Spark Plug feedthroughs 101

Things at the limits.

Re: Spark Plug feedthroughs 101

Postby Jerry » Sun Sep 26, 2010 12:48 am

johnf wrote:I had to do this with a triple start thread for one of my exams thirty four years ago as a fitter turner apprentice with no indexer on the lathe. I cheated by marking where the jaws on the chuck were positioned on the headstock with a pencil line and made sure the thread feed aligned -it worked --I saw the errors in my ways and transferred into a instrument making internship shortly after-- I had all I needed for machine shop so 2 years of electronics and a short stint 6 months of glassblowing finished it off (90% of this I have forgotten).


I have a guy that wants me to make him a tap for some odd ball triple start thread used in fountain pens. Figure since my lathe has a D1-3 spindle nose that has three cam lock pins I will do one thread, index the chuck, and do it again. Its actually something I cant do in my CNC as the machine only reads the index pulse on the spindle for timing.
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Re: Spark Plug feedthroughs 101

Postby Starfire » Sun Sep 26, 2010 9:03 am

Dud! - thanks Doug. :?
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Re: Spark Plug feedthroughs 101

Postby chrismb » Sat Oct 02, 2010 12:37 pm

I have been mulling this over and examining which plugs might be most suitable and cheaply available, seeing as I want several in my setup.

I think I might give it a go.

I am still quite concerned over the sealing and am not in a position to do any detailed work. I have a machinist friend who has done a little work for me when I've needed accuracy, but I don't think it is fair to lumber the job of doing up a dozen plugs.

So I would be grateful for views on this approach: I am considering using the very common BP5EFS. The 'FS' mean it has a tapered face rather than a gasket. I suspect it'll be trivial to set the machine up to put a matching taper onto the Al baseplates I have got. I note that the thread pitch of plugs aren't the standard pitches; the 'B' is 1.25mm, as it is for the 'D' plugs (the difference being D is M12 and B is M14). If the thread is the same pitch, I presume that leakage around the thread will actually be less with wider diameters, as molecules will have longer to go?

Given that I am neither capable of machining a seat on plugs, nor do I feel I want to ask someone to do such work, I think using these tapered plugs sounds a reasonable plan. As I am deviating from the recommendations here, I would be most appreciative of comments as to the issues/problems I might be creating for myself. (I'll also be using these with the sparky end to air as I require tall standoffs in my chamber.)
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Re: Spark Plug feedthroughs 101

Postby johnf » Sat Oct 02, 2010 1:07 pm

Chris I would see the tapered seat plugs as being a good option for you

Probably pay to make sure the plug is a non resistor type if you are feeding power through it
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Re: Spark Plug feedthroughs 101

Postby Doug Coulter » Sat Oct 02, 2010 1:22 pm

Yes, you should be able to get a seal, that way or with mere teflon tape, actually, at least if it's not going to get hot. There will be air trapped in the threads (that virtual leak thing all the vacuum books talk about) unless you grind a relief flat on one side and depend on the taper to seal. In Al, it probably will, if you don't strip out the threads tightening it. I'd suggest epoxy, but you might have to change it someday! You might want to look at a soft metal gasket (annealed copper washer) or one made of teflon or similar for the actual seal.

The resistor plugs really won't take even milliamp currents for very long -- I've blown a couple with a 12w CCFL inverter driving an ionizer. How they live in a car, I don't know, but they won't stand sustained currents well at all.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Spark Plug feedthroughs 101

Postby chrismb » Sat Oct 02, 2010 1:33 pm

Thanks for the feedback. There are BP5ES and BPR5ES, so I presume the former are sans resistor. I think I may buy one tomorrow and experiment with it first (electrically and mechanically) before deciding if I should accommodate it in my new bulkhead design.
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Re: Spark Plug feedthroughs 101

Postby Jerry » Sat Oct 02, 2010 2:56 pm

There are also ignitors like this:

http://cgi.ebay.com/CROWN-AUBURN-IGNITI ... 500wt_1156

They have an extended insulator and longer electrode.
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Re: Spark Plug feedthroughs 101

Postby chrismb » Sun Oct 03, 2010 5:24 am

I was chewing over Doug's suggestion. If I might pull on your thoughts a little more ( - and maybe this thinking-process will add to a '101' thread).

If I were to use copper washers to help with a seal, then would that work better with a taper in place on both, such that the copper is forced to undergo ductile working, or to have flat-flat, or even flat on the bulkhead and tapered on the plug so that the washer is forced to 'bite' the edge of the bulkhead?

All of this worries me a little becase I would presume that with enough torque to do this that I might strip Al threads.

I guess another question arising is whether the metal gaskets that come with spark plugs can achieve the same effect and whether anyone has assessed their vacuum sealing properties (again, some concern over whether this would involve excess torque to produce a successful seal).
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Re: Spark Plug feedthroughs 101

Postby Doug Coulter » Sun Oct 03, 2010 3:22 pm

I would try with taper on both, as that alone might work OK without a gasket. Kind of depends on the Al alloy and heat treat state. If I knew I'd use a washer for sure, instead of a taper in the hole, I'd step drill the hole just a little larger, and use the knife-edge that creates to get a good seal. In softer Al, that whole trick might be moot, however.
You're most likely going to wind up with 6061-T6 as it's common. Midrange properties.

Re stripping, use a thick piece of Al and deep thread depth. Finding anti-seize that won't cause other troubles might be an issue there. If low temps are expected, a little beeswax/MoS2 mix might do well. If not, I don't know what you'd use for the "fluid" part of such a mix that would have low enough vapor pressure. I would anodize it to give a hardcoat surface, which is generally a good enough anti-seize.

The gaskets that come with the flat base plugs kind of stink. They are hollow so they can be crushed against both faces (some are made of two pieces of metal spotwelded together). But it's not soft enough metal to conform well to rough surfaces -- they solve a different problem in an auto, which can tolerate leaks we'd consider huge. If you had both surfaces nicely shiny (under 25 micron surface roughness) you could probably get by with an o ring or teflon washer.

This is one of those weird things. Such a washer can tolerate a wavy surface -- it can bend and flex into the dips, but tiny scratches leak as it won't get down into them!

My first try would be to just screw in a plug, and maybe paint the outside with Glyptal (the real thing) to seal. It would be easy enough to break when replacement was needed, not require high torque, and has vapor pressure of nil once baked a bit (it cross-links and thermosets at something a bit over 100c). If that worked, I'd further file a flat or grove down the plug threads to let trapped air in them get out easier so I could get to purity quicker on pumpdowns.

Those igniters are cool, Jerry. Are they from oil burners or something like?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Spark Plug feedthroughs 101

Postby Joe Jarski » Sun Oct 03, 2010 8:32 pm

Does the plating on the spark plug body cause any problems on the vacuum side? Should it be removed?
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