What changed, why it changed, and what it means for your engine.
If you have been flying behind a ROTAX 9 Series engine for a few years, you will remember the familiar NGK spark plug. Many operators still ask us why they can no longer simply buy the old plug off the shelf. The short answer: it is no longer a ROTAX specified part, and that decision was never ROTAX’s to make.
The Background
NGK is explicit, both on its packaging and on its website, that its products are not approved for aviation use. Some years ago, NGK acted on that position and required ROTAX to stop specifying and using its plugs for aircraft engines. ROTAX had no option but to source an alternative supplier, and the result is the Genuine ROTAX spark plug, Part Number 297656.
It is worth repeating, because the misconception is widespread: this was not a ROTAX commercial decision. It was imposed on ROTAX by the spark plug manufacturer.
Why It Matters for Airworthiness
The NGK plug is no longer a specified part for the 9 Series. The airworthiness section of the respective Maintenance Manual Line (MML) requires the use of genuine specified parts for continued airworthiness. Fitting a non-specified plug is not a cost-saving measure. It is an airworthiness issue.
The Price is Offset by the Replacement Interval
The Genuine ROTAX plug does carry a higher unit price than the plug it replaced. That is offset by a significantly longer replacement interval.
Replacement Intervals
| Engine | MOGAS | AVGAS |
|---|---|---|
| Non-Turbo – 912 UL, 912 ULS, 912 iS | 400 Hours | 200 Hours |
| Turbo – 914 UL, 915 iS, 916 iS | 200 Hours | 100 Hours |
An inspection is required at half the replacement interval times. Always confirm the figures applicable to your specific engine and configuration in the respective MML.
Inspection Intervals
| Engine | MOGAS | AVGAS |
|---|---|---|
| Non-Turbo – 912 UL, 912 ULS, 912 iS | 200 Hours | 100 Hours |
| Turbo – 914 UL, 915 iS, 916 iS | 100 Hours | 50 Hours |
Torque: 16 Nm, Not 20 Nm
This is the single most important number in this article.
The tightening torque for the Genuine ROTAX spark plug is 16 Nm. The old NGK plug was 20 Nm.
Old habits, and old torque wrench settings, are a real risk here. Please make sure everyone who works on your engine knows the number has changed.
Heat Transfer Compound Is Not Optional
Heat transfer compound on the thread is essential. It is what carries heat out of the plug and into the cylinder head. Without it, the plug runs hot.
Historically, copper-based anti-seize compounds were widely used and accepted. ROTAX does not accept copper compound as an alternative, and this is not a paperwork technicality. We have seen cases of severe plug fouling directly attributable to copper compound.
Comet Aviation has sourced a very inexpensive heat transfer compound, available on our website. Given what a set of plugs costs, and for safety, it is the cheapest insurance you will buy this year.
Inspection: At Half the Replacement Interval
Plugs must be inspected at half the replacement interval. The procedure is straightforward, but there is one prohibition that catches people out.
- Clean gently. Use a non-abrasive method, ideally a plastic brush, to remove loose deposits. No wire brushes and no blasting.
- Inspect visually. Look for signs of overheating, fouling, erosion, or any other anomaly.
- Check the gap, but do not adjust it. Use a needle-type gap gauge. Adjusting the gap on these plugs can create a stress point on the electrode, which can then break off in service.
In practice, it is rare to find a gap out of specification, and plugs that check out can be refitted with confidence.
Removing the Plug Caps
The plug caps are a deliberately tight fit, and this is where avoidable damage happens.
- Pull the cap off in line with the spark plug. Pulling at an angle damages both the plug connector and the plug itself. A common symptom is a bent connector on the end of the spark plug.
- Never pull on the HT lead. Doing so can compromise the connection between the lead and the plug connector, which may not show up until you are troubleshooting a rough-running engine later.
Take the extra few seconds to avoid a potential engine failure.
In Summary
The Genuine ROTAX spark plug costs more up front, lasts considerably longer, and is the only plug that keeps your engine airworthy. Torque to 16 Nm, use the correct heat transfer compound, inspect at half interval, do not adjust the gap, and treat the caps with respect.
If you have any questions about Part Number 297656, heat transfer compound, or the intervals applicable to your engine, the Comet Aviation team is here to help.
ROTAX® is a registered trademark of BRP-Rotax GmbH & Co KG. This Tech Tip is provided as a general guide. Always refer to the current revision of the applicable Maintenance Manual Line (MML) for your engine.
