[Shop-talk] Spark Plug Cleaner

Sun Oct 21 16:21:19 MDT 2018

Answers here to many things you didn’t ask about…  About aircraft plugs, where blasting is the most common, but mostly applicable to car and small-engine plugs as well.  FWIW, I’ve been blasting car plugs since the 60’s and aircraft plugs since the 70’s.

Blasting is still used on most aircraft plugs, same as ever.  Aviation plugs cost $25-$50-$75 apiece and up, depending on type and how much of a bargain hunter you are.  No big sales at the local auto parts store.  Very few owners replace plugs at every Annual or 100-hour inspection.  Piston aircraft engines have 2 plugs per cylinder, and flat engines most commonly have 4 or 6 cylinders.  Radial engines usually have 7 or 9 cylinders (or 3 or 5 on some antiques), and the really big ones have multiple rows of 7 or 9 cylinders.  Lots of plugs.

The two largest aviation spark plug manufacturers, Champion and Tempest (originally AC) both publish blasting instructions and sell plug blasting/testing machines.  Quite obviously they approve.  Like a majority of individuals working mainly on their own planes, I don’t have one of these $1,000 to $1,500 machines.  The “Vixen” brand one that most auto parts stores still sell for under $20 works just fine.  It doesn’t have the impressive pressure-tester where you see the spark jumping while the plug is under pressure, but that test isn’t really helpful 99.9% of the time as long as you do a good job of cleaning.

As far as abrasive, I use a garnet abrasive because it indeed breaks down and exposes fresh sharp edges.   Most auto parts stores sell little bags specifically for use in plug blasters.  And if a garnet particle gets into the engine despite careful magnified inspection as part of my reconditioning, that particle won’t do as much damage as aluminum oxide.  But it’s probably not critical as long as you get all the abrasive out in the first place before reinstalling the plug.  Either way, abrasive is cheap to buy and lasts through many many plugs.

There isn’t actually a shiny coating or glaze on the center or nose insulator, and the ceramic itself isn’t degraded much by normal light blasting.  What the aviation plug manufacturers DO caution about, however, it to avoid eroding the cement that holds the center electrode into the insulator.  With extended blasting the cement erodes and a well develops around the center electrode.  Not sure how bad it needs to get to be an issue – I have some plugs with slight erosion and they work fine, don’t foul notably different than new ones, etc.

To get the initial carbon off (because its’ a really slow process to blast 14 plugs completely without doing some initial carbon removal), I first use Hoppe’s #9.  Just set the plugs upside down in a rack and drop in enough Hoppe’s to fill the plug interior.  Wait an hour (15 minutes is actually enough most times), dump out the Hoppe’s, and  spray carb cleaner or Brakleen into the plug to get much of the carbon out.  Then blow dry, and then blast.  Speeds up the process immensely, especially getting the deep carbon out or at least softened.

While cleaning, some of the carbon inside the plug’s outer steel shell is hard to remove.  Champion (etc.) sells a plug cutter tool that’s like a tiny curved saw blade, made to fit  into a vibrating engraver-type tool.  That makes fast work of the excess carbon around the outside.  Carbon on the center insulator usually comes off pretty easily.  Once you think you’re pretty much done.. you visually inspect the inside for missed deposits and any remaining abrasive.  A dental pick helps there.

The other thing that’s important in reconditioning aviation plugs is to check the resistance.  There’s a resistor in virtually every modern aircraft plug, not for RF suppression but for good running.  If the resistance decreases (it can over time) the plug will develop a misfire.  RF suppression is handled by shielding of the magnetos, cables, and the plugs themselves, which have a heavy steel outer case.  You never even see the ceramic outside like you do on an automotive plug (except for unshielded aircraft plugs, which are increasingly rare).

Massive electrode plugs have a large center electrode and (usually two) large side electrodes that are shaped to surround the center electrode.  The side electrodes can be bent in and out to set the gap.  While plug inspection and cleaning is part of every 100-hour and annual inspection, blasting can in time round off the edges of the electrodes and make the plugs less efficient, as the primary area for sparking to begin is at the sharp edges.  I’ve taken an ignition file to the tops of my electrodes a few times on older plugs to remove some of the blasting-induced rounding.  They look better, but I’m not sure it makes lots of difference in running.

Plug manufacturers don’t recommend blasting on “fine-wire” plugs, which are similar to platinum, etc. automotive plugs except the side electrode is a thin piece of platinum or iridium.  Those come pre-gapped and all you’re supposed to do is verify the gap, and if it’s incorrect replace the plug.  At $75 apiece minimum.  But they don’t foul much, which is why they’re used, and they last a long time.

Final caveat.  Shielded aviation plugs are all heavy steel on the outside.  All the ceramic is inside.  If you drop one, it looks fine.  Even after carefully inspecting it, all the ceramic looks fine every time.

But there’s an old aircraft mechanic’s saying that you only drop a plug twice.  First time on the floor, and the second time into the garbage can.  I’ve violated that a few times, but 75-80% of the time the plug is either bad immediately or goes bad quickly – within a few hours at most.  I’ve marked the plugs I’ve dropped and attempted to reuse, but after a few times realizing that the old saying is right, I haven’t reused any more dropped plugs.  A bad miss or worse really gets your attention in the air.

Have a blast !!

Karl

From: Shop-talk [mailto:shop-talk-bounces at autox.team.net] On Behalf Of John Innis
Sent: Saturday, October 20, 2018 8:02 PM
To: Al Fuller
Cc: shop-talk
Subject: Re: [Shop-talk] Spark Plug Cleaner

Only time I ever used one was on aircraft type plugs.  Auto and small engine plugs are cheap, I throw them away.  Aircraft plugs also have "massive" electrodes, and can tolerate some quite a bit more material loss.

On Sat, Oct 20, 2018 at 7:53 PM Al Fuller <al at bighealey.org> wrote:

Anybody still using spark plug cleaners – such as the small air powered sand blasters of old?

I’m looking at plugs for several cars, snow blower, etc. – and wondering whether the cleaning is known to damage the plugs.  I don’t recall that being an issue when I used them years ago, nut I do confess that the newer plugs seem to be made of more exotic materials.

Thoughts? 

Al Fuller

'65 BJ-8

'85 Rx-7

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