MTFCA Forum

I tried out a NOS set of Rentz Spark Plugs after running new Champion X’s for a long time. They have a longer electrode than the Champions which I assume makes them a hotter plug, so I set the gap a bit bigger on them, between .030 and .035 for the Rentz. The champions I have set at a snug .025

Upon driving I had to give the ignition more advance than before, at a cruise of 25-30mph I advance the spark all the way now, where I was previously setting it at 3/4 advance. Now at 3/4 advance I get an occasional stutter like I have too much retard.

Town driving at around 10mph I have it set at 1/2 advance where it was 1/4 advance before. Again anything less produces a stutter.
The engine doesn’t miss a beat at idle and free-revving is smooth and causes no stutter.

Sound normal? Maybe my gap is too big or too small…

Those plugs look similar to a Metz plug

I would go back to a smaller gap. The longer electrode would not give a hotter spark, but the plug tip would likely run hotter. Be sure the plug wires are not touching anything or positioned near anything. The Champion plug with the sharp edged electrodes would fire at a lower voltage than the Rentz plug with the rounded electrodes. The Rentz plug would probably hold an adjustment longer. Try the Rentz plugs at .025 to .027.

What is your ignition? Four coils? Magneto or battery? Distributor? Four coils on battery could require more dwell time with the wider gap therefore needing more advancement. A distributor with single point break should not effect timing. And the magneto's pulse cycles might not be affected by the wider gap, but could be slightly?
Regardless. I would try reducing the gap back to .028 to .030.

Isn’t it usually the simplest of things? Set gap between 25 and 30 now it’s running beautifully. ( I am running on MAG) Thank you all for the help! I am very fond of these plugs, neat stuff.

It is nice to hear that the Rentz plugs run well in your Model T. Back in the 1970’s someone found a old warehouse with large lot of new Rentz plugs fitting many different makes of cars from before the early 1930’s. The plugs were sold via Hemmings in the 1970’s and they had two reaches of half inch pipe plugs, the long reach that you show and a shorter reach plug more like the Champion plug you show. My dad and I bought several sets of each reach half inch pipe Rentz plugs but kept them for display. I always wanted to try out a set, maybe now is the time to do so.

You drive your T around town at 10 mph? Drivers behind you must love that.

Hope you get to try them out, I like the way they look.

I am driving at 10mph in a very small town, think Mayberry

There may be some combustion efficiency advantages in locating the spark gap farther into the chamber. Running the Rentz plug might cause "spark knock" issues if you were running T era gasoline at high speeds or high engine loading. I don't think there would be any problem with spark knock/detonation using typical modern day gasoline. If the Rentz plug design reduces the clearance volume, that would be advantageous in a T.

Some plugs with rounded electrodes tested poor to bad. I flattened the ends of the electrodes with a miniature grinder and most tested OK. I haven't tried them in a car yet.

Restoring the original electrode profiles can make a difference. The ideal electrode would probably be a needle point, but that's impractical for several reasons. The plug you pictured has very worn electrodes. Setting the gap would help that plug, but restoring the electrode profile as shown, and setting the gap would probably get it close to as-new performance.

...restoring the electrode profile as shown, and setting the gap would probably get it close to as-new performance.

Yes, mostly they test good or excellent. The plugs that mystify me are the ones that fire intermittently.

I'd guess the intermittent plugs have a hidden crack or flaw in the insulator or the unglazed end of the insulator may be deeply permeated with carbon or rust. If narrowing the gap on an intermittent plug improved its test performance, that would seem to point to an insulation leak of some kind, such as a hairline crack, rust or carbon contamination, or a metallic deposit on the surface of the insulator which can result from wire brushing or scraping with a metal instrument. This nwould most likely affect the unglazed end of the insulator. Some newer plugs have an internal gap or resistor within the insulator that could give trouble, but I think the older plug electrodes run straight through the insulator. I don't see how they could give trouble.