365 GTC/4 with AEC103 ECUs

I am using the bosch spark plugs. At idle these do work better than the recommender ngk 8 plugs.

People usually use ngk 7 or 6 in order to have better idle but these are hotter than the original champion, which could crack the fragile head.

The bosh are nice but they use a smaller hex. There are some nice beru, but they are now hard to find.

I gap the plugs "by the book", i.e small gap, otherwise the coil cannot generate enough energy for 12 big sparks at high rpm.

 

I switched from Champion to Bosch Platinum and went one step hotter. The heat range for OEM plugs are for high RPM driving; they are usually too cold for normal everyday driving. I recommend sticking with the OEM spark plug gap.

 

Given all the troubles I had on my 400 (same engine different crankshaft). I just don't want to use hotter plugs: the exhaust port is so thin that it may crack easily (had to bring my left head to the welder 3 times to cure all these leaks!). That's the reason I prefer to use the recommended heat-range (8~7) as the heat gets more evenly spread on the body of the plug.

Beru Z42 is just as cold as the NGK BP8ES, but it is more tolerant to overly rich mixture. That's a classic (copper) plug with single strap. The Beru UX 56 is basically the same with multiple straps (very nice but hard to find now). I've tried the Beru UPT8 (thin tip), no significant difference better to be honest. The Bosh with 3 straps are nice: my engine is grossly rich (did only 900km since the rebuild), and although covered with soot the engine still idles properly.

These are the plugs characteristics:

Spark Plug Thread Size: 14mm
Spark Plug Reach (mm): 19.00mm
Manufacturer Heat Range: 7 (hotter)
Manufacturer Heat Range: 8 (recommended by Ferrari)
Center Electrode Type: Projected

You can have a look at the sparkplug crossreference website (https://www.sparkplug-crossreference.com/convert/NGK_PN/BP8ES) unfortunately it does not take into account the heat-range of the plugs. Summit engine does provide a nice search engine for spark plugs

 

Your 63/72 OM specifies Champion N6Y spark plugs which cross-reference to NGK BPR7EIX spark plugs which is the electrode design that I'd use in any carb F to better prevent fouling. (Per NGKs cross-reference at: https://www.clubplug.net/index.php?route=product/category&path=24_78.)

Interestingly, the spark plug gap specification shown on the Vehicle Emission Control Information label on page 14 shows the spark plug gap as .023" to .035", but I believe that represents that .023" is the minimum gap for a new spark plug, and .035" is the maximum gap for a used plug for when they should be replaced. On page 15, it indicates that the spark plug gap should be set to 0.6mm (.024") whenever reinstalling a spark plug = I'd suggest that you use a new spark plug gap target of something like .023" to .027".

 

OEM for most vintage V-12 road cars is Champion N-6Y with a gap of 0.025". I found this too cold for normal driving. I went to the next hotter Champion N-7Y. Today, it's easier to find a resistor plug, RN-7Y. For less fouling I found the Bosch platinum equivalent, WR6DP, was a much better choice. These plugs come with a wider gap which I had to reduce to 0.025".

 

The heat range of spark plugs and heat transfer/possible damage to the head are not related or a consequence of.

In fact, a colder plug will actually transfer MORE heat into the cylinder head, as this is exactly how the design of the spark plug and the dissipation of the heat works.

There is no magic, the amount of heat that is generated within the cylinder and combustion chamber/head is the same irrespective of the spark plug, the only thing that changes with the heat range is that the tip of the spark plug is hotter or colder, but of course the heat removed from the spark plug needs to go somewhere

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It is true that the cold spark plug dissipates more heat, but it does so over a larger conductivity area. By contrast on a hot plug, all the heat has to pass through a bottleneck, so you end up with a portion of the spark plug thread that becomes an extremely "hot ring".

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(courtesy of https://carfromjapan.com/article/car-maintenance/spark-plug-heat-ranges/) ​

This is the worst possible place for thermal stress on an F101CL engine, as it is next to the water pocket that is prone to cracking. This plus the sodium valves that would make an absolute mess should a ground strap fell in the combustion chamber.

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I would personally stick with NGK8 or NGK7 and keep hotter plugs such as NGK6 for sedate driving or breaking in. For the record here are heat-range the cross references:

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Back to the OP question:

Multiple ground strap plugs and thin-tip plugs (Paladium/Iridium) are much more difficult to gap: inserting the gauge on a 4 straps plug is close to impossible. Paladium/Irdium coating are fragile and likely to be damaged by the gauge.

When I install the 12 plugs, more often than not I let escape one, which will fall from the wrench and into the cylinder head. Even such a very light tap modifies the gap! As a spark plug should not be gaped numerous times, I know accept a not so perfect gap, a long as it is more or less within the manual specifications.

Last but not least these V12 will gracefully idle despite one or two missing cylinders. If the idle is rough, you are probably missing quite a few more (mine was still running on, 6). In this case a laser gun does help identify which are causing troubles.

 

The heat range of the spark plug, is actually limited to the tip.

As you can see in your image you posted the "hot" plug actually keeps the body of the spark plug hotter by reducing the amount of thermal conductivity to the rest of the spark plug.

Fun fact: Each "heat range" is equivalent to approximately 100 degrees (measured at the tip).