Spark Plug Installation


Here are a few quick notes on spark plugs. Print this off and keep it in your maintenance notes. I have a 3 ring binder that I keep in the top of my engine building tool box. In it I keep any data that I am not going to memorize. In my case, this is part numbers for things that we repeatedly order by phone, CC vs. compression ratio data, and research notes and test data.  Lots of stuff, like this plug data, I obviously have memorized, but the point is that well organized builders have notebooks and reference data, and it is a good habit to develop, especially if your workshop and home are not at the same place.

What plugs should I use? A common question. At our place, I often use Autolite 275s just to run engines on the ground. People have flown them, but the primary use I put them to is break in runs. They are in stock at most chain auto parts stores, and are often on sale for less than $1.50 each. I still like AC R44Fs for everyday flying. People have flown a giant variety of plugs, and the engine is not that sensitive to them with one exception: Do Not Fly an engine that will use 100LL fuel on platinum plugs. Other than this, make sure the plug you are looking at is the correct application. For many years the Bosch catalog listed for the Corvair a plug that was 3/16″ too long, and actually hit the piston head. If you are thinking of trying a different plug, go with one that people have already flown in a Corvair like yours, with the same carb and the same kind of fuel. For example, Woody Harris has a lot of flight time in his 2,700cc and later 2,850cc engines using Denso iridium plugs, part number IWF16-5359. His plane has an MA3-SPA carb and flies on 100LL. If you want to eliminate variables, use R44Fs, as they have proven to work well on the broadest variety of engine configurations.

How much torque do I put the plugs down to? This is a very important question. People used to cars with iron heads always overtorque Corvair plugs. The Corvair Shop Manual says you can use 20 pounds, and I have had new builders ask if 25 or 30 was ok, as they didn’t want them to “get loose.” If you routinely torque them that much on installation, they will get loose, because they are going to strip out of the heads. A much better number to work with is 7 to 10 pounds. I use 7 pounds more often than 10. After an initial ground run, I will recheck the torque.  The Corvair’s plugs seal by a gasket, and it takes almost no pressure on this to get it to seal. Don’t overdo it; it isn’t a lug nut on a diesel truck.

Above, final prep work on Lary Hatfield’s 3,000cc engine destined for service in his Zenith 750. I built the engine for him in our shop this week. It has all our Gold Systems and one of the Weseman’s Billet 5th bearings. After careful set up, the engine fired up after 3 seconds of cranking and laid down a flawless and smooth 1 hour break in run. Notice how short my personal plug wrench is. It is a 13/16″ plug socket with a hex top. I apply the torque with a cut down 12 point offset wrench that is only 4″ long. This arrangement fits in a small storage space. Because the wrench fits on up or down, it is very easy to use in confined spaces like the front two plugs without the Nosebowl removed. The bottle on the head is Champion plug lube.

What should I use for anti-seize?  There is only one substance you should ever put on any plug in an aircraft: Champion 2612. This is the only stuff that aircraft mechanics use. It is black graphite liquid with a tiny brush. It does a neat, controlled job. Over the years, I have seen a great number of planes of all types with plugs coated in silver anti-seize.  I have seen people apply it in the thickness one might better use to put peanut butter on a sandwich. Its brush is sized to apply it to diesel truck lug nuts, and the stuff is messy, and conductive. I have seen builders get enough on their fingers and on the ceramic part of the plug to cause a short, and make the plug boot slip off the plug. Stay away from it, get the real stuff. Aircraft Spruce sells small bottles that go so far that I am only on my second bottle in two decades of being an A&P. They cost less than $10.

What gap should I use? The ignition systems that I build are not too picky about it, but start with .035″. Measure it with a wire, and use an actual electrode tool to open the gap if required. Resist the temptation to pry the gap open with a screwdriver or a feeler gauge. As always, if you have any questions, give a call or write in.

Thank you,


2 Replies to “Spark Plug Installation”

  1. Does your suggestion not to use platinum plugs with 100LL go for all engines or just Corvairs? Does lead cause a problem with platinum? Great site, lot’s of interesting info here! Thank you!

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