Thousands (millions maybe) of people have been fine with just jamming a new dry clean wire in to the hole.  You just need to get things to the way they're supposed to work.

Somebody mentioned arcing.  Maybe your cap is not seating well on the rotor and you're getting an arc across that gap.  I don't know how much excess heat that would produce, there are six arcs per revolution inside the cap anyway.  Check the inside of the cap, the center, to see what's happening and the contact point on the rotor.  Find the source of the heat.  Create a path of good contacts from the coil to the rotor top and your situation should become normal.

Attempting logic:

What causes melting? Heat.

What causes heat in electrical systems? Resistance.

Why is there too much resistance at that particular point in the system? My guess is that electricity cannot flow as designed past that point, so the point of resistance is the cap. 

Possible causes...?  Bad cap causing resistance. Spark plug wires causing resistance. Something inside the distributor causing resistance. Too much voltage or current coming down the wire to the cap, so the system can't pass it along.

That's all I have.

1 minute ago, Pilgrim said:

Attempting logic:

What causes melting? Heat.

What causes heat in electrical systems? Resistance.

Why is there too much resistance at that particular point in the system? My guess is that electricity cannot flow as designed past that point, so the point of resistance is the cap. 

Possible causes...?  Bad cap causing resistance. Spark plug wires causing resistance. Something inside the distributor causing resistance. Too much voltage or current coming down the wire to the cap, so the system can't pass it along.

That's all I have.

I have a non-stock coil/ignition box - could that be the issue?  Basically could the plastic used for these caps be rated for stock coil voltage/frequency?

11 minutes ago, chaseincats said:

I have a non-stock coil/ignition box - could that be the issue?  

What are you using?  Many of us are using high energy (voltage) ignition systems with stock distributor caps and no problems.  So, probably not.

10 minutes ago, chaseincats said:

I have a non-stock coil/ignition box - could that be the issue?  Basically could the plastic used for these caps be rated for stock coil voltage/frequency?

That thought had occurred to me, but if the ignition has been there for some time but the change occurred recently, it seems less likely.

I'm thinking OEM cap replacement and coil wire replacement would be a good idea.  Look for tracks inside the cap indicating arcing or heat. 

2 minutes ago, Pilgrim said:

That thought had occurred to me, but if the ignition has been there for some time but the change occurred recently, it seems less likely.

I'm thinking OEM cap replacement and coil wire replacement would be a good idea.  Look for tracks inside the cap indicating arcing or heat. 

 

3 minutes ago, Zed Head said:

What are you using?  Many of us are using high energy (voltage) ignition systems with stock distributor caps and no problems.  So, probably not.

The ignition box says "jacobs electric."  I took a look under the cap and it looked fine...

21 hours ago, chaseincats said:

Hi guys,

The plastic surround of the center spark coil lead (only) on my distributor cap has melted a bit for the 2nd time.  The car came with an aftermarket ignition box/coil and has run great for years.  Would putting ox-gard in the center terminal help deflect some of the electricity/heat awa y from the plastic surround?  Here's a picture of the melted center terminal (the other terminals don't have this problem).:

Back to the original post.  Most caps are made of a thermoset plastic, which does not melt.  That might just be degradation from some other cause.   Does the engine run well?  If so, why not just put the cover back over it and run it?

Jacobs is an older company that made ignition systems.  They made a CD system, like MSD's, and an induction system.  They only seem to be selling in Australia now.

http://www.jacobselectronics.com.au/

https://www.yellowbullet.com/threads/whatever-happened-to-jacobs-electronics.1405746/

https://www.summitracing.com/parts/jac-372546

 

 

51 minutes ago, Zed Head said:

Back to the original post.  Most caps are made of a thermoset plastic, which does not melt.  That might just be degradation from some other cause.   Does the engine run well?  If so, why not just put the cover back over it and run it?

Jacobs is an older company that made ignition systems.  They made a CD system, like MSD's, and an induction system.  They only seem to be selling in Australia now.

http://www.jacobselectronics.com.au/

https://www.yellowbullet.com/threads/whatever-happened-to-jacobs-electronics.1405746/

https://www.summitracing.com/parts/jac-372546

 

 

Ya the car runs great.  I put a glass of water on the valve cover the other day and got no ripples with it sitting at idle haha.

So you're thinking just shut the hood and move on?

8 minutes ago, chaseincats said:

Ya the car runs great.  I put a glass of water on the valve cover the other day and got no ripples with it sitting at idle haha.

So you're thinking just shut the hood and move on?

 I wouldn't  because I would obsess about it. Were the two caps that melted the same brand? I'd check the continuity of the coil wire (coil end) to the rotor on the bottom side of the cap as an assembly. Then separate the coil from the cap and check the continuity of the two components separately. Then check the continuity of the assembly again when you plug the coil wire into the cap. My first thought was something simple like the coil wire wasn't plugged into the cap all the way. The juice from hot coil wouldn't have any trouble jumping that gap but it would produce a lot of heat. I've not seen a dist. cap melt but I have seen a coil wire boot that kind of disintegrated caused by a poor connection. 

It might have not been in all the way - who knows.  Not sure if the dist caps were the same brand as the first one came on the car when I got it years ago.

A 510 at Watkins Glen had a small fire that upon investigation started by the distributor.  The owner and others  had never seen it happen before. Connected to this post?

18 hours ago, chaseincats said:

 

So you're thinking just shut the hood and move on?

Um, no.

It looks like the electric current is leaking from the coil wire where it inserts into the cap, and that is carving the plastic away (what you see that looks like burned plastic).

It also looks like the exterior of the cap, and the wires are dirty. The dirt can be conductive, providing a path for the current from the coil. Sure, the car may run OK, but that leaking electric energy is causing damage, and it will eventually stop getting enough energy to the spark plugs.

You said the inside of the cap looks fine. I would say that given the dirty condition of the outside, the inside is also dirty. Shine a bright light inside the cap, and look for arc tracking (it will look like irregular lines in the dust. 

Replace the cap, and at a minimum the coil wire (I would replace all the wires.

Also, use some silicone grease on each wire end when inserting them before sliding the boots down. That will keep moisture out, and should help maintain dielectric integrity, reducing the likelihood of the spark energy leaking and damaging the cap and wires.