Spark plug gaskets - replace or not?

Larry Vrooman said:
Regardless of what Tempest does, the FAA addresses it and flat side toward the plug is the *only* approved way to do it.



The rings on a CHT sensor are harder and less likely to deform, and it's just one potential bad seal out of 8 on a 4 cylinder engine. That said if it were cost effective to replace them they probably would. A larger issue is that CHT numbers in most flight manuals are based on bayonet probes, not spark plug ring probes and the latter usually read 25-50 degrees high.

I don't know why owners would be upset if a mechanic required new gaskets ever time a spark plug was removed or replaced. Spark plug gaskets are cheap. I buy them by the 100 pack for about $30.00. That's $0.60 per cylinder, a total of $2.40 every 50 hours, or $0.048 per tach hour, and probably down around $0.04 per flight hour. I'd be more upset if a mechanic wasn't following approved practices. What else is he cutting corners on?



My understanding is that when the flat (sharp edged) side is placed against the aluminum cylinder head it can start to cut into the head, which creates a stress riser. Is that a big deal? Maybe, maybe not. But since:
- cylinders are expensive;
- spark plug gaskets are cheap;
- the FAA approved and *requirted* method is flat side toward the plug; and
- it's just as easy to do it the right was as it is to do it the wrong way;

why is there even a question about what side faces the cylinder?
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As Mr. Thorpe says, please cite an FAA source for that. We're all wondering. As for the copper cutting into the head: really? In all my mechanicking years I have never ever seen that. The copper is much softer than the grade of casting aluminum used in heads.

And no matter which side goes against what, the washer always ends up thoroughly flattened on both sides once the plug is torqued to specification. It's why annealed copper is used instead of aluminum or anything else.

There are far too many OWTs in this business. And far too many unapproved maintenance practices. I've seen many of them and the damage they can do.
 
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I researched and never found any "FAA approved method" of the gasket orientation although my mechanic and A&P mentor says to install with the flat side towards the cylinder head. However, I DID find an easy method of rotating the spark plugs that I will now share. Using a spark plug tray which is labeled remove the spark plugs and place them in the cylinder slots labeled on the tray. Clean, inspect, gap, etc and place them back as they were when you removed them. When you install them, turn the tray 180 degrees to where the labeling is now upside down. Now what would have been 4 or 6 bottom is now 1 top and so on.
 
Larry Vrooman said:
Regardless of what Tempest does, the FAA addresses it and flat side toward the plug is the *only* approved way to do it.



The rings on a CHT sensor are harder and less likely to deform, and it's just one potential bad seal out of 8 on a 4 cylinder engine. That said if it were cost effective to replace them they probably would. A larger issue is that CHT numbers in most flight manuals are based on bayonet probes, not spark plug ring probes and the latter usually read 25-50 degrees high.

I don't know why owners would be upset if a mechanic required new gaskets ever time a spark plug was removed or replaced. Spark plug gaskets are cheap. I buy them by the 100 pack for about $30.00. That's $0.60 per cylinder, a total of $2.40 every 50 hours, or $0.048 per tach hour, and probably down around $0.04 per flight hour. I'd be more upset if a mechanic wasn't following approved practices. What else is he cutting corners on?



My understanding is that when the flat (sharp edged) side is placed against the aluminum cylinder head it can start to cut into the head, which creates a stress riser. Is that a big deal? Maybe, maybe not. But since:
- cylinders are expensive;
- spark plug gaskets are cheap;
- the FAA approved and *requirted* method is flat side toward the plug; and
- it's just as easy to do it the right was as it is to do it the wrong way;

why is there even a question about what side faces the cylinder?
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Speaking only for myself, it's question because I was unaware the FAA had ruled on the matter. Would you be so kind as to point me in the direction of the appropriate AD or circular, or whatever you are referencing? Thanks
 
Dan Thomas said:
As Mr. Thorpe says, please cite an FAA source for that. We're all wondering. As for the copper cutting into the head: really? In all my mechanicking years I have never ever seen that. The copper is much softer than the grade of casting aluminum used in heads.

And no matter which side goes against what, the washer always ends up thoroughly flattened on both sides once the plug is torqued to specification. It's why annealed copper is used instead of aluminum or anything else.

There are far too many OWTs in this business. And far too many unapproved maintenance practices. I've seen many of them and the damage they can do.
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What is OWT - I don't want to be that guy?
 
In my consulting days, we lived off the 2 x 2 chart. Scatter plot on 2 axes to try to frame up a concept. This thread reminded me of this. Here's my shot at analyzing the "which way is the copper washer supposed to go".

Picture1.png
 
Dan Thomas said:
Or the nylon zip ties used to secure cables and wiring to an engine mount, until the grit that gets under the tie eats into the mount tubing and requires a major engine mount repair.
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I used mostly Adel clamps for routing wiring and cables, but there are a few places where I used zip ties. The trick is to wrap the mount tube with silicone rescue tape, then tie wrap over that. No wear on the mount whatsoever.
 
SoCal RV Flyer said:
I used mostly Adel clamps for routing wiring and cables, but there are a few places where I used zip ties. The trick is to wrap the mount tube with silicone rescue tape, then tie wrap over that. No wear on the mount whatsoever.
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Yup. Got to keep the tie away from the metal. Any dirt or sand that gets under it embeds in it and the tie becomes a file. When you consider that the tubing wall is typically only .035" thick, and that the limit for such damage is 10% of wall thickness, and that 10% of .035" is .0035", the thickness of a sheet of paper or a bit more than the thickness of a human hair, it's serious stuff.

Loose Adel clamps do it, too.
 
When you buy a harness you get the proper clamps, and they are not Adel-clamps they come in 4 sizes for 1,2,3 and 4 wire size, and you get new proper screws and washers.

I wonder why a A&P doesn't know that ?
 
On spark plug gaskets: you've got the rounded edge side and the sharp edged side. So let's agree that the sharp edged side has a small lip the needs to abrade against the opposing surface when the plug is torqued. Granted, the copper to steel abrasion is minor but over the life of the engine would you rather have that repeated abrasion on your thirty dollar plug or your eleven hundred dollar cylinder?

So that would be an argument for rounded edge side down. Anyone have a compelling argument for sharp edged side down? (outside of "it doesn't matter")
 
Silvaire said:
On spark plug gaskets: you've got the rounded edge side and the sharp edged side. So let's agree that the sharp edged side has a small lip the needs to abrade against the opposing surface when the plug is torqued. Granted, the copper to steel abrasion is minor but over the life of the engine would you rather have that repeated abrasion on your thirty dollar plug or your eleven hundred dollar cylinder?

So that would be an argument for rounded edge side down. Anyone have a compelling argument for sharp edged side down? (outside of "it doesn't matter")
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In all my years I never saw damage to either the head or the plug from the sharp edge of the flat side of the gasket. Never. That copper is so soft it's not funny. And I looked after flight school engines from cradle to grave, so to speak. From new or overhauled to TBO. No gasket damage.
 
Dan - I know but you gave an "it doesn't matter" answer and the question was are there any arguments for sharp edge down?

For instance you might argue that, when torquing, the sharp edged side will remain stationary to whichever surface it contacts whereas the round edged side will abrade onto the surface it contacts. This could be true or complete bs but it's the kind of thing you would find written in a forum such as this. Just some guys idea that has zero empirical data to back it up.

Like you I've been doing this for over 40 years and honestly I never really thought about it nor do I remember anyone ever making any mention of it outside of POA.

and FWIW I usually put it flat side down because it looks prettier. :)
 
Silvaire said:
Dan - I know but you gave an "it doesn't matter" answer and the question was are there any arguments for sharp edge down?

For instance you might argue that, when torquing, the sharp edged side will remain stationary to whichever surface it contacts whereas the round edged side will abrade onto the surface it contacts. This could be true or complete bs but it's the kind of thing you would find written in a forum such as this. Just some guys idea that has zero empirical data to back it up.

Like you I've been doing this for over 40 years and honestly I never really thought about it nor do I remember anyone ever making any mention of it outside of POA. :)
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Yup. One of those topics invented by someone unfamiliar with the metals and the whole application. The "sharp edge" of a copper gasket is sharp enough to cut paper, maybe. Maybe. But it's so soft that it sure isn't going to cut hard cast aluminum alloy. It won't even cut the skin on your fingers if you grab a handful of them. I have never come across any published data as to orientation of that gasket in any manual or SB or AC or any other authoritative source. If there is any data I wish someone would post it.
One look at a used gasket tells the story. That gasket gets thoroughly flattened and extruded and its edges bulge out convex. I'd be willing to bet that the outer edge against the head even lifts up a bit as the plug crushes the center. It can be annealed and reused, but its hole gets bigger and it gets thinner every time it's used again. Reusing it once without annealing it might be fine, but as it gets harder (work-hardening) and its hole gets bigger it starts ending up off-center and the outer ridge formed on it can interfere with its sealing ability.

There are topics far more important than sparkplug gaskets. The constant stream of posts detailing alternator, magneto and vacuum pump failures are vivid proof that these things are often being run to failure, a practice not very smart when it comes to airplanes. Instead of proactive maintenance that can actually cost less in the long run, an owner finds himself with dead electrics or gyros at night or in IMC, or a dead mag at some really inconvenient place and time. There is the topic of ground-running the engine to supposedly keep it healthy during non-flight seasons, a practice condemned by the manufacturers and overhaulers since it grossly accelerates internal corrosion. There's the topic of topping up your battery electrolyte to the bottom of the split ring when the battery is just sitting in the airplane, not on charge, which just results in acid boiling out once the alternator goes to work, corroding the box and sometimes the airframe. Stuff like this has lots of published data to back it up, and yet it's commonly ignored while we argue sparkplug gasket orientation.
 
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