8 COM Spark Plugs

[Gannet]

Thanks for your comments, Nudge.
I have them apart and they are clean enough to give me consistent resistance readings. The issue is whether they have a resistor in their core and whether an open circuit reading from a low voltage meter is relevant to their reliability and performance.
I plan to fit them to an engine today and see if the open circuit ones work.

I have had success in getting them apart by putting them in a vice and using an extended socket (1/2" BSF ie the 14mm spark plug size) with a very long lever and they then suddenly loosen -so far!

Jeremy

[Gannet]

I found one of my 8 COM plugs had a fractured insulator. So I smashed it up to determine how the central conductor was constructed. The main core went from the tip to the HT threaded connector. It was simply loosely engaged into the bore of the threaded connector by approx. 1/8". There was plenty of diametrial clearance (probably about 0.010) between the conductor and the bore. This would explain the lack of electrical conductivity and also some resistance with some plugs. It would have no adverse affect on performance or reliability, which was my concern.
I have now run all of them for a very short period in the workshop on an FV. They all started on first pull and have been fitted to FVs. I expect - well hope really - that they will perform reliably in use. How can they possibly fail? I don't want a long list in reply to that!

Jeremy

[Charles uk]

Jeremy may I have the metal parts of your 8 com, as that will save me destroying one of mine, please don't throw them away.

Charles

[Hugz]

Any chance of a photo please Jeremy?

[Gannet]

Charles,
That's no problem as l never throw anything away! If you have got any of the internal copper sealing washer, I would find that very useful.
The internal costruction of the conducter explains the variable electrical conductivity of the plugs and the irrelevents of that to their performance.

Hugz,
Certainly, I will post one tomorrow. Obviously my description was a bit vague!

Jeremy

[Hugz]

Excellent. This is so exciting. Resistors were later installed to suppress radio interference (EMI) and the interference also affect solid state circuitry ie motor control mother boards. Not something a Seagull has to worry about. I believe carbon fibre replaced copper in HT leads for a similar reason.

[Charles uk]

I haven't got an old 8 com, just a very tired 7 com (twin electrode & very different shape).

[Keith.P]

Looking at a 30's spark plug books, the inner core was steel, with an outer copper sleeve drawn over the electrode core and held in close contact, thus increasing its heat conductivity, so nothing to do with the spark, but for removing heat in the spark plugs core.
As a side note, suppressed plugs are already available in the 50's, and radio screened plugs are the fully enclosed military type plugs.
Even found spark plugs for diesel engines that use a spark ignition system.

[Hugz]

That's interesting. I wonder if valve (glass tube) radios where more prone to interference than transistor radios?

[Charles uk]

See how Marconi's transmitter worked.

[Hugz]

I'm still working out how a string and two cans work!

[Nudge]

Is this what you are talking about?

This is the 8COM that I cooked to get apart. It did work after I cooked it, but I wanted something to find top dead center.

[Gannet]

Hugo,

Here is the photo of the stipped down/ smashed up 8 COM, showing the central conductor and the top electrical conection into which the central conductor goes.
I have also e mailed the photo to you directly.

Jeremy

[seagull101]

Nudge, I would say that that is too flimsy to find TDC, i borrowed something similar to do my Marston timing and it was a M10 bolt threaded through the nut part of the plug.

[Hugz]

Excellent, thanks Jeremy. The join of the connector and the electrode does seem to be a weak point. You would think it would be threaded or similar. A bit of arcing there would cause pernament problems. I'm assuming the electrode assembly was fired (in kiln) within the porcelain. Not as l imagined.

I just use a screwdiver to find TDC. Am l missing something?