Spark Plugs
 

If you are using carburetors the recommended spark plug is the NGK BP5ES.

I was looking for some Weber Parts and stumbled on a website

Weber Carb Parts

As you can see this site suggests the use of an NGK BP5ES for engines with carbs.

I had never considered using such a 'hot' plug in a 911 engine and would worry about them melting.

We tend to use a BP6 on basic engines such as a 911T, we use a BP7 on a 911S and a BP8 on a Rally or Trackday car.

Does anyone have any insight into using a plug with this heat range and its reliability?

JMHO, but an NGK5 is far too hot for any air-cooled 911 engine.

We use 6's in gently-driven street cars and 7's in ones used for track days. NGK8's (or Bosch 3's) for race engines.

I've been using BP7ES or BP8ES in my 914-6 (now with high-compression 2.3L) without any issues. There was very recently a thread on this on 914world and yes there are people using '6' or '5' for some reason.

What about dual plug engines? I have been using NGK B7ECS In a not so gently driven 2.7 10.3:1.

Generally don't you use cooler plugs in dual plug engine?

I am not sure if it makes much difference but would be interested in any other comments.

I see you have a Lotus Se7en - which one?

I have a 1969 Series 3 Twin Cam SS and still has a great 'grin factor' :D

It is a little older than yours, 58 series one MK7-410 tenth one made I think.

Back when we dual plugged BMW motorcycles, I believe we used colder plugs.
My memory is fading fast!

I nearly bought an S1 with an OHV Aquaplane conversion about 20 years ago but ended up building an Historic Rally car instead - I still regret the decision. :(

I do have a 63 A Frame Cortina which I bought when I was 19 for around $500.