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Spark plug tubes and spark plug replacement on a slant six engine

One 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.

I should have done this when I changed the plugs, but it was only at that point I realized they needed to be replaced - because 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 remove a spark plug. Well, the first thing to do is to get the gaskets. I special ordered a box from AutoZone; FelPro rubber gaskets set me back less than $5. You can see the part number in the photo, which is why I took a picture of the box. And hey, look, they were made in the United States! They must be old stock...

For the rank beginner, the first major step — getting the plugs out — presents a challenge, because of operating space. Three plugs are extremely easy to reach; two are moderately easy; and one presents a problem. In each case 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.

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

The next step is to get the gasket off the tube. This is generally easy — 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.

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 1974 or later 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.

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 recommendation until 1974 (also for the 1970s V8 engines; I didn’t check the Hemi) and starting in 1974, when they went to the tapered seat plugs, 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.

Ben Deutschman of the Slant Six Club of New York/New Jersey wrote that owners should be careful not to put on spark plugs with the "crush washer" (the loose metal washer at the end), but to take off the washer first, if they have a 1960-1974 engine. After 1974, the head was redesigned, the plug tubes were eliminated, and taper seat plugs were used. (Before 1975, as Ken Mayer pointed out, the tube acts as the washer.

For Valiants, the plug specified was Champion N147 through 1973, and Champion BL13Y from 1974 on; you might as well use the inexpensive Champions in favor of later, pricier Bosch plugs, which some people have actually had less favorable results with.

Most people who still have these engines now already know how to do such things; but there are always new owners and “haven’t done this in 25 years” owners, and I hope the reference is handy for you.

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