Failed plug

[soxinbox]

I just had some engine instruments installed, and I was going to return my
plane to service. I did two power runs, aborting takeoff right before
rotation. These were normal and uneventful. I popped the cowling and
inspected the engine and all was well. I then taxied out for a test flight.
During the run up the mag check failed, and was severe enough that I did not
suspect carbon fowling. I taxied back and had my mechanic help me check the
plugs. One plug was bad and we replaced it. The engine worked fine and the
flight went on without incident.

Later I took a close look at the failed plug. It has a crack from about 1/4
inch from the seat to 1/8 inch from the threads for the plug wire retainer.
The crack is not opened up, but is a distinct fine crack about 2 inches
long. Is this a normal failure for a spark plug? What can cause this kind of
crack? Is this indicative of a manufacturing problem?

[[email protected]]

Sounds to me like this sparkplug had been dropped onto the floor
sometime before it was installed. Once an aircraft sparkplug had been
dropped, it then belongs in the trash dumpster.

[Michelle P]

Contact with the floor will do this. really hurts when it is a fine wire
plug.....
Michelle

soxinbox wrote:

>I just had some engine instruments installed, and I was going to return my
>plane to service. I did two power runs, aborting takeoff right before
>rotation. These were normal and uneventful. I popped the cowling and
>inspected the engine and all was well. I then taxied out for a test flight.
>During the run up the mag check failed, and was severe enough that I did not
>suspect carbon fowling. I taxied back and had my mechanic help me check the
>plugs. One plug was bad and we replaced it. The engine worked fine and the
>flight went on without incident.
>
>Later I took a close look at the failed plug. It has a crack from about 1/4
>inch from the seat to 1/8 inch from the threads for the plug wire retainer.
>The crack is not opened up, but is a distinct fine crack about 2 inches
>long. Is this a normal failure for a spark plug? What can cause this kind of
>crack? Is this indicative of a manufacturing problem?
>
>
>
>

[nrp]

Sounds more to me like a raw material problem. I can't imagine simply
dropping a plug would open up a longitudinal crack of that magnitude in
the case - or do I have the wrong image? A picture would help.

If the spark plug case is made of a free machining resulfurized or even
leaded steel alloy, these alloys are somewhat prone to having stringer
defects in them. On the other hand such steels are also brittle at
higher temps & I wouldn't think would be appropriate for sparkplug
useage.

[George Patterson]

nrp wrote:
> Sounds more to me like a raw material problem. I can't imagine simply
> dropping a plug would open up a longitudinal crack of that magnitude in
> the case - or do I have the wrong image? A picture would help.

From his description, the crack is in the ceramic.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.

[George Patterson]

soxinbox wrote:

> What can cause this kind of
> crack? Is this indicative of a manufacturing problem?

In addition to dropping the plug (as mentioned by others), this can be caused by
using a standard deep-well socket instead of a plug socket. Tilting the socket
when trying to remove it after tightening the plug is another way to break one.
Hitting the plug accidentally while working on something else is another.
Properly trained mechanics will not do either of the first two; amateurs might.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.

[Scott Skylane]

soxinbox wrote:
/snip/
>
> Later I took a close look at the failed plug. It has a crack from about 1/4
> inch from the seat to 1/8 inch from the threads for the plug wire retainer.
> The crack is not opened up, but is a distinct fine crack about 2 inches
> long. Is this a normal failure for a spark plug? What can cause this kind of
> crack? Is this indicative of a manufacturing problem?
>
>

Is the crack in the outer, metal shell of the plug, or the inner ceramic
lining?

Happy Flying!
Scott Skylane
N92054

[soxinbox]

Sorry for the late reply, I tried to send a reply message, but it got lost
along the way, so I am trying again.

The plug cracked in the outer metal case. The ceramic part inside looks
fine. The crack is smooth and slightly curved and extends longitudanlay from
almost from one set of threads to the other. I will try to put a picture up
if anyone is still interested. I don't think this could have been caused by
over torquing, because this usually creates a crack at an angle to the
longitudianal axis. I also think it is unlikely to have been caused by
dropping it since there is no evidence of an impact. I am thinking it was
caused by cheap metal and repeated temperature cycles, but then again I am
not a meteurlagist.

[nrp]

I have since found another engineer who also thought sparkplug blanks
were indeed machined from screw stock (i. e. resulfurized or leaded
free machining steel alloys). That would still suggest a possible raw
material fault. I certainly wouldn't want any plugs made from that
particular bar.

Be sure you save it. I'll bet you could exchange that plug with its
manufacturer for a new one...........!

[soxinbox]

They already offered that. I am more interested in getting an opinion from
them. My mechanic had them look at an oil filter that had a problem, and he
said they actually took a close look at it and let him know what the problem
was. I hope they are as helpful on this.

"nrp" > wrote in message
ups.com...
>I have since found another engineer who also thought sparkplug blanks
> were indeed machined from screw stock (i. e. resulfurized or leaded
> free machining steel alloys). That would still suggest a possible raw
> material fault. I certainly wouldn't want any plugs made from that
> particular bar.
>
> Be sure you save it. I'll bet you could exchange that plug with its
> manufacturer for a new one...........!
>

[nrp]

Be a little careful - they might "accidently" lose it!

These longitudinal crack-like defects are called sulfur stringers and
sometimes we found them in poor quality resulfurized free machining
steels. It makes the steel very brittle - especially when the
temperature is elevated, and that's why they machine so easily.

I'm not a metallurgist - which is what you really need for a valid
opinion.

If this is what has happened though, as a mechanical engineer I am
appalled - from both a design and especially from a quality monitoring
standpoint.