8 COM Spark Plugs

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Gannet
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8 COM Spark Plugs

Postby Gannet » Thu Sep 28, 2017 6:42 pm

I like originality and have been encouraged by Keith P to fit an old spec spark plug to an FV.
But in trying to sort one out, I have found that some old plugs have a different thread where the HT lead goes. This seems to prevent the Seagull plug cap from tightening down correctly. Most of the old 8 COM that l have, appear to have a 4 mm thread. A few others including a Lodge 33 and an 8 COM have a slightly coarser thread with a slightly bigger dia.
Why is this? Was the change made at a certain date?
Anyway I fitted an 8 COM to FV2682 and it unsurprisingly started ok. I then used it on the back of my small dinghy on the estuary earlier this week for a couple of days and all went well.
Do we know what plug was fitted to engines of this era (1950 ish) when they left BS?

Jeremy

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Re: 8 COM Spark Plugs

Postby Keith.P » Thu Sep 28, 2017 7:57 pm

Black seagull plug caps are later than the motor, Modified Champion caps are seen on the early little forty motors, Champion caps are seen on most motors before that, Yes, later plugs have a different thread, that's why you find seagull caps with the thread snapped off in them.
When, why, don't know.

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Re: 8 COM Spark Plugs

Postby Keith.P » Thu Sep 28, 2017 8:22 pm

The only way to tell what spark plug to use, is to find the right era manual for the motor, KLG book 1935 says KM1, also told Lissen MC1 for seagull Marston and the lodge book 1955 says Seagull C1, that has most probably changed to something else by now, I haven't found an old Champion book yet.

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Re: 8 COM Spark Plugs

Postby Collector Inspector » Fri Sep 29, 2017 9:47 am

It is always important to have the correct "Model Year Spark-Plug" when restoring any Old Dear of course.............believe me in that I started a side collection of vintage spark plugs correct model year spec for nearly everything I have.

Some may realise what I have is rather eclectic.............(Means LOTS of bloody sparkplugs as an aside collecting thingy)

Thing is that I have found that while being a nice static display, nice restore, proper plug/leads/caps you know "Lookin the Goods" there is one little problem in that;

The Ye Olde Plugs look nice but unless you have a personal tested "Runner" the vintage new old stock plugs that are available at huge cost by now, are seconds from the initial factory or, faulty for reliable general use on a transom.

Never trust an Ebay Seller!

By all means collect Plugs (Doing that is Sexy) but if you want to use a mechanically sound Old Dear in anger.........New modern plug fresh from the the great spark man in the cloud aye?

Hmm what extra loud music to educate my street is of the moment decision.........

Collect it!

BnC
Just took a leaflet out of the letterbox, saying that there's no reason I can't have good sex at 65.

Brilliant - because I live at no. 49 so it's not far to walk home afterwards!

Gannet
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Re: 8 COM Spark Plugs

Postby Gannet » Fri Sep 29, 2017 10:49 am

Keith,
Thanks for your comments. Yes, I am reasonably sure that Fs and early FVs and probably early FVPs were fitted with the modified Champion plug cap.
So you are saying that the later 8 COMs had the slightly bigger and coarser thread which do not fit the Champion or the Seagull plug cap?
Do you agree that the earlier ones were 4mm thread?

You are the one with lots of early data sheets etc. Is there no info on what make of plug was fitted? Were 8 COMs available in 1950?

Bruce,
Thanks for your comment. Surely a spark plug which works well is going to carry on working well for a reasonable length of time? Although I accept that eventually it will fail, but if one uses an engine for say, 20 hours a year, won't the plug last a good few years?

Jeremy

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Re: 8 COM Spark Plugs

Postby Keith.P » Fri Sep 29, 2017 11:42 am

The older plugs had a 8/32” thread and later changed to M4x0.7, when or why, I don't know, I'm pretty sure that it all changed some time in the early to mid fifty's.
My 1955 book lists still show both, the modern push on cap type spark plug top, with and without a knurl thumb wheel, still 8/32” I suspect.
I have no paperwork on Champion plugs, but I suspect Seagull started to make their own plug caps when the plug thread changed and M4x0.7 was more common, plus it's their own plug cap they are selling, or thereabouts.

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Re: 8 COM Spark Plugs

Postby Collector Inspector » Fri Sep 29, 2017 11:51 am

That is the ticket as above both aye!

BnC
Just took a leaflet out of the letterbox, saying that there's no reason I can't have good sex at 65.

Brilliant - because I live at no. 49 so it's not far to walk home afterwards!

Gannet
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Re: 8 COM Spark Plugs

Postby Gannet » Fri Sep 29, 2017 5:13 pm

Thanks Keith, a bit more work sorting through plugs and plug caps in the workshop has revealed what you were saying that the brown Champion plug cap has the bigger thread ie the 8 x 32 UNC.
So as Keith has pointed out the Champion 8 COM of this era ie around 1949/ 1950 ( and perhaps earlier and a bit later) had the 8x32 UNC thread. Interestingly some modern reproduction brown 'Champion' knulled thumb wheels that I have, actually have the M4 x 0.7 thread form.
A pink Lodge CB3 and a C3 also have the 8x32 UNC thread.
I have the M4 x 0.7 and the 8 x32 UNC thread plug gauges which certainly helps in sorting the bits out.

Is it possible that the spark plug industry changed over to metric thread form in the 1950s? Although I cannot imagine the North American industry willingly using a metric thread. Apart of course from the M18 thread! Perhaps Bruce could comment?

Jeremy

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Re: 8 COM Spark Plugs

Postby Collector Inspector » Sat Sep 30, 2017 6:30 am

Now that you ask Jeremy........I have no idea!

Seems logical to change to ISO standards (Manufacture) at some point. I have two virtually identical Champion L10 plugs. Black bodies and round copper gaskets. One has UNC and the other is M4. The UNC has a brass nut and the M4 has the plated steel. I know that both are Australian and from 1960 or so.

The modern equiv is L86c which is M4.

I consider this a homework in progress as what an excellent assignment thank you Mate.

There are reproduction knurled nuts in UNC, M4 and 2BA...all brass.

BnC
Just took a leaflet out of the letterbox, saying that there's no reason I can't have good sex at 65.

Brilliant - because I live at no. 49 so it's not far to walk home afterwards!

Gannet
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Re: 8 COM Spark Plugs

Postby Gannet » Mon Feb 12, 2018 5:26 pm

I have just acquired a few used Champion 8 COM plugs. In the process of stripping and cleaning them etc, I decided to measure the resistance of the central core. Several of them were effectively open circuit and others had a resistance of between 5 and 30ohms. I was using a cheap analogue meter. Are these results (ie open circuit or low resistance) indicative of whether they will function? Or is it due to a feature of the construction of the plug internals?
Maybe l should just get on and put them in an engine and see if they work, but what about the reliability of the 'open circuit' ones?

Jeremy

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Hugz
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Re: 8 COM Spark Plugs

Postby Hugz » Mon Feb 12, 2018 9:30 pm

That is strange. The inner core from top to bottom should have no resistance ie dead short, the meter needle flies across to hit the other side. It is just a conductor. More of a concern is if there is a line of conduction via a hairline crack in the porcelain which cause conduction from electrode to body rather than arcing across the gap. Try testing with a magneto in the dark to see if anything untowards is happening. Also the shape of the firing end of the electrode slowly burns down with use which causes sluggish spark. Maybe a megger tester whould be a better way of testing the insulation resistance. A megger tester puts a very high voltage across the plug to check what voltage escapes. They are a bit pricey though.

Gannet
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Re: 8 COM Spark Plugs

Postby Gannet » Mon Feb 12, 2018 9:45 pm

Thanks Hugz. I have used a megger in the past, but haven't got access to one now. I believe some plugs have a resistor in the central core, so I wonder if these 8 COMs have this feature and failure of this could be the problem. Anyway, I guess the best thing is to test one on an engine and see what happens. It seems such an unlikely failure to me though.

Jeremy

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Re: 8 COM Spark Plugs

Postby Keith.P » Mon Feb 12, 2018 9:56 pm

I have come across 14mm pink Lodge spark plugs with carbon inside them under the top thread, I have had the thread still attached to the plug cap on a number of motors, any corrosion around that thread, can point to a future, as any corrosion would force them to separate the thread from the core.
So I try not to use second hand plugs myself.
Early plugs normally had a steel or copper core.

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Hugz
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Re: 8 COM Spark Plugs

Postby Hugz » Mon Feb 12, 2018 10:05 pm

Yes, I wonder which plugs had the resistor. Pretty sure ours don't. Might do a bit of research.

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Nudge
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Re: 8 COM Spark Plugs

Postby Nudge » Tue Feb 13, 2018 7:30 am

If the plug is dirty then that could be giving you a bad reading.
If you can take it apart, then get out your gas torch and heat the end up.... Dont be shy with the heating,you will be supprised how much the plug can handle.
I have heated one up to red heat (it was a rusted up 8 com) and got it apart. All of the carbon burns off leaving you with a clean plug.

It worked for me, not saying it is the only way. Or the best way!
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