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Checking Spark Plugs


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#1 MrMorden

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Posted 13 September 2013 - 08:29 PM

How often should spark plugs be pulled and checked? I have about 20 hours on my plane since its annual and Rotax 100hr service. Should I check plugs every annual? Every oil change? Every pre-flight (joking)? Or only if I note some type of roughness or performance change?

Is there a detailed online walk through of how to properly check plugs and gap them for a Rotax 912?

Also, can somebody shoot me a link to the correct plugs to get? IIRC there are two very similar plugs with different heat ranges, one correct and one not correct...
Andy Walker
Athens, GA
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#2 Jim

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Posted 13 September 2013 - 08:58 PM

Just replace them every annual works for me.

"It is impossible for a man to learn what he thinks he already knows"
--Epictetus


#3 Doug G.

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Posted 13 September 2013 - 11:23 PM

Annual seems to be fine. I will be re-gapping mine this fall because of our temperature extremes, I have them on the wide end of the range now.
I ordered mine from CPS. I don't have the number handy but I am certain it is in at least one thread on the forum. I know you need them with the removable caps (evidently you can get them non- removable).
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#4 Roger Lee

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Posted 14 September 2013 - 12:41 AM

This question has a couple of variables and can even have more than one answer.

First the book answer. It says to check them at 100 hrs. and re-gap and you can use them to 200 hrs. I wouldn't even think of doing this and I have never been in a school that advocated it.

The next part is how many hours a year do you fly. So for some the annual may be too soon and for some the annual may be too late. The annual may work well for some. If it flies more than a 100 hrs. a year then it may be too long.

Then we have 100LL adding its wonderful lead coating and 91 Oct. unleaded to add to the equation.

So what the majority of us do (you will get different opinions) is change ours at 75 hrs., but you could go to 100 if you wanted, but I would never go past that. I'm sure someone will chime in and say they haven't changed theirs for the last 10 years and the engine runs. :huh:

Gaps are to be between .023 - .027. The wider gap for the hot summer months and the narrower gaps for the colder winter months. If you live in a real winter state then I would use .023 this winter. Do not forget the thermal conducting paste. That alone can make a 40+F difference between the plug and head temp sync. Do not put thermal paste on the last 1/3 of the plug. More or less the top 1/2 - 2/3 (closest to the gasket). If you get it on the tip it will not burn off as it is silicone based and will cause a misfire so clean it off well if you get the tip in the paste. Absolutely no anti seize.

The plugs are NGK DCPR8E with the removable insulator tip. There is only one heat range for this specific plug. You can buy them any where so shop around and try to find a good price. I would bet you can get them local at an automotive store or a motorcycle shop.

Even with older plugs you may not as a human feel the difference, but the engine can tell internally. Don't be a penny pincher and skimp on plugs and oil. Do we change oil and plugs a little early, sure, but you can't tell as a human by looking at either when it's gone too long. You have an $18K engine that should go way past TBO don't do anything by act or omission to cause it a shortened life.



I had a guy ONCE that bragged he hadn't changed his oil in his tractor for 20 years and it still ran. No one in southern Arizona would work on his plane so he sold it.
Roger Lee
Ryan Airfield (KRYN)
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#5 Jim Meade

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Posted 14 September 2013 - 03:31 AM

You have an $18K engine that should go way past TBO don't do anything by act or omission to cause it a shortened life.

I agree. And one act is unnecessary maintenance. If it ain't broke, don't fix it.
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#6 Roger Lee

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Posted 14 September 2013 - 04:34 AM

"And one act is unnecessary maintenance. If it ain't broke, don't fix it."

I'm all for not doing un-necessary maint, but who gets to decide?

With that train of thought why do any maintenance at all until it breaks.

We have always disagreed on what is necessary and what is not, but I try to put the other peoples or my wife's safety in front of all my decisions as a mechanic and a pilot and money or time involved should never be a consideration against safety. Don't fix it if it ain't broke doesn't work when you crash because you didn't want to fix it until it broke. Lots of dead bodies in the cemeteries since 1912 when the Wright Brothers flew and of course things break even when good maintenance practices are followed so why add to the failures. Poor education and poor maintenance practices from mechanics and owners cause many a crash then you dump poor pilot decisions on top of that and we kill way too many people a year un-necessarily.

Don't fix it if it ain't broke is a 20 year oil change.
That's why we have rules because people don't do what's necessary or right. They make poor decisions based on money and John Doe's neighbor's comment from next door. So rules get made to protect the 20 year oil change people and keeps the 20 year oil change mechanic from using their own rules of maintenance being used on the the under or uninformed. Most owners make maint. decisions without enough knowledge because they aren't in the business or don't keep up on a continuing education program.
I hate rules as much as the next guy so blame the guy who doesn't do the right maintenance and so the government steps in to protect the rest of us. It's certainly the shotgun approach, but we all get to see the ones that need protecting from themselves and need to protect the others those same people will injure. You can't say it only affects me and I'll only kill myself because that same guy takes passengers or kills the people on the ground when he goes into the house or shopping center. If he makes it through his ownership then he sells his mess to the next guy.
Money should never be a deciding factor when aircraft safety and good maintenance is at stake. We bought into an expensive elite sport. If someone can't afford the maint. then it was a poor choice of a money based activity.

It has always been and always will be the acts of the few that dictate the rules to the masses.

My job as a mechanic or a pilot is to make sure everyone gets to go home at the end of the day and even then there is no guarantee.

Safety first and continuing education for the life of the owner and mechanic while they are flying.
Roger Lee
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#7 Jim Meade

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Posted 14 September 2013 - 12:59 PM

Oh, Roger, good grief. There are standards for most things and sensible people can understand and apply them. Oil has a life that most of us could agree on. As do spark plugs. Rubber hoses can be inspected as can nearly any part on the airplane.

Your problem on this forum is you insist on putting forward your own interpretations and preferences as the best practice or the preferred way even when it is in contravention of Rotax and Flight Design publications.

The airline industry and the military are two area that use the approach of periodic inspection as opposed to time based maintenance. Only part 91 still does things the old way. People like you don't help any because you confuse your personal comfort level and operating practices with good, sound condition inspections and appropriate maintenance and actually make the old way even more restrictive.

You make a continued practice of talking down to the members of this forum as if you have special knowledge and they don't, so they should trust you and do it your way. This practice serves your purposes because it gives the person who is not sure the same feeling as they get when they talk to their doctor or priest. The sad thing is, it becomes a matter of faith and trust rather than a matter of knowledge and science.

Your job as a mechanic is to apply good, legal maintenance practices to your work, not to convince someone to replace their plugs at 75 or 100 hours when the book calls for 200 UNLESS THERE IS A REASON. Rotax, in all it's experience and wisdom, doesn't know better. Roger does. I'll stipulate that more and more you will put in the book numbers before you go off on your own tangent, which is better than you used to. So, there may be hope for you yet.




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#8 Jim Meade

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Posted 14 September 2013 - 01:02 PM

Annual seems to be fine. I will be re-gapping mine this fall because of our temperature extremes, I have them on the wide end of the range now.

Doug,
A question for you and everyone. If you are going to pull the plugs every spring and fall based on temperature, what are the considerations of setting up a spring and fall set of plugs? They are cheap, after all. Then you don't have to regap them. Set them once. I'm not a great fan of regapping plugs although it is not a big deal.
The disadvantages of having two sets of plugs might include keeping track of how many hours are on them. If you are going to go on time based maintenance, you could us them each for two seasons if you fly less than 200 hours/year. Or you could date them. Or, of course, you could write the engine hours down on the storage box. It may be it's not worth the trouble. But, I have to confess, I might toy with the idea of having a summer and winter set of plugs.
What do you all think, pro and con?
CTSW E-LSA

#9 FastEddieB

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Posted 14 September 2013 - 01:12 PM

I made a little spark plug holder from an old battery box I had lying around (click for larger image):

Posted Image

Originally had 12 larger holes for my Cirrus plugs, which I sleeved down for the smaller ROTAX plugs.

Anyway, plug condition can be an indicator of problems with a given cylinder and overall mixture and potential lead issues - good thing to take a look at and monitor any time you have them out.

You can see where I applied the conductive paste.
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#10 Dan Kent

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Posted 14 September 2013 - 01:34 PM

When I was getting my 100 hr service the mechanic sent me to O'Reily's to get the plugs. I think the total cost including tax was less than $22 for a dozen (may have been 10) plugs. Very inexpensive.

#11 FastEddieB

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Posted 14 September 2013 - 01:46 PM

Less than $2.50 each at both Advance and O'Reilly's, IIRC.

Make sure they have the removable "barrels" before leaving the store - some do, some don't.
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#12 Tom Baker

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Posted 14 September 2013 - 01:58 PM

Roger already gave the plug number, but the stock number for the one with removable top is 4339. One thing that no one has mentioned is in reality the gasket on the plug is intended to be a one time use gasket. It is designed to crush and make the seal and after it has been torqued down it doesn't crush again. I buy 40 at a time boxes 4x10 to make 5 sets of plugs. I change them any time I pull the complete set from the engine.

#13 Roger Lee

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Posted 14 September 2013 - 02:30 PM

"Oh, Roger, good grief. There are standards for most things and sensible people can understand and apply them. Oil has a life that most of us could agree on. As do spark plugs. Rubber hoses can be inspected as can nearly any part on the airplane."

"If it ain't broke, don't fix it."

But that's not what you said and unfortunately too many take this to heart. You don't see it because you don't maintenance those 20-30 LSA a year

" And one act is unnecessary maintenance. If it ain't broke, don't fix it."

"There are standards for most things and sensible people can understand and apply them."

But they don't or we wouldn't have as many issues with some of the products. This fully bears out with all the request and problems that Rotax and aircraft Mfg's deal with on a daily basis. I talk directly to the MFG's quite often so I get to hear first hand.


The LSA mechanics that do 20-30 CT's and or other LSA a year and take continuing and on going classes and talk among themselves to share other technical info may vary well have a better insight or handle on issues, new ways of thinking, new technical data before it goes public from the Mfg's (both plane and engine MFG) and those mechanics tend to talk to the Mfgs on a regular basis and all that info may not make it down to an individuals level. So some of us on different sites try to help disseminate that info. I'll will be the first to tell you that not all info is shared down to the individual level from MFG's and some Mfg's call us so we will post the info without them being directly involved.

You always said there must be some inner loop. Well you were right and it works that way so info can get out to all without any one MFG trying to write a book and every item and it can stay ahead of waiting for it to be a once a year manual change.

I don't pull numbers or items out of a hat, they come from the Mfg's and the many ongoing classes and some of this info is new and a head of what is in the manuals. So I was taught what I talk about or given the info from the MFG. You always want documentation, well it isn't there so we rely on each other and help each other where we can.

People have helped me my whole life and I have realized how good those people were to me. So now I want to give back and this is my way.

It isn't written in stone from me, nor do I ever profess to know everything, but if you happen on an issue I can help on and ask a question I'll try and do my best to give back and you can choose to use the info or not. I don't talk down to anyone I just give as truthful and straight forward answer as possible. Everyone on this forum has areas of expertise and areas that they would like more info. I'm no different I learn from many here and help where I can.


I take from your statements of me you have never benefited or I have never helped you with your LSA (CT or Rotax).

"Your problem on this forum is you insist on putting forward your own interpretations and preferences as the best practice or the preferred way even when it is in contravention of Rotax and Flight Design publications."
"Your job as a mechanic is to apply good, legal maintenance practices to your work, not to convince someone to replace their plugs at 75 or 100 hours when the book calls for 200 UNLESS THERE IS A REASON. Rotax, in all it's experience and wisdom, doesn't know better. Roger does. I'll stipulate that more and more you will put in the book numbers before you go off on your own tangent, which is better than you used to. "


If I have led you astray and caused you to fail at knowing a little more about your Rotax and CT or it's maintenance I'm sorry.
Roger Lee
Ryan Airfield (KRYN)
Tucson, AZ.
LSRM-A Specializing in LSA Maintenance
Authorized Rotax Repair Center - Heavy Maint Rated
520-574-1080 Home Try home first
520-349-7056 Cell

#14 Roger Lee

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Posted 14 September 2013 - 02:42 PM

Roger already gave the plug number, but the stock number for the one with removable top is 4339. One thing that no one has mentioned is in reality the gasket on the plug is intended to be a one time use gasket. It is designed to crush and make the seal and after it has been torqued down it doesn't crush again. I buy 40 at a time boxes 4x10 to make 5 sets of plugs. I change them any time I pull the complete set from the engine.

Less than $2.50 each at both Advance and O'Reilly's, IIRC.

Make sure they have the removable "barrels" before leaving the store - some do, some don't.

-------------------------------------------------------------------------------------------------------------------------------------------------------------

Good info guys,

Tom,

Sometimes I use my bore scope to look inside the cylinders to see how the burn, carbon build up and leading are inside. Because of our water cooled heads and thermal paste our plug burn is fairly good and consistent.
Roger Lee
Ryan Airfield (KRYN)
Tucson, AZ.
LSRM-A Specializing in LSA Maintenance
Authorized Rotax Repair Center - Heavy Maint Rated
520-574-1080 Home Try home first
520-349-7056 Cell

#15 sandpiper

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Posted 14 September 2013 - 05:17 PM

You make a continued practice of talking down to the members of this forum as if you have special knowledge and they don't, so they should trust you and do it your way. This practice serves your purposes because it gives the person who is not sure the same feeling as they get when they talk to their doctor or priest. The sad thing is, it becomes a matter of faith and trust rather than a matter of knowledge and science.


I went to LSRM school with Roger. Time together was not only class but also after hours as we had our recreational vehicles parked next to each other on the ramp. Since then, 2008 I think, I have communicated with him on this forum, phone, face time and face to face. He has never talked down to me even though he easily could since his maintenance knowledge and skills far exceed mine. He has always gone out of his way to help, me and others, making sure I knew what was required and never once made me feel "talked down to" in the process.
John Horn
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Flying a 2007 CTSW
Building a Vans RV-12 (it's going to fly on Tuesday!)
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Rotax service, maintenance, and heavy maintenance trained

#16 Doug G.

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Posted 14 September 2013 - 05:55 PM

Although I do not always agree with Rodger, I do find the info he provides helpful, and he has always been more than willing to assist me in any way he can over the phone. As he said, it is up to me to decide to accept his advice or not - most times I do. I find him exceptional in that we can argue here on the forum, but if I call him he is more than willing to help me out. A lot of people don't seem to deal with life that way.

Jim and Eddie, as far as using two sets of plugs - that makes a lot of sense. On Roger's advice, I gapped to .27 for the summer and will be going to .23 before the weather starts getting cold. Using two sets (and a good plug stand) would make that an easier task. Thanks
Doug Grant
LSRM-A
CTLS-N962LS
KFAR-Fargo, ND

#17 Tom Baker

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Posted 14 September 2013 - 06:26 PM

Although I do not always agree with Rodger, I do find the info he provides helpful, and he has always been more than willing to assist me in any way he can over the phone. As he said, it is up to me to decide to accept his advice or not - most times I do. I find him exceptional in that we can argue here on the forum, but if I call him he is more than willing to help me out. A lot of people don't seem to deal with life that way.

Jim and Eddie, as far as using two sets of plugs - that makes a lot of sense. On Roger's advice, I gapped to .27 for the summer and will be going to .23 before the weather starts getting cold. Using two sets (and a good plug stand) would make that an easier task. Thanks


Those are rather large gaps. You might want to try .027 and .023. It might work a little better. :)

#18 sandpiper

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Posted 14 September 2013 - 08:07 PM

Good catch Tom!
John Horn
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#19 Doug G.

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Posted 14 September 2013 - 10:34 PM

:blush: Good thing my feeler guages don't go that large or I might have been in trouble. lol
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#20 Doug G.

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Posted 14 September 2013 - 10:39 PM

On the other hand, maybe I was using a different number base - I love a challenge. :)
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