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Spark plug dropped.... Why throw away?

William

Well Known Member
I've read if a aviation spark plug is ever dropped to drop it again in the trash. Never understood what part on the spark plug gets damaged and why It can't be inspected visually or with a ohm meter to test if it was damaged. Reason I'm asking is I have a vibration issue at idle up to about 900 rpms. My engine is freshly overhauled o320 with new plugs but I did drop one or two of them. Above 900 rpms the engine is smooth. I have a engine monitor. Is there anything specific to look at to indicate if a spark plug is damaged. My mags were overhauled by myself with all new internals except the coil. My coil ohms out fine, but could it be the problem as well?



Thanks
Bill
 
On my Beech's last annual, I found one of the plugs had a large section of the ceramic insulator missing. The A&P said that was typical of what happens when a dropped plug is re-installed.

We borescoped the cylinder and could find no damage, so perhaps the piece broke off outside the engine.
 
When a spark plug is dropped......

The ceramic insulator can be cracked internally and usually cannot be seen by visual inspection.
 
Eventually dropped plug will show its ugly face. Since airplane in service from 2011 I dropped three spark plugs. I thought I would use the third one there was no visible damage. After about 20 hours it was running rough I removed the wire and pieces of shuttered insulator came out.

 
It will fail somewhere other than your home base.

I did the same thing once. Plug hit my shoe on the way to the floor and was a fine wire plug ($$$) so I decided to test it out. Put it in everything tested fine during run up and quick test flight.

1.5 hours into the next cross country the plug started failing causing an extra stop and having to find a plug out of town on a weekend (also carry a spare now).

There was never any visible damage to the plug even after the failure.

Andy
 
Because spark plugs aren't made for drop tests. Auto or Aviation. It's sad, but true.
 
Failed Ceramic Bond

I've read if a aviation spark plug is ever dropped to drop it again in the trash. Never understood what part on the spark plug gets damaged and why It can't be inspected visually or with a ohm meter to test if it was damaged. Reason I'm asking is I have a vibration issue at idle up to about 900 rpms. My engine is freshly overhauled o320 with new plugs but I did drop one or two of them. Above 900 rpms the engine is smooth. I have a engine monitor. Is there anything specific to look at to indicate if a spark plug is damaged. My mags were overhauled by myself with all new internals except the coil. My coil ohms out fine, but could it be the problem as well?

Thanks
Bill

The most insidious failure of a dropped plug is one where the bond between the ceramic insulator and the steel barrel of the plug is fractured. When this happens, the insulator can no longer efficiently conduct heat to the plug barrel and the cylinder head. This can result in the plug getting extremely hot and causing preignition, usually with devastating results. If the plug is dropped, smash it with a ball peen hammer and throw it away. Period. It's just not worth the consequences to try and use it.

Skylor
 
Dropped Plug

Don't use it! Will eventually get ugly at the worst possible time.

Was taught by the late Frank English of Macon Georgia, former chief of maintenance USAF recip engine maintenance and a WWII Spitfire mechanic as part of the Eagle Squadron.

Frank said "If you ever drop a plug once, drop it twice...into the trash!"

From experience I agree!
 
Thanks for the informative responses everyone. Forgot to mention I have new plug wires as well. This vibration is only between 700 and 900 rpm. Any ideas to check for?

Thanks
Bill
 
I have never purchased airplane plugs so I don't know how they are packed. I hope they are packed better than auto plugs and have more padding than a cardboard box. I can't believe that if a guy working in shipping dropped a box of plugs that he would fess up and take the heat damaging them.

Is there a way to test them that is fool proof or you just take your chance?
 
I have never purchased airplane plugs so I don't know how they are packed. I hope they are packed better than auto plugs and have more padding than a cardboard box. I can't believe that if a guy working in shipping dropped a box of plugs that he would fess up and take the heat damaging them.

Is there a way to test them that is fool proof or you just take your chance?

The EICommander will tell you if a plug is bad, but you have to run P-mags for it to work.
 
Vibrations could be from a rough idle. Carbureted? Maybe you have an induction leak. If a small leak, engine will smooth out with increasing rpms. Just a thought.
 
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