Spark plug gap size, 216 Autolite

Tom's36coupe

Tom's36coupe

Well-known member
I have been running with a gap size of 35 in my Autolite 216 plugs on my Edelbrock aluminum heads, any advantaged or problem with going bigger like 40 or 45? I have a 12v system with Mallory electronic distributor and 40k volt coil. Thinking bigger spark is a good thing, especially with they way Flatheads combustion chamber is???
 
i dont think 40k volts will handle .040 or .045 gap, but it might in a ford flat head, the higher the compression ratio the more volts needed to jump the gap , i think 50k is the min needed to handle those gaps, but your right the wider the gap the better the combustion will be because of the longer spark, giving the fuel charge more spark exposer
 
Anything more than about 25 could cause tracking in the dist. cap on the old crab cap. If thats what your using? The old flathead will run just fine at .025.
 
Heres my take on the plug gap discussion.

Spark plug working gap is determined by the combustion chamber design, the compression ratio and the actual air fuel mixture. NOT the ignition system itself.
A stock engine at .025 with correct air fuel mixture and a fresh set of spark plugs would need about 5000-8000 volts to ignite the air fuel mixture. If the coil has 20,000 volts available it would leave approx 15,000 for reserve needed for acceleration,load changes , wear and tear etc. The only thing a change to .035 would do is raise the actual needed voltage and eat up some of the reserve..
Now if the compression, air fuel is changed then you may want a longer burn time ie a greater plug gap.
Wide plug gaps were created for lean air fuel and emission controls.
 
You should be going smaller not bigger.

As has already been said, 25 thou is a nice gap. You'll find the car starts and runs better with a smaller gap. Also a smaller gap places a much reduced load on the whole ignition system making it more reliable.
 
Makes sense to me now how the gap works, my Edlebrock aluminum heads have 8-1 compression so not very high. Thanks to Charlie in NY my carbs are working great and Bubbas for my distributor being dialed in. Guess I am just greedy, want more power!!!
 
You might consider using 437s on your flathead with aluminum heads. They have a longer reach than the 216s, which are usually used on standard heads.
 
The 437s are made by Autolite. Vic Piano says that he uses 216s in his standard flatheads and 437s in those that have aluminum heads. If anyone should know, he would.
 
Thanks for the insight, will give the 437's a try soon. Yes, Vic does know a thing or two about our Flatheads! My current 216's are gapped at 030, will try the new plugs with 025 and see how it runs.
 
The longer reach would be taken up by the deeper thread in the aluminum head, the plug would not be closer to the valve, right? ..B.
 
So, actually with the 437 plug the electrode would be in more of a stock position like the iron heads? And since I have been using the 216 plugs for the last 15 years it has been a little farther away because of the thickness of the aluminum heads..... Bought a set of the 437's tonight and just put them in, gapped at 030, car seems to run better, though it might just be my imagination??
 
Tom, were you able to use the Mallory 12V electronic distributor right out of the box without any problems. I ask because I have one but have yet to fire it up. Thanks, Bob
 
BobM, I attempted to adjust the springs myself over the past few years on the Mallory. I sent it to Bubbas a month ago and now it runs great. They told me that the unit is set up from the factory to Chevy specs, not Ford. Do yourself a favor and send it to them for the recurve.
 
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