Spark plugs

[wannafly]

The pictured spark plug is from my 912s. The plugs have about 80 hours on them with almost all 100LL. I used Daclin at every fill. Just curious is that a normal amount of lead build up? Can I sand blast the plug and reuse them? The engine is running great even with the buildup.

[kitfox2009]

New plugs are SO cheap. We are all flying 50 to 90 thousand dollar equipment over mostly hostile terrain.
Why skimp on plugs?

On a related issue. Today I flew through the Rogers Pass on the way home from Alberta.
Very smoky all the way. High winds above 8000 so stayed around 55 to 6500.
Anyway on run up in at my friends farm strip one module acted up. Seems a plug wire had fallen off a lower plug. Never happened before but it might be a good idea to check more often.
Cheers Don

[wannafly]

Looks like a smokey flight Don. We actually had ash falling from the sky last night it was so bad here in Lethbridge.
On the spark plugs I always buy new. Someone had just mentioned I should just blast and use again. Was wondering what others were doing. I'm like you. Why risk my self and my Kitfox for a few dollars of spark plugs.
fly safe.

[kitfox2009]

Hi Mic
I almost got down your way this trip. Would have liked to get caught up on your epic adventure. Just ran out of time.
I did greet the Vimy boys in Hanna of all places. That was quite an adventure across Canada in VW powered Newport replicas from WW1. The wind was about 20+ knots that day.

[Flybyjim]

Hi Mic,

Yes, I am always amused when I see folks trying to save a couple $$ on spark plugs and Rotax oil filters when they spend more than $40,000 for a plane. We only get one ride in this rodeo, better have a good saddle and a sound plan.

How was the harvest?

Jim

[efwd]

Replace. :)

[Russell320]

One thing I have found is when the spark plug time is between 75 and 100 hours, the engine gets harder to start.......not bad, but I can tell. That's when I replace the plugs. I also run 100LL and decalin. As said before, plugs are cheap......buy them in bulk, and don't waste your time refurbishing....unless you enjoy that type of thing.....with beer.😉

[wannafly]

Jim
Harvest went well, have gone none stop since I got home. Finally got a 3.5 hr flight in today with the new spark plugs.

[kitfox2009]

Hi Mic
Glad to hear you had a good harvest. Great weather for it.
I even made a few rounds in the combine. Quite different from when I was a kid.
Another spark plug pic. 120 hours 912 UL using marked non ethanol premium fuel. 100 'LL only when traveling.
Cheers
Don

[Rodney]

Plugs are cheap, but it is also very easy to clean them

I keep a brass wire brush on one of my bench grinders. It just takes a few seconds to clean a plug.

On trips, I have to burn 100LL like every one else, so I usually change oil and filters every 25 hours. I keep an extra set of cleaned plugs so when I change oil, I put the cleaned plugs in, and take the dirty ones home to wire brush and check the gap.

Seem a shame to throw away good plugs just cause they are dirty.

Rodney

[efwd]

Anybody using an alternative spark plug to the
ROTAX SPARK PLUG 297-656 used on the 912iS which cost $21 each?

[Av8r3400]

I'm still using the same NGK DCPR8E plugs (gapped to .020") I have always used in my engine. No plans to change to the "Rotax" brand plugs.

I have the same opinion on continuing to use Mobil-1 Racing 4T.

[efwd]

Thanks Larry. I happened to note that those NGK plugs are what is in the Rotax Parts list of Old. Not that old but they have since started putting their name on the side of what is likely the same plug. I have just ordered my new NGK plugs from AS&S.

[Hawkertech]

Hi Mic,

Yes, I am always amused when I see folks trying to save a couple $$ on spark plugs and Rotax oil filters when they spend more than $40,000 for a plane. We only get one ride in this rodeo, better have a good saddle and a sound plan.

How was the harvest?

JimI used to work on corporate jets, The annual/Phase 4 inspections were about 150,000 or more. we had owners not blink at $150,000 for an inspection then argue the price on a $20 part.

[efwd]

Yeah, I would like to have their bank account as a result too. That is likely why they have $150K available to do the work in the first place. When NGK plugs were the prescribed plug and all was good, why would I need to pay 5X the price because it is labeled differently? Rotax is obviously not differentiating between what they sell for their certified engine and what is fine in their experimental engine. I don't need to pay certified prices for my engine as it is not certified. No problem here spending money when it is warranted.

[jiott]

Eddie, there are many, many of us staying with the tried and true NGK plugs.

[jiott]

In fact, I would argue that the new Rotax plugs are LESS proven and have the possibility of problems yet to show up, compared to the NGK plugs track record of reliability for umpty-ump years. I could be wrong, but I don't believe the Rotax plugs are just rebranded NGK plugs.

[Dave S]

FWIW - According to a detailed look at Rotax documentation on this deal, The new plugs are designed to go with the new boots; however, Rotax also says that the NGK branded plugs are fine to use, till they are no longer available (from Rotax).

The NGK plugs can be found from many vendors dealing with motorsports like quads, snow machines, motorcycles, etc; and, they vary in cost from $2.52 to almost $5 depending on who you buy them from.

I am sure it is not possible to determine to what extent the actual engineering is different with the new boots and plugs other than the shape of the boots and the name on the plug. That information is internal company info that will not be shared, I am sure.

My suspicion is this probably has more to do with dueling lawyer over the aviation use of NGK products than anything else. No idea who makes the new Rotax plugs, however, it is not beyond possibilities that NGK could make them but brands them for Rotax. The branding basically moves some of the product liability to the entity who's name is on product from the manufacturer.

On a sidebar comment about plugs - no matter new or used or whatever brand, I have always checked the resistance of the center conductor ( should be 5K ohms on these) and ocasionally find a plug, not necessarily used, which doesn't meet spec. A high resistance plug will still fire; but, who knows when/if it will fail. New plugs are not necessarily without flaws.

Certainly a person is generally OK with Rotax branded parts (although they have had a few bad parts of their own in the past), I see no reason to switch to the Rotax branded plugs on our 912ULS.

Jim's comment about the reliability of the NGK branded plugs rings true.

[DesertFox4]

Dave S- no matter new or used or whatever brand, I have always checked the resistance of the center conductor ( should be 5K ohms on these)

Thanks Dave for this. 👍. Will be checking every new spark plug in the future. Added it to my iPAD notes on regular Rotax 912 ULS maintenance.

[efwd]

Okay, a follow on to the issues. I pulled my plugs and found them to be the NGK plugs. This engine was delivered from the factory with these plugs. I first bought replacements at Aircraft Spruce and found them to be the incorrect model of the correct part. Only one model has the metal top that will unscrew and leave you with the threaded post that is required. It is my understanding that NGK chose to not accept the liability for aviation use so now they have a big red X over an airplane and a trike right on the box. Problem with this was they were still manufacturing plugs for an aircraft engine manufacturer so... they quit Rotax. So, either they have actually quit, or they manufacture the plug ( or someone else does ) and label it as a Rotax manufactured part and up the price 10 fold to cover the liability.

[jiott]

The NGK DCPR8E plugs are a standard NGK plug used in many motorcycles. So even though NGK has decided not to take the liability on aircraft, these plugs are still readily available from auto parts and motorcycle shops. In fact for 6 years now I and many others have been purchasing these plugs at these stores, rather than from aircraft parts suppliers like ACS, etc. They are still the same plugs; you will just have to ignore the red X thru the airplane, and of course not try to collect from NGK if you hurt yourself using these plugs. To me personally it changes nothing.

[jrevens]

Old thread but new question - I got several boxes of new NGK plugs (DCPR8E 4339) and have checked the resistance on 8 of them with a Fluke 76 meter. Almost all of them are measuring greater than 5K ohms, with the majority around 5.6K, a few around 6.6K, and only one at 4.6K. I've read that 5K or a little less is most desirable. Is anyone else measuring and seeing similar results? Would you find these resistances acceptable? I've got 2 more boxes of 4 that I haven't opened up yet, all having been purchased very recently. I haven't been able to find any information on acceptable limits specifically for these plugs.

[Dave S]

John,

I am probably the one guilty of introducing the idea here.

Certainly your Fluke meter can be considered to be a very accurate instrument. The device being measured (spark plug) is more certainly prone to greater variation in materials and manufacturing. Small resistors in conventional electronics are often rated in terms of +/- variation from their labeled value (such as +/-10%, +/- 15% etc). Spark plug resistors will very seldom be found to be exactly one and only one precise value.

Quality Aircraft Accessories has a description of their take on spark plug resistance at:

https://www.qaa.com/resource-center/tempest-tech-tips/the-right-way-to-check-spark-plug-resistors

According to QAA, the value of 5,000 ohms is a reference and not an absolute value. Readings of +/- 20% on spark plugs are probably not significant. Testing spark plug resistance helps us exclude spark plugs with cracked internal resistors (like the one that got dropped on the concrete), which can often show infinite resistance at the test voltage.

The QAA article also includes the comment "Nothing is magical about the 5,000 ohm value"

I would be confident using the plugs you measured with the stated variations. I find new plugs I use in the same range you indicated.

[jrevens]

Thanks for the reply & comments, Dave. I was mostly curious & concerned about the ones in the 6.6K range, but probably unnecessarily. That’s about 30% over 5K. As the article suggests, some problems with rough engine operation were found with plugs having higher than the 5K resistance limit. I believe that may be attributable to the probability that those issues were encountered with magneto ignition systems. A more modern, relatively higher energy solid-state system, like that used by Rotax, probably decreases the probability of any problem with that higher resistance. I was just hoping for feedback about what others who check the resistance have been finding. Thanks again!