the spark point is moved away from the quenchout areas and more towards the centre of the combustion chamber where the mixture is less likely to be difficult to ignite. That's the whole reason why these extended-electrode plugs were developed in the late 1970s, to reduce misfiring. ... They've been used since the early 1990s on 4-litre Jeeps, 3.2 and 3.5 litre Chrysler V6s, etc. I use them in my slant-6s, in my 2.2/2.5 engines, I used them in my 318 1989 Ram, all to great effect. [These are, again, only for 1960-74.]

And in August 2011: “The catalogue recommendations for the ’75-up head suggests a plug that runs much hotter than necessary with today's clean fuels. You can buy yourself quite a bit of ping-resistance without running into plug fouling if you will use an NGK #5 or even a #6 rather than a #4 (in the NGK number system, lower number means hotter plug). The regular normal plug would be a UR-5 or UR-6; the Platinum would be UR5GP or UR6GP, etc."

Installing the slant six spark plugs and tubes (1974 engine)

fel-pro gasketsOne of the more interesting aspects of auto mechanics is watching a fairly simple, small repair get larger as one tries to figure out how to get to the components involved, or traces down a problem to its root cause. In this case, the problem is stemming oil leaks on a 1974 slant six engine whose internals are all original. After going after the biggest offender, the valve cover gasket, we attacked the spark plug tube seals, partly because it’s easy to do.

A couple of the tubes came right out, and the gaskets had no pliability at all. They’d practically fused to the aluminum tubes. This made their oil sealing properties somewhat suspect, and the plugs were covered in oil.

The first thing to do in this job is to get the gaskets. I special ordered a box from AutoZone; a set of six FelPro rubber gaskets set me back less than $5. Dan Stern wrote that the NAPA part number is B45384 — they sell them individually so order six. The FelPro ES12794 is sold in boxes of six. And hey, look, they were made in the United States!

For the rank beginner, the first major step — getting the plugs out — presents a minor challenge, because of operating space. Three plugs are easy to reach; two are not so bad; and one is a problem on air conditioned cars. You need a standard socket driver, an appropriate sized spark plug socket, and an extender. It helps to have extenders of varying sizes (mine are roughly one inch, two inches, four inches, and six inches). You can stick them together to make a longer extender in this case; you won't be applying that much torque.


That last plug is actually the first plug if you're going from the grille to the cab; it’s the one right up front underneath all the hoses and wires, next to the alternator, if you have a 1974 model with air conditioning, like I do. Where do you fit the wrench? You can do the goofy thing, which is put it between the two big antifreeze hoses, or you can do the smart thing, which is extend it out just beyond the alternator, but not so far that you'll hit the fender.

wrench fitting

Once the plugs are out, you can take the tubes out (on applicable years). Any moderate resistance can be overcome by wiggling the tube from within, which knocks it free. Finger pressure should be enough.

The next step is to get the gasket off the tube. They come out without any resistance in my experience, unless the valve cover gasket is pressed against them. This was the case for the final tube, and I gave up and left it there rather than compromise the valve cover gasket. Next time the valve cover comes off, I'll do that tube.

original spark plug tube in place old gasket

The gaskets come off the tubes pretty easily - you just have to push them down as I did in the illustration at top. They require some effort, but if you go nuts you can bend the aluminum of the tube (I didn’t!), so don't go nuts, just keep at it and push down the part of the gasket that sticks out around the tube. Then you make sure the surfaces are both clean and undamaged, put the new gasket on, and when you replace the spark plug, it'll make the seal.

Ideally you will also have a torque wrench to get those plugs in with exactly the right amount of pressure. And, if you have a 1963-1974 slant six, make sure you remove the little metal gaskets from the ends of the spark plug bolty-bits - because you don't use them and if you put them in, it will cause problems (see the top of the page explanation).

Michael Buddenhagen wrote: I have a 225 Slant Six that’s 35 years old and recently used this procedure. The harder to reach sparkplugs can easily be done with a swivel adaptor. Also, some of the plug tubes on my engine were pretty hard to get off, but if you use a hook pick or allen wrench you can easily pull them out via the spark plug hole in the back of them.

If you are a rank beginner, you should probably read a standard guide to changing spark plugs. To dash through the rules, put a small dab of antiseize stuff (it comes in little tubes at any auto parts store) onto the start of the spark plug threads so you can get the plug off again, and read the box for torqueing instructions, unless you were smart enough to get a good 3/8 inch torque wrench, in which case use torque specs from the service manual.

According to my manuals, Chrysler used a 30 foot-pound tightening recommendation until 1974; in 1974, when they went to the tapered seat plugs, they went to 10 lb-ft. The spark plugs all had a gap of .035 inches, but like many people with healthy ignition systems I raised it slightly to .040 for slightly higher (I hope) efficiency.

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