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Spark plug gap for Autolites

bryan

Jr. Member
Joined
Dec 22, 2004
Messages
61
Loc.
Charlotte
Hello all- I have a 72 Bronco w a 302- currently I have the autolite ap26 plugs installed and was wondering what the proper gap is for them- my 70s Chilton’s manual says .034 for the 72 v8, which is how I have them gapped now- when I pulled them they were at .044, which Chilton’s list as spec for LATER years- SEEMS like it’s running smoother but maybe in my head- any help? (Chiltons actually calls for BRF 42 plugs w the aforementioned gap, not sure if these Autolites are the same?)’
 
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bryan

Jr. Member
Joined
Dec 22, 2004
Messages
61
Loc.
Charlotte
Forgive my ignorance, but I’m not sure if you mean the ignition switch on the dash? That has been replaced w what I believe to be an exact stock replacement 🤷*♂️
 

Broncobowsher

Contributor
Total hack
Joined
Jun 4, 2002
Messages
33,357
Electronic ignition is more powerful, capable of jumping a bigger gap. That is why the later spec is a wider gap.

Points ignition isn't as powerful. The gap has to be smaller in order to get the spark to be good. Running the larger gap may cause issues at times with a weak spark that can't jump the gap.

That is the reason for two different specs for spark plug gap on the same basic engine.

The larger the gap the more molecules of air/fuel get ignited during the spark. That is a good thing. It turns into a game of running a large gap for better running engine, but not so large that it isn't reliable. The larger gap stresses the ignition system more. Plug wires are more likely to arc (to an easier ground). Rotor buttons have to take more energy and like to melt. Coils want to break down more. As plugs wear the gap gets even larger and may get to the point where there is just too much space to jump the gap.

The gap, while a fixed dimension, isn't a fixed load. As cylinder pressure rises there gets to me more and more molecules stuffed into that gap. Under load the effective gap can be as if the spark is trying to jump half an inch through air. That is quite the multiplier. Going up .010" is like asking the ignition system to jump a spark through an extra .100~.200" of regular air.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
43,567
did you mean that it's seeming to run smoother now, with the smaller gap? With a stock ignition setup that is very likely true and not just your imagination.

You should check what's under your distributor cap just to be sure. That way you know for future reference. Not many are still running points and condenser ignitions as there once were. Lots of aftermarket electronic replacements available for the last 40 years or so.
I bet my first one was almost 50 years ago now!

Next time you're under the hood, pop the distributor cap off (just two metal spring clips) and take a peek down inside. If you're not familiar with the different setups, take a photo of it and let us see it and someone will be able to tell you what you have.

Paul
 
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bryan

Jr. Member
Joined
Dec 22, 2004
Messages
61
Loc.
Charlotte
did you mean that it's seeming to run smoother now, with the smaller gap? With a stock ignition setup that is very likely true and not just your imagination.

You should check what's under your distributor cap just to be sure. That way you know for future reference. Not many are still running points and condenser ignitions as there once were. Lots of aftermarket electronic replacements available for the last 40 years or so.
I bet my first one was almost 50 years ago now!

Next time you're under the hood, pop the distributor cap off (just two metal spring clips) and take a peek down inside. If you're not familiar with the different setups, take a photo of it and let us see it and someone will be able to tell you what you have.

Paul


Yessir I meant that it seems smoother w smaller gap- took pics gonna try to post them now
 

B RON CO

Contributor
Bronco Guru
Joined
Jun 29, 2016
Messages
2,325
Hi, you have the original points and condenser ignition. The spark plug gap should be @ .035.
The points and condenser only last about 10,000 miles.
It is not easy to find good quality tune up parts so many guys convert to a Pertronix ignition and coil.
Then you would open up the spark plug gap.
Good luck
 
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bryan

Jr. Member
Joined
Dec 22, 2004
Messages
61
Loc.
Charlotte
Thank you guys so much! I drove this thing forever w the wrong gaps😖. Should be much better now
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
43,567
I believe the picture shows that the struggle it had trying to produce a spark that could jump that larger gap.
The rotor has lots of black on it's contact, and perhaps might indicate the same thing on the underside of the cap. Can't see the rotor tip that well, but if it's crusted over with carbon buildup it needs to be cleaned. More likely replaced though, since it's of unknown age and not expensive.

And if I'm not mistaken, your distributor's points cam is trashed! It should be smooth with just the indication of the points fiber bit riding over it. Not grooved and pitted like it looks from here. Could just be the light and angle of the camera, but it looks worse than most.

If you are going to keep this style of trigger, you should definitely invest in a good dwell meter and feeler gauge set.
But inspect that cam and other things too first, since changing to electronic will negate their use 99% of the time.

You should also test the distributor's bearing surface and shaft tolerances by hand at least, by removing the rotor and pushing the shaft end side to side and up and down to see how loose it is.
There should be almost no sideways movement from pushing (though it will twist easily against spring pressure of the advance weights) and very little up and down movement.
And while you're at it, twisting the upper part with the cam on it will give you a feel for how the advance mechanism is functioning. If it's smooth and tight when you twist, that's a good sign. If it's floppy, chunky and sticks at all, then it's time for some serious TLC because your spark events will never be consistent or accurate.

Paul
 
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