Spark plugs

[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.😉

[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.

[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.

[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/...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.

[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.