A Tale of Two Spark Plugs……


“They were the best of plugs and they were the worst of plugs; It depended solely on which engine you screwed them in. Applications be damned, to hell with gaps! We’ll run what we like and blame the results on GM and William Wynne”

……The following story is true, the names have been changed to protect the guilty…………

Guy “A”, Sends several notes, calls etc. Reports his engine is running poorly after a number of hours. He is a technical kind of guy, interested in concepts like lean of peak operation, analysing performance, fuel additives, etc. We run through a pile of different possibilities. Thinking that it must be something that troglodyte ww did, he gets inside his distributor and starts changing stuff to see if he can cure an intermittent hard miss on climb out. It doesn’t do anything. (but mess up the distributor)

Along the way, It is discovered that one cylinder is blowing 50/80 on a compression test. This is now the new suspect for Guy “A.” He is sure of it. I carefully explain that a leaking exhaust valve will not produce an intermittent miss. I go in to explain that even if the engine had 40 over 80 on a differential compression test, this in no way is indicative that cylinder is producing 1/2 power.  Even if a cylinder indicates this low, I seriously doubt that any more than 2% of the flammable air fuel charge escapes past the leak in operation. Just think about how short a period of time the compression stroke is on an engine turning 3,000 rpm.

After all kinds of communication and tests, some how we stumble over spark plug gaps and Guy “A” casually says he gapped the plugs correctly at .025″. I ask why on earth he would do this? Guy “A” feels that this is the correct gap for reasons he cannot explain. A moment later he reveals that he has NGK spark plugs in the engine! He thinks they are great plugs. I agree, pointing out that I always liked them in two-stroke dirt bikes, but that has nothing to do with Corvair engines in aircraft. I ask how he decided which NGK to use, he tells me he tried several, and it ran best’ on the one he picked out. (I wonder to myself how badly it detonated when he found the plug that was too hot.)

Guy “A”  Complains that they don’t sell AC-R44F plugs in his local auto parts store. He goes on-line and finds out that the correct plug we tell people to use cost a whopping $1.44 a plug, far less than the incorrect ones he is using. He goes back and regaps his plugs while waiting for the correct ones, and Volia! intermittent miss gone (at least on the side of the ignition he didn’t mess with.) Today he sends me a picture of the incorrect plugs, he has 83 hours on them. The includes the opinion that they “look pretty good.” I point out that they are the worst looking plugs I have seen in a while. His return question is what is wrong with his motor that it makes the plugs dirty? HOW ABOUT THIS: ITS THE WRONG PLUG FOR THE ENGINE, AND IT HAD A GAP THAT WAS CORRECT FOR A LAWN MOWER.

Trick question: A guy tells you that he has been driving Corvairs for 25 years, he knows engines real well and he has had great results with NGK plugs in his Corvair. They are great to use right? WRONG. Don’t take advice from car guys. Show me the car guy who runs is car on 100LL all the time and cruises at 75 HP. A 1965-69 Corvair will break 100 mph on 75 applied HP. It is not valid data, it doesn’t apply. You can’t show me a land based corvair that only fills up at airports and always drives at 100 mph. 

Here is my major problem, at Guy “A”‘s airport, a lot guys have watched his engine do this intermittent miss on many, many take offs while Guy “A” has been ‘testing.’  Do you think that each and every one of them will now be made to understand that the root of all of Guy “A”‘s issues is caused because he willfully ignored the correct plug for his engine, and then made up a new gap? No way, I am sure that the lesson that each of these people learned is “Car engines suck, Corvairs are bad, they are promoted by a fool, Since guy “A” has a PhD in engineering, if he couldn’t figure it out, no regular builder could.”

My second major problem is with anyone who has an issue with their engine, and the very first thing they decide is “It must be something William did or sold to me.” Statistically speaking, the last 50 times some one jumped to this conclusion, it was something they were doing 49 times. I don’t mind questions, but I do mind the assumption that I don’t know what I am doing, especially when it comes from a person who isn’t even willing to follow my spark plug recommendations. Think it over, Civility is a two-way street. Would you like me if I went to your place of work and always started communications with the assumption that suggestions you made are not worth following and the work you produce is the root of all my issues. How long are you going to be civil with me?

I like teaching people what we have painstakingly learned. Mistakes are ok, we can correct them, but let me point out that this entire wild goose chase was avoidable. It was caused by an attitude problem, a perspective that asking some people on the net and guessing was technically just as valid as all the reasearch, testing and operational data that I have gathered since 1989. -ww

PS: Lest anyone think I am just picking on Guy “A”, let me share a story of Guy “B”. Guy  “B” is an old friend, and he owns a certified aircraft with a 150 hp Franklin, (Which has magneto ignition.) He is not an a&P , but does some Pilot preventive maintenance on it. When it is cool, it runs fine, but after a while, it runs poorly. Winter weather leads him to understand that he must have carb ice. This goes on for several flights, some of which almost end up as forced landings. Everything is considered the culprit, even the basic Franklin design. Steve Upson, old member of the ‘hangar gang’, and very stubborn mechanical detective makes it his personal mission to figure it out. After a long day, he asks the operator about the kind of work he did. He goes into how he cleaned the plugs and gapped them at .035″ Steve points out that this is more than two times the limit allowable. Owner chimes in that it works on all his cars. Steve asks how long he has had Bendix mags on all of his cars. Plug gaps corrected to .016″ plane flies perfectly.



4 Replies to “A Tale of Two Spark Plugs……”

  1. Thanks so much for your thousands of hours of dedicated research and attention to detail. Aviation is definitely the better for it. Have a great Christmas and new year.

  2. I enjoyed this. Small things can bite you. I am amazed how so many people can be so “ALL KNOWING”. I also hunt and reload cartridges for my rifles. If these folks were to reload like they work on engines, they would be dead very quickly! You gotta follow instructions.

  3. yes .016 is a great gap for performance Engines. from 75 to 1000 hp Air Cooled Engines run hot so to speak and Plug gap’s do get wider. Also air craft engines do not need to meet Federal Government Standards for emissions. Less gap less strain on the Electrical System.

    1. Greg, the only engines I have seen in 30 years of flying that the manufacturer recommended .016” for a gap were sparked by a Magnito. I just want Corvair builders to use my recommendations for plugs and gaps.

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