Aluminum head | tap vs chase

The spark plug hole threading on an automotive alu head that I'm working on is not consistently conducting/grounding spark energy back to the
battery negative terminal. There is high resistance and/or open circuit between the all 4 thread holes, and other parts of the head (which is well-grounded to the battery negative terminal), whereas all other parts of the head show virtually no resistance (between two probe points on the head and/or to the battery negative terminal). The threads appear undamaged, never repaired, reasonably clean, but have a goldish hue to them and appear 'hardened' (for lack of a better description). The plugs seatwell on the head - no blow-by - and other ignition components and wiring have been ruled out as best as possible.
I'm not sure if correlation will extend to cause and effect of a particular driveability/misfire problem I'm trying to sort out, but is theresomething that could cause the alu to 'harden' and to change/impede, at least at the thread surface, its electrical conductivity properties?
--
What is the difference between a 14M X 1.25 tap, vs a 14M X 1.25 chaser?
Specifically, how will they cut differently (if they do), which hole
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LanceM wrote:

I suggest you connect a temporary ground wire to the threaded metal section of each spark plug to rule out this possibility as a contributor to your symptom. The aluminum threaded holes will not stay clean, but will oxidize, eventually. And this is not a place where you want ot weaken the threads by removing material. The coating may also perform and anti galling function.
--
John Popelish

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    A chasing tap will remove less metal than a regular tap.
    How are you measuring the resistance at the plugs? If you're trying to touch the inside of the plug hole with meter leads, then you might not be able to get a reading because aluminum develops an oxide coating (similar to a thin "rust") that can be a very good insulator.
    The best way to measure that resistance is to torque a plug into the hole and measure from the plug shell to the battery negative. The threads on the plug should cut through the oxide on the head because the oxide will be very thin and then you should see a low resistance.
    Even if your meter doesn't show a low resistance, the voltage at the plug is 10,000 volts or more and it should make the oxide "break down" and conduct with no problems. The voltage at the tips of your meter leads is probably only a couple of volts and that probably won't make the oxide break down electrically.
    In short (pun intended) if you have a problem it's *not* because the plugs aren't grounded.
Jim
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Gave us:

Exactly. If the spark makes it to the electrode tip, it WILL make it to ground.
Hell, the plug wire has a higher resistance than that.
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James Meyer wrote:

By both of the methods you describe below

What concerned me was a comparison to another sample car/spark plug hole threads/alu head, which showed virtually no resistance. But the "oxide coating" has been helpful

Yes - have done - and it does show very little to no resistance

A bit more background which I perhaps should have stated previously. At night, _directly beneath_ all 4 spark plug bases, I'm seeing mild blue glow rings, or a HT spark energy leak of sorts. I do not see any evidence of spark energy leaking out the spark plug wire boots, running down the spark plug porcelain, to the head. Or any other leaks throughout the ignition system. I've used various techniques with a scope, water misting, ground probe, etc
All of the 2dary ignition components have been replaced a few times with quality components. Everything is tight and secure
The intensity of the glow rings somewhat increases at a specific problematic mid-range RPM, and decreases at higher RPMs, surrounding where the spark plug gaskets make contact with the head
Thanks for the comments. I have other possibilities to pursue, too (but the glow(s) bugs me, as they shouldn't be there, afaik)
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"LanceM" a mis l'ide suivante :

I don't think the glow will be a result of a high resistance from plug to body. The voltage is so high and is intended to jump quite a gap across the plug points, it would easily overcome any resistance in the threads.
Just a thought, I've never actually seen this.... Perhaps the blue glow is a result of seeing the combustion through the plug insulation, or even a poor connection/break in the HT lead close to the plug.
--
Regards,
Harry (M1BYT)
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    I would use a short piece of wire connected to a ground close to the plug to probe around the base of the plug. If I could get the blue glow to follow or be attracted to the tip of the grounded wire, then I would more convinced that it was electrical leakage.
    I have seen spark plug wiring schemes where the HV wires are made from coaxial cable. The center conductor carries the HV of course and the braided outer shield is terminated at the plug in a sleeve that slips over the plug and makes connection to the plug's shell. It looks somewhat like a giant RF connector. The other end of the coaxial plug wire goes to the distributor which is enclosed in a full metal can with holes for the cable and the outer shield bonded directly to the can.
    The special wiring was intended to be used in cars with 2-way radios installed. The coaxial wires contained the spark noise that would otherwise interfere with reception.
    A set of wires and a distributor cover like that would eliminate the need for the plug to make electrical connection to the block at all since the spark current would flow to the plug through the inner conductor and return through the outer shield connection.
Jim
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<snip>

Get a long screwdriver with a plastic handle. Slide the scredriver blade against the head, down into the space next to the sparkplug. Turn the blade so it shorts between the head and the hex-nut base of the plug. When it shorts the two, see if the 'glow' goes away. If you keep the blade in contact with the head at all times, you wouldn't even need to wear any electrical glove.
I kind of doubt this, but a corona discharge around the plug base would mean the plug is 'floating' several hundred volts above the head that it is threaded into. This seems very unlikely. A quick short with screwdriver between plug base and head will tell you a lot.
If you prefer not to do this while engine is running, wedge something like a boxed-end wrench tightly against the plug base with engine off, then start engine and see if 'glow' is still there.
Used to be a spark-plug by 'auto-lite' that the tower was translucent. You could see the 'flash' of the combustion through the porcelin. As I recall, the color was orange-yellow, not blue. The light from the orange combustion far overwhelmed the blue spark. So I don't think *that* is what you're seeing coming through the porcelin.
daestrom
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"daestrom" a couch sur son cran :

There used to be a diagnostic glass spark plug, intended to allow you to set the mixture accurately by looking at the colour. As I recall you set it to a neon blue colour, yellow would indicate far too rich a mixture.
--
Regards,
Harry (M1BYT)...
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'Aluminum head | tap vs chase', on Sun, 7 Dec 2003:

AFAIK, a tap is for cutting a new thread, whereas a chaser is for cleaning up a damaged thread, so it substantially doesn't remove metal, it rearranges it.
The top end of the tap MAY produce a slightly over-size thread. Think 0.1 mm (4 mil) or less.
1.25 IS the thread pitch, so there is no difference.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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about

It's been a while, but I think if you look at the 'taper', you'll also notice that a 'tap' has a long, narrow taper to aid in starting the thing 'square' in the hole. Chasers have a much more 'blunt' look to them with just a few threads on the end tapered. With pre-existing threads, it isn't too hard to get started without 'cross-threading'. Then just a length of straight threads to allow chasing through a deep hole. Also good for 'bottoming' a blind hole.
daestrom
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It sounds like oxide coating similar to anodizing alluminium oxide is an insulator which can cause strange readings on a resistance meter, however I think it's unlikely to resist a good 12000volt spark Does the plug body show high resistance to the head when properly seated?
Does the problem dissapear if you hook up some temporary grounding wire to the plug bodies?
You could try to clean the plug seat where the washer goes with emery

All taps should cut the same final size they just vary in the taper start.

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LanceM wrote:

Thanks for all the suggestions. I'll try some. I also borrowed a better quality scope
Lance
---------
alancemor....AT..yahoo.....DOT...com
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