Using ptfe in spark plugs

[keith5700]

I don't think the "normal" materials used for model engine plugs will work.
The Megasquirt uses a wasted spark system, which means it sparks on every plug on every rev.
So the plugs get a spark on the exhaust stroke as well as the firing stroke.

When I was struggling to get the engine started, with a dizzy based ignition system I made a small pressure chamber from clear Perspex. Some of you will remember this.
It showed that it is possible to have a nice fat spark at atmospheric pressure, but as the pressure increases in the chamber  the spark gradually gets snuffed out. In my case there was no spark above about 70 psi. And this showed up why the engine wouldn't start.

The Megasquirt, and the coil packs, are powerful enough to spark well above 100psi, but this must mean at atmospheric pressure the spark is huge.

If you look at how the ptfe has worn away, it is eroding from underneath the electrode, which is where I would expect the spark to be eminating from. The ptfe opposite the electrode is barely showing any evidence of melting. So my theory is that it's not heat from combustion which is melting the ptfe, but the spark on the exhaust stroke which is blasting it away.
Which inevitably means the insulator needs to be ceramic or glass based.

My current idea, which I've only just thought of, is to go with Graham's idea, but to get one macor rod and make just the end, say, 6mm with this, and fill the rest in with ptfe.
I have already made the outers and can't find any fish spine tubes to fit.
I could get 10 plugs out of one 4" rod.

That mac master site won't let me view the items and I can't seem to register on there either. Thanks for the info though.
Keith.

[Graham Meek]

Hi Keith,

The very first plugs I made were using Macor, I managed to get a free sample from RS. I went over to using Fish Spines as I get about a 1000 spines as a free sample, if you ask nicely, (that will keep me in plugs until the next millennium).

My best regards
Gray,

[steamer]

Well   Roland is an awesome model engine builder   ( Rolly's dad's method of lathe alignment ect)   His 1/4 scale Bentley BR2 runs awesome and has so for oh the last twenty years at shows all over the east coast on Corian plugs...

<a href="https://www.youtube.com/watch?v=0vmq83LdGrk" target="_blank">http://www.youtube.com/watch?v=0vmq83LdGrk</a>


Dave

[Ramon Wilson]

Hi Keith - first off congratulations on such magnificent work  it really is difficult to find the words to describe what an awesome impact it makes - as others have said just ignore the 'idiots' - there are far more who will appreciate what you have created than they will ever number.

Now - Macor - I have in my tool chest a piece 25 mm square by 150mm long. I bought it from RS through work long ago to make the plugs for the Bentley. At the time I split the cost of the length with my mate Sid who made plugs from it for his Anzani.  I intended to slice it up with a slitting saw into little square sections for turning but as I have some ceramic tubes for the Bentley it's unlikely I shall use it - Any good to you? PM me if so.

Regards - Ramon

[///]

Apologies to Keith for going slightly off topic.

Would just like to ask Graham what you use to black/blue your plug bodies?
It's a very nice finish.

The PTFE erosion was one of the reasons I opted to keep the PTFE out of the combustion zone. The other is it can give off some rather nasty vapours when exposed to heat above it's comfort zone. This is how I went about making my plugs below. The smaller plug is 4.5 mm thread shown against my standard 1/4-32, this smaller plug uses the same type of construction.

My best regards
Gray,

[keith5700]

That's fine, I'd quite like to know too.

[Graham Meek]

Hi Both,

I use a bed of sand to get my small items blue, a small tin with some sand in is heated with a small gas torch, the items when they reach the desired colour are picked out and dropped into some "Fresh" engine oil. You can if you like Oil black by dipping in the engine oil and then burning the residue off in the torch flame, but this treatment tends to be like a "Shellac" and is easily chipped.

My best regards
Gray,

[keith5700]

thanks, I'll give that a go tonight.
Keith.

[///]

Thanks Graham,  I would never have thought you could get such strong colouring from simple heating!
Sure sounds like a more appealing method than the cold blackening kits with nasty chemicals.
Definitely giving this a go, cheers!