Installing CHT Probe Under Intake Flange Bolt

[AMK53]

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!

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[rv9ralph]

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

[efwd]

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.

[Dave S]

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.

[109JB]

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/catalog/inpages/eischtprobe18mm.php Probably a K type thermocouple, so any K type should work fine regardless of brand.

[AMK53]

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


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?


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!


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/catalog/inpages/eischtprobe18mm.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.

[jrevens]

... 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.

[AMK53]

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

[rv9ralph]

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

[Xengineguy]

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.

[AMK53]

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.

I've never used stepped washers. How would this work exactly? Wouldn't the thermocouple start spinning as soon as there is friction contact between it and the step washer?

[PapuaPilot]

I've never used stepped washers. How would this work exactly? Wouldn't the thermocouple start spinning as soon as there is friction contact between it and the step washer?

I think it would get loose eventually and spin, which leads to a fluctuating reading on the gauge. I would do as others have suggested and get it sandwiched between washers using a high temp self locking nut. Check it for looseness each inspection and tighten if needed.

[Xengineguy]

I've never used stepped washers. How would this work exactly? Wouldn't the thermocouple start spinning as soon as there is friction contact between it and the step washer?

Wish I could draw what I’m talking about. Think of a standard washer with the outer od large enough to
cover the probe, plus some. Grind a flat on the od of the washer so you need to index it on installation.
The flat keeps it from turning.
Now think of the same washer with a step that fits the ID of the probe, thick enough to not quite extend all the way through. When you torque the probe down the probe tightens, then the step contacts the surface to protect the probe from over crush. Hope this make sense

[AMK53]

Thanks for all the advice. It looks like using a clipped oversized washer did the trick. It prevented the probe from being spun about its center axis and it stayed intact.

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