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Thread: Automotive Spark Plugs

  1. #1

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    Automotive Spark Plugs

    Lots of experimental guys use them, me included. Unknown to me until now the Lycoming 390/580 cylinders require long reach spark plugs and adapters. I assume the head must be thicker? The IO-400 didn't use long reach.

    Things that make you go hmmm. Long reach NGK iridium plugs are $36 each. The Denso equivalent? $9 each. WTF?

  2. #2

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    You will find the Denso's to be better made with many different versions not that that matters in a plane. I can get most of the Densos for half the money you state.

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    skywagon8a's Avatar
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    N1PA

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    Iíd never seen this warning label or the yellow paint on cylinders before so I had to do some research. I found long reach adapters easily enough but didnít know which plugs to use. I emailed Pmag who referred me to sparkplugs.com for the Denso plugs. Ordered and shipped, so Iím not shopping. I found the long reach requirement interesting and was surprised that NGK equivalent plugs are $30 each more expensive. Given the spark plug reach issue now Iím wondering about injector nozzles. Email sent about that. Fun with airplanes continues.


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  5. #5
    180Marty's Avatar
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    Little different but here is an experience I had. After running my O470 K for 500 hrs, I discovered it had one long reach cylinder ( maybe 470U) that I had short reach plugs in. Never knew I had a problem.
    Last edited by 180Marty; 01-05-2020 at 10:23 AM.

  6. #6
    hotrod180's Avatar
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    Quote Originally Posted by 180Marty View Post
    Little different but here is an experience I had. After running my O470 K for 500 hrs, I discovered it had one long reach cylinder ( maybe 470U) that I had short reach plugs in. Never knew I had a problem.
    Very interesting.
    Esp since the 470U uses 8.6:1 pistons & the K uses 7:1.
    FWIW I'd be interested in an STC to drop the higher compression pistons into a 470K,
    kinda sorta like the Norland STC (basically a carbureted IO470),
    but maybe the K cylinders aren't up to it?
    Cessna Skywagon-- accept no substitute!

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