Spark plugs - replace on hours?

Katamarino

Katamarino

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Katamarino
2 of my plugs (massive electrode) were just replaced due to small cracks in the ceramic. The others have ~500 hours on them.

Would it be good practice to replace the remaining 10 at the same time, given their age? Or just soldier on and replace as needed.

If it makes any difference, I will be setting out soon on about another 100 hours flying including Pacific crossing, but I do carry a set of spare plugs with me.
 
If the electrodes are still fine according to the wear gauge, and they pass the pressure bomb spark test, they should be fine. Are they getting that level of maintenance?
 
Dan Thomas said:
If the electrodes are still fine according to the wear gauge, and they pass the pressure bomb spark test, they should be fine. Are they getting that level of maintenance?
Click to expand...

Every 100 hours I clean and gap them. If there's a spark tester available, I use it. I have a full engine monitor so identifying a bad plug on the mag check should be straightforward.
 
Katamarino said:
If there's a spark tester available, I use it.
Click to expand...
Agree they should be good to go provided the remaining original plugs have been pressure checked this inspection cycle given your intended flight schedule and two of the originals had cracked insulators.
 
Was the ceramic cracked on the outside? If the crack was on the inside at the center electrode insulator, I’m wondering if it’s worthwhile to trend your past CHT and EGT in those two cylinders? I ask because in automotive circles, cracked ceramic is a symptom of detonation. Just a thought. A&Ps would know better than me though.
 
I watch the CHTs and EGTs pretty obsessively, didn't see anything out of the ordinary. No CHTs over 380.
 
I clean, check, and gap as necessary my spark plugs at every oil change.
Check resistance, pressure check if you have access to the gear, and check electrode wear.

The links may have aged ...

PRECISE GAP SPARK PLUG SET TOOL 12-11600 $18.90
http://www.aircraftspruce.com/catalog/topages/precisegap.php
http://www.aircraftspruce.com/catalog/topages/1201682.php

ELECTRODE SPREADING TOOL 12-51520 $17.95
https://www.aircraftspruce.com/catalog/topages/expansiontool12-03391.php

CT-450 RETRACT. GAP GAUGE S 12-00797 $22.50
http://www.aircraftspruce.com/catalog/topages/gapsettingtool2.php

Spark Plug Erosion Gauge 12-00791 $4.35
https://www.aircraftspruce.com/catalog/topages/ct482gauge.php
Go-NoGo gauge - hole is 0.260" in diameter (you can make your own). If it fits over the electrodes when properly gapped, the plugs are beyond acceptable wear.
 
I know the discussion is on certified aircraft but to answer the question for some of us "experimental" engine guys - I use Iridium auto plugs and change them out every annual. Cost is < $35.00 for a set of six.
 
As mentioned, check the resistance. Don’t forget this easy step.
 
If one has a bad plug, cracked ceramic, resistance, whatever, I’d just replace the one or two bad ones. I now have all fine wire plugs, a few years ago I had a mix, massive & fine wires.

It’s kinda like mixing oil, no biggie as long as each type is suitable.
 
I pull my plugs, clean, check resistance, and swap them top to bottom every oil change.
I replaced my plugs in the middle of March 2022.
They were in horrible condition.
 
I see the two cracked ones as your sign that the others are quite possibly not far behind.
Maybe you'll get another year out of them, but there is also a decent chance one will fail while you are not at home base, during a run-up. That's unpleasant.
I would change them all now.
 
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