Author Topic: Using ptfe in spark plugs  (Read 6783 times)

Offline keith5700

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Using ptfe in spark plugs
« on: February 09, 2015, 04:28:25 PM »
I gave the V8 a bit of a thrash yesterday,whilst filming a new video but it didn't seem to be running quite right.
I pulled a few plugs out afterwards and found these.
The PTFE looks like it's just been sparked away, rather than burned from combustion.

I suspect it's because I'm using the Megasquirt ecu, which is a wasted spark principle, so the spark it gives when on the exhaust stroke is just too big and fat for the ptfe. I'm assuming when the pressure is low the spark will be much bigger than on the compression stroke.

Anyway, I need a better insulator, cheaper than Macor!




Offline ///

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Re: Using ptfe in spark plugs
« Reply #1 on: February 09, 2015, 04:52:33 PM »
I found a supplier of Macor with very reasonable pricing.
I've not purchased anything yet as they do not ship to Australia and I've not yet sorted out a forwarder, but I am definitely going to give them a go.

http://www.technicalproductsinc.com/rods.html
Clicking any of their stock takes you to Amazon.
As an example, a 12" length of 1/4" rod, $27.
Best price I've ever seen by a huge margin, but is it too good to be true? I don't know yet.
http://www.amazon.com/Macor-MAC1-0412-Machinable-Ceramic-Diameter/dp/B00I2U5LTA

Edit: From memory they have a $50 MOV
« Last Edit: February 09, 2015, 04:56:26 PM by /// »
Simon

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Offline Jasonb

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Re: Using ptfe in spark plugs
« Reply #2 on: February 09, 2015, 05:26:19 PM »
Graham Meek went for "fishspine" ceramic wire insulators at the spark end and kept the ptfe/corian on the outside. Seem to work very well.

Offline keith5700

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Re: Using ptfe in spark plugs
« Reply #3 on: February 09, 2015, 07:38:26 PM »
Simon, that size would be perfect, and the price looks to be correct. I tried to order some from Amazon in US but they won't ship to UK either.
Here in rip of Britain, RS want $37 for a piece 6mm dia x 100mm long, which is $111 for 12"! I reckon I need 3 x 12" pieces to do both engines.

Jason, I actually have some small ceramic tubes which may be suitable but they're not really ideal, this is my backup plan though.

Cheers both.

Offline Allen Smithee

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Re: Using ptfe in spark plugs
« Reply #4 on: February 09, 2015, 10:21:16 PM »
Looking at that and wondering if it correlates with my experiences from my motor-racing days (early 80s  :old:) it could simply be that the central electrode is too heavily shielded, so it's overheating. You could try simply shortening the PTFE shroud and leaving more exposed electrode to see if that fixes the problem. It could also be too much spark energy; mechanical ignitions were essentially self-limiting, but when we went over to CDI/Plasma ignitions we found that they could achieve ridiculous spark energies if asked so it was important to ensure the HT voltage wasn't too high. This (as I'm sure you know) is controlled directly by the plug gap, and if you set the gaps too wide with a halfway decent electronic (simple CDI, let alone plasma) system the HT voltage gets very high leading to an oversized (and over-hot) spark. So you could try simply dropping the plug gaps to see if that stops the damage.

0.03 supplied,

AS
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Offline PStechPaul

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Re: Using ptfe in spark plugs
« Reply #5 on: February 09, 2015, 10:34:47 PM »
I don't know if McMaster ships to the UK, but they have various ceramic rods that might work:
http://www.mcmaster.com/#standard-ceramic-rods/=vu7uy2

1/4" Macor (1470F) is $72/12" and a lower temperature 930F is $58.

You might also consider ceramic molding compounds:
http://www.mcmaster.com/#molding-compounds/=vu7xo1

http://www.mcmaster.com/#8498k41/=vu7y7s  This is 4 pounds of silica which will handle 2700F, for $55.

Offline ///

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Re: Using ptfe in spark plugs
« Reply #6 on: February 10, 2015, 03:22:01 AM »
Simon, that size would be perfect, and the price looks to be correct. I tried to order some from Amazon in US but they won't ship to UK either.
Here in rip of Britain, RS want $37 for a piece 6mm dia x 100mm long, which is $111 for 12"! I reckon I need 3 x 12" pieces to do both engines.
......

Hi Keith,
Yes I've seen similar pricing at RS Australia, but then these companies cater to an industry where if you need something now and price isn't a major concern, just lead time, then they've got it. My own company has purchased from them often for this very reason.

Regarding needing 3 off 12" lengths, I had figured it would probably be better to get a 2x6" sheet of 1/4" for much less cost than 3 off 12" rods, then with a hacked together table saw(using a thin slitting saw or similar) rip it into 8 lengths of ~1/4" wide. You would end up with the equivalent of 48 lineal inches versus 36, minus the loss for chucking of course, and save roughly $30.
There are 5C collets for chucking square stock, my only concern however is how well macor would behave with interrupted cuts while turning the corners off the square.
Simon

"The reality is that without cheap imported machines, I would be spending my time doing something less creative and less enjoyable" - Captain Jerry

Offline Graham Meek

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Re: Using ptfe in spark plugs
« Reply #7 on: February 10, 2015, 07:57:35 AM »
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,

Offline PStechPaul

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Re: Using ptfe in spark plugs
« Reply #8 on: February 10, 2015, 10:04:08 AM »
Would it be possible to use a threaded ceramic insulator? I found some at McMaster that are 1/4" OD threaded for #6-32 and 1-1/2" lomg for $6, but it's only threaded 1/4" on each end:
http://www.mcmaster.com/#ceramic-standoffs/=vugk7r

An unthreaded ceramic spacer 1/4" diameter and 3/4" long is about $3:
http://www.mcmaster.com/#96109a180/=vuglno

The ceramic tubing I found at McMaster is high alumina and poor insulator. But perhaps these 5x3x25 mm ceramic tubes would work, for less than 25 cents each ($5/20):
http://www.ebay.com/itm/5x3x25-5mm-Cylinder-Shape-Ceramic-Insulation-Protection-Tube-White-20pcs-/311183397250

I found some micro spark plugs with 1/4"-28 threads:
http://www.ebay.com/itm/STITT-MICRO-SPARK-PLUG-1-4-X-32-THREADS-FOR-IGNITION-MODEL-AIRPLANE-ENGINES-/171667880735

I'm interested in this because I am working on an ignition module for model engines and I'd like to know more about what is needed, such as voltage and spark gap. Thanks.

Offline Ian S C

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Re: Using ptfe in spark plugs
« Reply #9 on: February 10, 2015, 11:48:38 AM »
The plugs with the fish spine ceramic insulator were featured in an article in "Model Engineer", those plugs used the little glass tube that is found inside a domestic incandescent light bulb, melted into the steel body of the plug, with the electrode in place.
Ian S C

Offline keith5700

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Re: Using ptfe in spark plugs
« Reply #10 on: February 10, 2015, 12:45:54 PM »
Thanks for all replies/suggestions, I'll have to do a bit more reading to see which is the best way forward.

1/ Macor
2/ceramic casting
3/ceramic tube, fishspine
4/melt glass around electrode

The Megasquirt has got an adjustable spark energy setting, but I haven't got a working PC at the moment and I can't remember what I set it to when I got it running initially.

Cheers.

Offline IanR

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Re: Using ptfe in spark plugs
« Reply #11 on: February 10, 2015, 12:50:40 PM »
Len Mason built some small spark plugs, down to 2BA thread I think, using epoxy resin as the insulator, and wrote it up in ME. It worked, but got flexible when warm.

Offline gbritnell

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Re: Using ptfe in spark plugs
« Reply #12 on: February 10, 2015, 01:49:14 PM »
There's always the option of using Corian. Steve Huck, myself and others have used it for some very small plugs with good success.
gbritnell
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Online steamer

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Re: Using ptfe in spark plugs
« Reply #13 on: February 10, 2015, 05:16:25 PM »
Keith,

My friend Roland Gaucher uses Corian very successfully for spark plug bodies.    Go to the corian dealers, and bring the cylinder head with you....bat the eyes...let them fondle the head, and I bet you a pint they'll give you a nice slab of cut for you...probably what ever color you want!... :lolb:


Dave
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Offline PStechPaul

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Re: Using ptfe in spark plugs
« Reply #14 on: February 10, 2015, 06:02:37 PM »
Corian seems to be rated only up to 212F.
http://en.wikipedia.org/wiki/Corian

There are some grades of Glastic (fiberglass) that work up to 155 and 210C:
http://www.professionalplastics.com/FIBERGLASSRODS-BARS

You can get electrical grade fiberglass (GPO3) rated 265F for $5 for a 1/4" diameter x 3 foot piece
http://www.mcmaster.com/#2591k31/=vumvxo

Perhaps the insulator could be made from borosilicate (pyrex) glass tube rated 445F, 1/4" x 1 foot for $3.44
 http://www.mcmaster.com/#8729k31/=vumxa3

Also look into high temperature epoxy (pyroputty) 450F to 2000F:
http://www.mcmaster.com/#high-temperature-epoxies/=vumypk