Broken Spark Plug

by Diego Fonseca in 912 / 914 Technical Questions 8 years ago

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Re: Broken Spark Plug

by Diego Fonseca » 8 years ago

Diego Fonseca

2 3 0

Thank you all for the advises!

I've thrown the towel and called a mechanic to take the head off (I will seize the opportunity and do the top ovehaul, since my engine is at 1300 hours).

Roger Lee, the plugs are there for about a year and 130 hours. I used milk of magnesia as thermal paste on the threads. About the
over torque, I used the 177lb/in, so the only way that it could have happened is if the torquimeter lost the calibration.

Kevin Kane, thanks for the advice for the heli-coil. I'm located in Brazil! B)

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Re: Broken Spark Plug

by Roger Lee » 8 years ago

Roger Lee

125 5.12k 4.53k

If you aren't kidding about the milk of magnesia then my guess is that is what caused corrosion and or the plug is cross threaded.
You should only be using the correct thermal paste and nothing else.
For others that don't know that also means no anti seize. It has no thermal transfer properties.

Milk of Magnesia? The same stuff you drink for an upset stomach?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Broken Spark Plug

by Diego Fonseca » 8 years ago

Diego Fonseca

2 3 0

Roger Lee,

Milk of magnesia is an approved alternative lubricant (anti-seize) for many jet engines' igniter plugs. For example, the CF-34 AMM calls out to use milk of magnesia when installing igniter plugs, and does not mention at all other anti-seize compound.

Yes, it is the same Phillips stuff you can buy at the pharmacy in the blue bottle!

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Re: Broken Spark Plug

by Roger Lee » 8 years ago

Roger Lee

125 5.12k 4.53k

This may be acceptable in very specific circumstances in certain aircraft, but it isn't acceptable here and this is why most engine MFG around nod the world do not use Milk of M. It is very alkaline and has water in it so it may lubricate somewhat going in because it is wet, but because of the high alkalinity against the steel plugs and aluminum heads it will not luricate comings out and will seize up. This has no thermal transfer properties. To nation The silicone thermal paste we use not only stays soft for anti galling to the threads, but prevents dry thread galling going in and coming out. Its main purpose is to transfer heat away from the plug up to around 40f-50f. This helps prevent detonation.
If you had used the silicone paste my bet is you wouldn't have twisted a plug in half. That took exceptional force due to the plug being seized in the head from both drying, water corrosion in your milk and the high alkaline mixture on the aluminum. The silicone thermal paste we use prevents that. If you keep using the milk of M you may revisit your seizure scenario in the future and could set yourself up and destroy a cylinder or two if you get close to a detonation scenario. $20 of the silicone paste may save you thousands of dollars.

These is a reason Rotax wanted the silicone thermal paste used. For about $20 you most likely could have prevented all your frustration right now.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Broken Spark Plug

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