I saw your question but was waiting for TeamRush to answer. I know he prefers a narrower gap than what I recommended. I got my values from MSD.
Here is what MSD says:
http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/Ignitions/6420_instructions.pdf
On page 3 of that PDF they say for compression up to 10.5 to 1 start out with a gap of 0.50" to 0.60"
Compression of 10.5:1 up to 13:1 use a gap of 0.40" to 0.50"
And for higher compression than that narrow the gap down more.
But those are just starting points. Here's what MSD says:
Like I said TeamRush prefers a narrower gap so I'd like to see what he says.
Over and out.
I have also posted a few times about the need for smaller gaps in our AMC engines.... ONLY go to the larger spark plug gap of .040-.045" if you have the TEAMRUSH tune up parts and aux grounds..... if not stay with the .035" plug gap using stock copper AUTOLITE spark plugs..... Your ignition parts will last longer, normal life, and your ignition will work and work right... Learn how to read your spark plugs and use the NORMAL stock heat range plugs.... Get your igntion healthy, carb adjusted, and read your plugs... they should be tan....
Once you have this..... look at your spark plug ground stap..... there will be a color change half way up/down the strap.... usually adjusted with the time or AF/carb.... I have found tooo much advance and the cylinder gets hot and often comes with too much fuel since trying to burn the extra fuel needs more time.... NO STAP COLOR and usuall tooo much time, lots of initial, and adjusted wrong or "not corrected"... Computer X users need to fix their advance curve, take out all the inititial advance and get this so it works right again... will mention this below in more detail
I agree with TeamRush that the spark plug gap is often opened up toooooo far and not necessary... I have posted several times since I joined Jeep-CJ.com that once the TeamRush larger tune up parts are used the spark plug gap can be opened to .040 to .045" from the stock recommendation of .035". The only reason we can open the gap a little bit is the LARGER Diameter Cap and TALLER Rotar with PREMIUM Spark Plug Wires will get the high voltage spark plug energy toooooo the spark plug gap.... With out cross fires in the smaller cap, or spark jumping to ground in the cap via the metal parts and ground path in the distributor (So common on HEI distributors after yr or few months of use get the red dust), or even the High Voltage energy jumping from the wires to the engine ground or any metal contact..... even spark plug wires that touch or are too close or tooo parallel to each other...
The IGNITION is a inductive, resistive, capacitive circuit and could be called a IRC circuit. The spark plug gap becomes a capacitor..... the larger the gap the bigger the capacitory value. HIGHER VOLTAGE is needed to jump the larger spark plug gap..... so a larger capacitor with larger gaps and not linear voltage goes up quick..... Further, larger spark plug gaps SLOW DOWN the ignition circuit TIME..... because it takes more time to build up the higher voltage. The higher voltage is also hard on the other part in the ignition. Higher voltage also jumps the gap quicker at higher voltage..... like static discharge on carpet is also 20k to 40k volts.... Lower voltage will discharge across the spark plug gap for longer time.... amperage. Longer spark time.
Spark plug wires will not last as long and tune parts with higher voltage.... kinda like hitting them with a min welder rew times a second.... if VOLTAGE jumps to easiest ground via a plug mount or engine metal ground the spark plug wire can ground out and will fail pretty quick.... I had one fail with .045 gap in less than two years.....
Also notice MSD said you need to run 8.5m Spark plug wires like theirs or theirs and IS >>>>> VERY IMPORTANT..... TeamRush has tested lots of wires and the only ones he likes are the premium MSD wires and even the parts stores Street Fire versions did not pass testing... how many jeeps CJ run MSD wires... $75 wires.... there is resistance buying $7 adapter so TEAMRUSH tune up parts can be used
Further many do not use the MSD ignition modual
DuraSpark Modual..... the more time the stock modual is ON and getting ready to fire the hotter the modual electronics get.... Use .060" or more and the VOLTAGE gets up there pretty quick, the ON time increases and you are going to over heat and fail your DuraSpark..... The CHEAP Clones of the MotorCraft Duraspark Ignition Moduals will die from excessive heat much much much quicker. USE large plug gaps with the CLONE durasparks modual types and you will be walking once the summer temps come.....
When I reading the MSD Instructions.... did you catch they say distributor fire signal wires need to be protected from noise... use the metal ground plane, twist the wires to min cross talk and induced currents... I would twist at 2-3 turns per inch, MSD missed that part of recomendation, how much to twist...
Kill a DuraSpark modual and the replacements are much weaker..
Large plug gaps will over heat modual. Poor gound in, poor head gound, leave ignition on with out engine runnning will alll kill the duraspark in short order...
WE are trying to get the spark energy to the spark plug gap....... the spark plug gap wants to jump to any ground location that is easier to jump to than the spark plug gap..... it is like flowing water ... it will flow to the least resistance and easiest path.... in any part of the ignition.... Big Gaps will drive up the heat in coil, heat up formvar insulation, and can cause shorts and early failures in the coil too....
NET NET.... with large spark plug gaps going above the .040 to .045" with the stock AutoLite Copper plugs will just wear out your tune up parts..... cap,rotar, wires, spark plugs, coil and DuraSpark Modual quicker than it should.... will cause more failures and will not make the 100 hp I6/
AMC 258 i6 / 4.2l or the 130HP
AMC 304 /V8 run any better, worse actually in short order if read this with understanding.
MUCH more success for our 35 year of Jeep CJs can be had by the study of the computer controlled Jeeps that are newer than the 1981 or 81 (cali). Once we take the computer out, or sometimes called NUTTERED, and happens if ignition sensors not working right...... engine would be in LIMP HOME MODE the IGNTION CURVE is not right and needs to be adjusted.... the stock distributor can be recurved easily and is completely adjustable..... the proper advance curve is listed in the Jeep FSM, Field Service Manual for the non computer years and 1978, 1979 , 1980 are perfect years to look at. Engine size, vehicle weight,
Transmission type, vehicle under load/ lugged often/ pull trailer will have different advance curves.... Learn how to do this and your engine will wake up big time....
Note ..... this is why HEI gets used so much, and talked up too much in my opion.... BUT the HEI clones for the AMC have toooo much advance.... lots of knock issues, should be recurved and tested completely for AMC, and there are gear issues, and there are physical fitment issues too, too long, gear not in right spot, some even bind the oil pump up.... Duraspark is easier to adjust so might as well fix what you have the stock stuff....
TeamRush is tune up parts...... larger cap, taller rotar, premium spark plug wires..... then let it work and get spark energy out to the plugs and back to the battery.... This means aux ground all over the jeep duraspark modual, the head (s), the spark plug threads using copper antiseize, and routing wires so they do not touch each other, run parrallel, or touch engine or metal that is grounded.... simple engine that can be tweeked to run pretty well.... even your alternator needs aux ground
CJ
AMC 258 i6 / 4.2l with normal tires, healthy engine can get 20 mpg++++ on the hwy, 3500-4500 rpms pretty easy, and go down the road at 65 mph.... TeamRush added 20% pep, better starts, 1000 rpm range, to my new long block build..... My comparison with brand new tune up parts.... I can rev to 4500 rpms when shifting pretty easy and cruise down the highway at 65-70 for hours.... I get 23 mpg hwy doing 55/60 and used a vaccum gauge and maxed the reading to learn how to stay out of gas pedal and max the vac levels...
YES...tune up stock parts or better TEAMRUSH tune up parts.... will need to adjust the time and the carb settings.... YOU have change the electrical time of circuit and the better ignition needs to have all parts adjusted to take advantage of the TUNE UP>>>>
Will need to adj the time and then carb.... go back and adj the carb, and then do the time again..... This would include recurve your 1982 or newer AMC DuraSpark Distributor advance curve if want to run well...... FAR FAR FAR too man helpers are recommending 10-12 or more initial advance settings and that is why there are hard starts..... that is a lot of advance and when slow cranking to start motor that can cause lots of cranking........ problem so big with HEI Dissy in AMC that not too un common to have ignition OFF switch so not spark till cranking than allow ignition ON to allow spark after cranking...... Band AID for HEI Dissy so fix up your duraspark stuff and boy can you be a happy camper.... I had a guy travel 10 hrs one way to have me fix his ignition.... Learn how to do this and you will be the best bud in your local jeep club.
The DuraSpark has two spark circuits.... the start & run.... the start when cranking retards the time 8 deg or so for easy starts and min the chance of damaging the engine and get easier starts.... nice very nice benefit of stock ignition.
At min I would also suggest you test your stock style coil....
the resistance measurements are in your FSM and give you good indication if there are faults or short internal to coil.... When you take plug wires off while running or drive voltages tooo high.... the coil will get small voltage jumps in the coil windings.... there are two windings, one big winding set and one small winding set.... This is thin wire with a shelack coating they call formvar.... Voltage jumps cause HEAT in coil and the forvar melts and you get SHORTs since the copper wire is now touching... and coil will not get the right voltage out..... failed coil and you are walking home...
WE can all agree the stock recommendation for spark plug gap is .035"..... if we go to .060 or keep going as the MSD said to do.... we will nearly be 2x the stock recommendations..... not necessary... and your duraspark modual will not last long... Also adding lots of heat energy to the spark plug and not sure how that will affect the reading of plugs and the location of the spark plug ground strap color reading..... my gues plug gets so hot it will drive the color change off the end of stap and no color change..... guess on my part.... and intersting test if you want to play some....
TeamRush has some pretty good ideas.. and good teacher....
me... eng with mech, electrial, physics education and 30 years in electronics.... his stuff is pretty goood!