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Thread: Installing CHT Probe Under Intake Flange Bolt

  1. #1

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    Default Installing CHT Probe Under Intake Flange Bolt

    The recommended CHT probe location on my Rotec R2800 is under the intake flange bolt. When I tried to install it there, the probe started to pancake out due to the large pre-load required at the bolt (~ 18.5 ft-lbs).
    In addition to the pancaking due to compression stress, the CHT probe also spins around the bolt axis due to the rotation of the bolt as it's being tightened, and in one case it sheared in one spot (see photo). The original probe shows the same compression, but it did not seem to have been cut. I'm not sure if this was just luck or some other reason. I've tried sandwiching the CHT probe under two washers to protect them a bit, but did not help all that much.

    Does anyone have any techniques that they can recommend for installing this without breaking the temp sensor? I think the thermocouple spinning around the bolt axis is what is causing the largest damage. I'm not sure how I can prevent it from spinning.

    I'm all out of ideas so any advice is appreciated!

    RecommendedInstallLocation_1_LI.jpg
    .EICHTProbes_LI.jpg

    OriginalProbe_LI.jpg
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  2. #2
    Senior Member rv9ralph's Avatar
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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    Can you use a stud in the hole and torque a nut to hold the manifold and a second nut to hold the probe?

    Ralph

  3. #3
    Senior Member efwd's Avatar
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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    I assume the probe has been installed between the washer and the exhaust flange and when tightening the bolt the washer turns and the probe goes with it? In that case I would get a washer with a wider outer diameter, shave one side down and place that edge next to the exhaust. This would prevent the washer from rotating while tightening.
    Eddie Forward
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    Senior Member Dave S's Avatar
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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    Quote Originally Posted by rv9ralph View Post
    Can you use a stud in the hole and torque a nut to hold the manifold and a second nut to hold the probe?

    Ralph
    Ali,

    Ralph's idea would solve a couple things.

    1) Avoiding smashing the probe loop

    2) Preventing the problem from developing down the road where a soft material bearing the torque under the bolt head (the probe) will often wear/thin with time causing the bolt to lose torque and possibly developing an exhaust leak.
    Dave S
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  5. #5
    Senior Member 109JB's Avatar
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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    Also looks like the probes being used are for install under a spark plug. Likely 14 or 18mm. Grand Rapids sells then for 4mm, 10mm, 12mm, 14mm, and 18mm. Get on that matches the bolt size better. You can buy them at ACS https://www.aircraftspruce.com/catal...tprobe18mm.php Probably a K type thermocouple, so any K type should work fine regardless of brand.
    John Brannen
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  6. #6

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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    I'm sorry for the late reply. Work has had me slaving away recently.

    Quote Originally Posted by rv9ralph View Post
    Can you use a stud in the hole and torque a nut to hold the manifold and a second nut to hold the probe?

    Ralph
    That's not a bad idea, but I would worry about maintaining a strong preload on that flange. The problem with using a stud is that I would have to use some kind of lock nut to ensure that the nut doesn't just come off with some good vibration. It would be hard to mount a temp probe on top of a lock nut. Maybe if I can find a lock nut that's flat?

    Quote Originally Posted by efwd View Post
    I assume the probe has been installed between the washer and the exhaust flange and when tightening the bolt the washer turns and the probe goes with it? In that case I would get a washer with a wider outer diameter, shave one side down and place that edge next to the exhaust. This would prevent the washer from rotating while tightening.
    I think that's what is happening. That is not a half bad idea. I didn't think about tapering the washer so it can't spin on its own. I have two washers sandwiching the probe right now, but they're barely bigger than the temp probe. The compression strain on the thermocouple is not a big problem as long as it doesn't split in half due to the shear. I'll have to try this out and see!

    Quote Originally Posted by 109JB View Post
    Also looks like the probes being used are for install under a spark plug. Likely 14 or 18mm. Grand Rapids sells then for 4mm, 10mm, 12mm, 14mm, and 18mm. Get on that matches the bolt size better. You can buy them at ACS https://www.aircraftspruce.com/catal...tprobe18mm.php Probably a K type thermocouple, so any K type should work fine regardless of brand.
    This probe is 3/8" ID and the bolt is a #8 (0.164"). It's the closest size that I could find to fit. Everything else is either too large or way too small.

  7. #7
    Senior Member jrevens's Avatar
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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    Quote Originally Posted by AMK53 View Post
    ... This probe is 3/8" ID and the bolt is a #8 (0.164"). It's the closest size that I could find to fit. Everything else is either too large or way too small.

    Ali,

    A #8 machine screw is a very small size for that intake flange... I don't think that's right.
    John Evens
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  8. #8

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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    John, you are right. I meant to say M8 and not #8. This is what I get for typing half asleep.

  9. #9
    Senior Member rv9ralph's Avatar
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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    The problem with using a stud is that I would have to use some kind of lock nut to ensure that the nut doesn't just come off with some good vibration. It would be hard to mount a temp probe on top of a lock nut. Maybe if I can find a lock nut that's flat?
    Expanding on my thought. Use a standard nut to appropriate torque value, install CHT sensor, washer then high temp locknut to hold everything in place. Torqueing down the current bolt, with a compressible material (CHT sensor) could result with losing your torque as the sensor reduces in thickness.

    Ralph

  10. #10

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    Default Re: Installing CHT Probe Under Intake Flange Bolt

    Quote Originally Posted by Dave S View Post
    Ali,

    Ralph's idea would solve a couple things.

    1) Avoiding smashing the probe loop

    2) Preventing the problem from developing down the road where a soft material bearing the torque under the bolt head (the probe) will often wear/thin with time causing the bolt to lose torque and possibly developing an exhaust leak.
    Build a stepped washer with one flat side to prevent it from turning. The id step protects the thermocouple
    from crush. Torque it down as normal.

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