MTFCA Forum

New Autolite 14MM threads, what should the gap be?

Thank you, Steve.

Bill Everett

Our '26 runs better with .025" gap , running on magneto

I prefer .02657"

Just teasing... any of the above should work well for you.

I prefer .25 mm.

I meant .55 mm.
Art Mirtes

Depends on which ignition system - stock timer & coils in the .030 neighborhood - a distributor system usually in the .035 + neighborhood.

The guy who rebuilt my engine thinks they run better with a 0.040 gap. I run mine a bit less.

My experience .... Having a .025" spark plugs gap works better on battery or magneto starting and running. ... Time wise, running, gives less time for coils to fire to give a good running spark, and less over strain . Just my opinion.

A wider plug gap is best for firing a lean mixture. However, the wider the plug gap, all else being equal, the more voltage will be required to fire the plug. A Model T with a stock ignition system is better off sticking to factory recommended plug gaps. A magneto-only system can be expected to start more easily with recommended plug gaps. If you have a distributor system with a modern coil, you might get benefit from running a wider plug gap, perhaps around .035. An engine with higher than stock compression will tax a stock system, and might perform better with a plug gap around .025. The voltage needed to fire a plug under any specific set of conditions will be affected by the condition of the plug. Worn plugs with rounded, eroded electrodes require higher voltage to fire. Such plugs can benefit from careful filing of the electrodes to restore their original profile. The wider the plug gap, all else being equal, the cleaner the insulator will have to be to prevent grounding. Running wide gaps with a stock system will increase peak secondary voltage and may increase the risk of secondary coil breakdown and leakage from plug wires. It may have an negative effect on how long vibrator points will last and hold an adjustment. I read somewhere that Model T coils will put out about 15,000 volts at the secondary. The later, single coil 6 Volt systems would put out about 18 to 20,000 volts. 1950s and 60s 12 volt sytems could produce 20 to 25,000 volts, which was barely adequate for engines running 9.5 to 11.5:1 compression with conventional plugs. Today's systems run 35,000 volts and more. Many current engines run compression around 10:1 and wide plug gaps Around 1973, ignition systems were improved and secondary voltages went up to 30 to 35,000. This allowed plug gap increases to around .042 with (emission) compression ratios back down around 8:1. Plugs today often have platinum tips with sharper electrodes to reduce voltage requirements and permit very long service intervals.

Pat - what sort of gap would be best with new Champion Xs, a Z head & the E-Timer? This is a confusing combination of features to my mind.

Paul

Many variations in ignition systems don't make much difference in gap requirements. One does.
Bosch, recommended a .018 gap for the DU4 magneto without an impulse. .020 with an impulse. I found that .020 was okay on the speedster I ran for years with a DU4 no impulse. Although that may have been a bit wide, and may have contributed to two mags I burned out during those years. The Bosch DU4 likely cannot run a gap over .025. I tried it. Had to tow the car to start it, and often the magneto's inner gap (designed to fire before burning out the armature) would fire (I could hear it over the engine's roar!)

For all other ignition systems I have used? I wound up exactly where Steve Jelf is. I like .028 the best overall.

An E timer operating stock coils would probably do best at the factory recommended gap. Adding compression might warrant reducing the plug gap slightly to avoid pushing the secondary winding voltage too high. I doubt there is any perfect setting, and a stock engine and ignition system in good running order can be expected to work well at recommended settings. If you modify your engine or ignition system, or both, changes in plug gap and other settings may give better results than the standard spec. Generally, if you have a modified system that can provide higher secondary voltage, you could probably run a wider gap than stock. Modern platinum plugs with more refined electrode design might allow a wider gap than stock without allowing secondary voltages getting too high for stock coils. I don't think a wider gap offers much benefit except that it can give better results on very lean mixtures. Regular plugs tend to develop wider gaps under use, so I'd lean toward setting the gap on the tight side with new plugs.

Thanks for the explanation. The motor is mostly stock but does have a older Chaffin Driver cam, the Chaffin inlet manifold & 351 Cleveland valves. With the Z head & I -Timer its a peppy T.

Run an anode in your radiator to help reduce the enevitible electralasis.