New Owner of a M20E

[Hank]

The spark plug probe reads higher because it is in a different location than the other probes, NOT because it reads wrong. Put spark plug probes in the other three cylinders and they would read fairly close to each other, but their readings would then not match the numbers in your Owners Manual, which was developed based on the other location. Drop the spark plug TC into frezing and boiling water, it will read correctly . . . . 

Your other option is to spend the $$$ to upgrade your JPI to one that is certified to replace your factory gages, then you can use the factory location for all cylinders. Be sitting down when you get the installation quote, and tell them the model and probes, etc., that you want to trade in.

Good luck!

[carusoam]

Next steps...

See if I understand this correctly...

1) you would like all four JPI TCs sitting in each of the cylinder wells... the ship’s gauge can be supplied by a TC sitting under the well/TC holder.

2) this is great for the most even numbers... extra helpful for problem solving the hot CHTs...

3) when finished, put the ship’s TC back in the well.

4) live with the one 25°F shift that you will get.  It will always be there. Because it is measuring a different location.

5) the reason the ship’s gauge is in the well and is important to have it stay there long term is it’s redline. The redline is the temp to avoid as defined by that TC, in that location.

6) since the ship’s gauge is actually calibrated and reading actual temps on an analog gauge... you will get real results when you temporarily move TCs around...

7) Today’s big challenge is the TC holder is possibly cross-threaded in the well.  You want it removed to put in a piggy-back thermocouple along with the ship’s gauge resident in the well...

8) the TC holder is probably steel possibly mounted in an aluminum hole... remove it...

9) the mechanic can tell if the threads can be recut/tapped...

10) Expect this wouldn’t be the first time this has happened.  Exchanging cylinders won’t be the expected acceptable answer...

11) the TC holder is probably a five dollar part, that comes in a few sizes or done with conic NPT threads.... take a look in the Aircraft spruce catalog site...

12) expect the cylinder head threads to be recut, and a new TC holder to be put in place... have the right piggyback TC to go under it...

13) for better background call a cylinder manufacturer and ask them how this gets done... when people cross thread things...

14) it’s a tough situation, sounds like you know what you want to do to get the best data available...

15) 14 lines to say pull the TC holder and install a new one...

16) how that gets done with minimal damage is where the trained mechanic really helps.... they do things like warm the engine and locally cool the TC holder for removal...

Get a good look at a TC holder, you don’t need to be an engineer to understand this challenge.

Good luck, it would probably cost less to swap one cylinder for the other... remember to pull a cylinder that the TC holder comes out of...

they're probably not cross threaded, just dissimilar metals, gone through extreme heat cycles.... somebody somewhere has a solution for this...

PP thoughts only, not a mechanic....

Best regards,

-a-

[Guest]

40 minutes ago, DustinNwind said:

Carusoam,

All has been done with a mechanic, in fact we had 3 mechanics on the ground and a right seat pilot monitoring the JPI as I flew today to document temps. I appreciate the notion that I should "trust" what people are telling me on a forum is correct but I only accept things that are verified - even more so things need to be verified if I'm getting temps that could cause an engine failure. I verify not only for my pocket book but for the life of my passengers and the my life. So please respect that I'm going to go the extra mile to confirm why something is happening. I don't consider anything I've done extra work, it's just work. Also, I'm so ecstatic there is such a helpful group of folks here willing to tell their knowledge. I'm confused on where you got my aggression towards MS? The only thing I'm aggravated with is who ever rethreaded the probe into this engine muscled it in and torqued it the wrong way so I can't take it out with out having to tap a new hole due to ruining the treads. 

1) All TCs have been identified and that's how we trouble shot today.

2) I want all TCs to work properly as the JPI should be real numbers.

3) I'm not a mechanic nor do I want to be a mechanic, thus I have a mechanic + it's the law.

4) It took 2 for your guidance. 

5) As I said in the first sentence and I'll say it again,  thank you everyone for your words, it has made the trouble shooting easier. Your help is invaluable. As for JPI technical department I will and can not accept their answer "it's normal for the spark plug probe to read roughly 50 degrees warmer than it is. You just have to figure on that in your calculations". I believe in doing something the right way or don't do it at all. Making a probe that reads wrong is not making an accurate piece of equipment.

 

 

From the engine type certificate, the cylinder head temperature is meant to be read with a well type thermocouple not a gasket thermo couple like your engine has.  

Your OEM CHT is reading in the correct location, the JPI is not.  As your current JPI is not a certified replacement you can either live with the inaccuracy or buy a different JPI probe.

Clarence

 

 

[carusoam]

@M20DocClarence,

Is the TC well actually in an aluminum part?

The TC holder is probably a piece of steel?

How does a mechanic pull out the TC holder without destroying the aluminum threads?

How easy is it to recut threads and put a new TC well in place?

This could solve the two thermocouple challenge for the one cylinder...

Best regards,

-a-

[carusoam]

1 hour ago, DustinNwind said:

1 hour ago, DustinNwind said:

  

5) As I said in the first sentence and I'll say it again,  thank you everyone for your words, it has made the trouble shooting easier. Your help is invaluable. As for JPI technical department I will and can not accept their answer "it's normal for the spark plug probe to read roughly 50 degrees warmer than it is. You just have to figure on that in your calculations". I believe in doing something the right way or don't do it at all. Making a probe that reads wrong is not making an accurate piece of equipment.

 

 

Dustin,

It’s the probe’s location (under the spark plug) is actually hotter than the usual measured location.  The probe isn’t reading wrong. The equipment is accurate...

what you want is two probes that fit inside the same well... but that doesn’t exist, yet...

what is the next best thing is the piggyback that sits under the TC holder...

The ship’s gauge has to do its thing, the way the FAA approved it... nobody wants to pay for this change or it would have been done after JPI Built their first engine monitor...

The hottest point of the cylinder is where the exhaust valve is....

What we are doing with the CHT gauge is... reading the temp at the same spot, the same way, all the time and the POH has all the limits when doing it this way....

If we do it a different way, the redline is probably going to no longer be correct...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[Guest]

6 hours ago, carusoam said:

@M20DocClarence,

Is the TC well actually in an aluminum part?

The TC holder is probably a piece of steel?

How does a mechanic pull out the TC holder without destroying the aluminum threads?

How easy is it to recut threads and put a new TC well in place?

This could solve the two thermocouple challenge for the one cylinder...

Best regards,

-a-

The OEM CHT probe is a bayonet style which locks on to an adapter which is screwed into the threaded hole in the aluminum cylinder head.

[DustinNwind]

9 hours ago, M20Doc said:

The OEM CHT probe is a bayonet style which locks on to an adapter which is screwed into the threaded hole in the aluminum cylinder head.

Exactly - the best way to get an accurate reading on the #3 cylinder is to piggyback this adapter. Unfortunately whoever put this engine back together cross threaded the adapter into the cylinder. Now I have a pickle - do I unscrew it out and add the piggyback and hope it goes back in, otherwise I'd be looking at changing out the #3 cylinder. 

 

@carusoam the reason it reads so hot on the spark plug is because it's next to a piece of steel that is generating very high heat and not dissipating it like a block of aluminum would. From what my mechanics are saying at least. 

[jaylw314]

9 hours ago, M20Doc said:

The OEM CHT probe is a bayonet style which locks on to an adapter which is screwed into the threaded hole in the aluminum cylinder head.

 

Out of curiousity, is there a helicoil that size that can be used for this?

[Guest]

1 hour ago, jaylw314 said:

Out of curiousity, is there a helicoil that size that can be used for this?

Yes, it’s a 3/8-24 NF thread.

Clarence

[gsxrpilot]

2 hours ago, DustinNwind said:

Exactly - the best way to get an accurate reading on the #3 cylinder is to piggyback this adapter. Unfortunately whoever put this engine back together cross threaded the adapter into the cylinder. Now I have a pickle - do I unscrew it out and add the piggyback and hope it goes back in, otherwise I'd be looking at changing out the #3 cylinder. 

 

@carusoam the reason it reads so hot on the spark plug is because it's next to a piece of steel that is generating very high heat and not dissipating it like a block of aluminum would. From what my mechanics are saying at least. 

I'd figure out how to calculate -50 degrees off the JPI for that cylinder and go fly.  There are obviously several reasons why that CHT is reading differently than the others... all of them can be fixed... and none of them are JPI's fault.

[jaylw314]

 

57 minutes ago, M20Doc said:

Yes, it’s a 3/8-24 NF thread.

Clarence

If that can be used, then even if the adapter threads strip the cylinder well, it's not irrecoverable.

 

25 minutes ago, gsxrpilot said:

I'd figure out how to calculate -50 degrees off the JPI for that cylinder and go fly.  There are obviously several reasons why that CHT is reading differently than the others... all of them can be fixed... and none of them are JPI's fault.

I think there is still some benefit to the piggyback probe, because the spark plug gasket is not actually inaccurate, but it has a measurement problem that is nonlinear, e.g. you can' just subtract 50 degrees, it may be different at different power settings and airspeeds. 

[DustinNwind]

1 hour ago, jaylw314 said:

 

If that can be used, then even if the adapter threads strip the cylinder well, it's not irrecoverable.

 

I think there is still some benefit to the piggyback probe, because the spark plug gasket is not actually inaccurate, but it has a measurement problem that is nonlinear, e.g. you can' just subtract 50 degrees, it may be different at different power settings and airspeeds. 

@jaylw314 You are correct, it's not linear. The peak of the difference is on take off and the lowest difference is upon landing. 

[carusoam]

Dustin,

Have you gained enough confidence to pull the little adapter out?

Fix the threads, or use a helicoil... to put a new adapter in.

The threads for that are not holding the cylinder on.  Using a helicoil should be fine...

Your mechanic should be OK with this idea.  Unless one of our mechanics says it’s a nogo...

Most people would be doing the math if they couldn’t find a solution to getting a new adaptor in place. There are plenty of reasons that all the cylinders are not reading exactly the same...  FF, EGT, and air cooling differences....

it definitely helps to have all the TCs mounted as even as possible... Same for EGTs... Check their mounting locations as well...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

 

[Yetti]

I was thinking those were NP threads ie with a taper.  But Mooneydoc knows his stuff.   I would hope they are not that buggered.   You are going to need a bottom tap to clean them up.

https://www.amazon.com/Irwin-Tools-2636-Piece-Carbon/dp/B003A2UPYG

alum is always tricky to cut and can go south fast.   Heavy duty sulfer based thread cutting oil (makes a mess)  seems to work best.

Since it is not holding liquid you can have a little more grace in tolerances.

 

 

[Guest]

11 hours ago, jaylw314 said:

 

If that can be used, then even if the adapter threads strip the cylinder well, it's not irrecoverable.

 

I think there is still some benefit to the piggyback probe, because the spark plug gasket is not actually inaccurate, but it has a measurement problem that is nonlinear, e.g. you can' just subtract 50 degrees, it may be different at different power settings and airspeeds. 

The spark plug holes have helicoils, then installing one for the CHT probe shouldn’t cause issues.

Clarence