Help with EGT Probe and a Suggestion on Fire Extinguisher

[Ernie Martin]

1. I'm having trouble finding the EGT probe that is clamped on the exhaust pipe of the engine, feeding temperature to the EGT gauge. Looks as shown in the picture below. Cessna PN is C668501-0110 (the PMA version is Alcor 86156 but it's no longer made). Any info would be appreciated. Aircraft Spruce sells an MM-112 but not sure it works. Thanks.

2. In a post years ago I suggested Halon hand-held fire extinguishers for the cabin, as being far superior to the conventional powder extinguishers, which make a mess and impair visibility to the point where it's hard to fly the airplane. They are expensive, however, with $213 being the lowest for a suitably sized unit (Halon is no longer manufactured so these items use reclaimed Halon). That led me to investigate Halotron units. They seem to perform like Halon units but at lower cost. I was focusing on a Buckeye 70258 on Amazon at $141 until I found it at WebstaurantStore.com for $87. At this price you should consider it if you either don't have one in the cabin or have the powder type.

Ernie

[patrolpilot]

Ernie, right after I purchased my 337G in September, the rear probe failed on the way home. My mechanic replaced it with this probe: Aircraft Spruce ALCOR BY Tempest EGT/TIT Type K Thermocouple & Clamp 3-3/8 Max

My airplane has the optional Cessna Fire Extinguisher under the seat. No age, date, or gauge. So, I replace it with a like-sized bottle. The bracket was different, but my mechanic changed it out in 15 minutes.

I live in a small town, so I took the old bottle into the fire station and discussed it with them. It turns out there was a young Air Force firefighter veteran! He helped me chose a replacement through Aircraft Spruce: H3R Fire Extinguisher Model A344T.

[DrDave]

I went to the local fire extinguisher/safety equipment store and replaced all of my dry chem extinguishers with Halotron clean-agent extinguishers. The 2.5lb model was $118. The best part is that they sold me several re-conditioned units for half-price. The 5lb re-con ones were $90. That's a deal. I did buy a couple of big CO2 extinguishers for the shop/garage/hanger.

Dave

[edasmus]

Hi Ernie....

I have been doing a little research on these EGT probes as my front engine is out for overhaul and I would like to replace as many of these probes as I can.

So what I have learned is the same as you as far as the PMA Alcor 86156 no longer being made. My rear engine has this probe. The Alcor 86156 is a Type "E" probe. I have no idea what that means (vs. the seemingly more common Type "K" probes) but the Spruce probe you mentioned is also a Type "E" probe. The Universal MM-112-1 and MM-112 differ only by the connector on the wire lead, otherwise they are identical. I just purchased two of the MM-112-1 EGT probes hoping they will work.

I spoke with both Alcor and Universal on the phone and they were both very helpful. From what I understood, there is a resistance difference between the old Alcor 86156 and the new Universal probe. They both agree the Universal probe will cause needle movement on our old Cessna EGT gauges that I believe you and I have the same units in our 1973 C337G models. My old Cessna EGT gauge has adjustable pointers so I can align them after leaning which makes any changes during cruise obvious. I'm hoping that the Universal probe causes enough movement on our old Cessna gauges to make them useful. I will not know for another 6 weeks or so if they work.

The Type "K" probe that a previous post mentioned works with my JPI EGT/CHT display. Those Universal EGT probes I have had success with in the past.

Let us know if you have any other success with this.

Good Luck,

Ed

[Ernie Martin]

For PatrolPilot: did the Aircraft Spruce type K probe work with the original factory gauge? The original is a type E (which I believe has one lead grounded) and I was under the impression that a type K won’t work with the original gauge.

For Ed: when you are running again, let us know if the probe worked.

Thanks,
Ernie

[edasmus]

Will do...

[GAdams]

I know from first hand recent knowledge that a type K does not work with the type E old style gauge we have in our aircraft. The reason is the value of the signal is stronger in a type E than a type K. A type E system must be entirely type E. The problem is the probe, the wiring and the gauge must be the same. That also applies to the K. No mixing of systems. Try this link for further details. Perhaps I misinterpreted the information. https://alcorinc.com/index.php/troubleshooting/

[wslade2]

Correct. Not an expert but recently did a little "learning" while thinking about replacements for my gauges. There are basically two types of thermocouple wires/systems. Type K and Type E. The systems are not interchangeable. Since I put in a "K system", everything was replaced from exhaust to panel.

I took the easy way out and just replaced it all. Stuff for the K wire seemed easier to get.

Note that thermocouple wire is a specialized type of metal wire alloy and not your typical stranded variety. The probes have to be compatible with the wire/system you use.

Long term plan is replace everything with modern engine monitoring system

[patrolpilot]

Originally Posted by Ernie Martin

For PatrolPilot: did the Aircraft Spruce type K probe work with the original factory gauge? The original is a type E (which I believe has one lead grounded) and I was under the impression that a type K won’t work with the original gauge.

Ernie, yes, it is working. I've been at Center Line Aero (PKV) the last several days, let me talk to Lou about these probes if you don't mind as he might shed some light on them as he keeps a variety of probes in stock.

Honestly, I would think that the difference between the two lead lengths would necessitate a difference between the two.

Originally Posted by edasmus

I spoke with both Alcor and Universal on the phone and they were both very helpful. From what I understood, there is a resistance difference between the old Alcor 86156 and the new Universal probe. They both agree the Universal probe will cause needle movement on our old Cessna EGT gauges that I believe you and I have the same units in our 1973 C337G models. My old Cessna EGT gauge has adjustable pointers so I can align them after leaning which makes any changes during cruise obvious.

Originally Posted by The C336 Skymaster Story

To aid in identifying a failed engine" Charlie Tanner's power plant group designed a microswitch assembly that sensed fore and aft engine motion in the rubber engine mounts. With a rearward motion (from the drag of the windmilling propeller) a red warning light would illuminate in the related propeller knob. Although the aforementioned microswitch unit multiplied the actual engine movement by a factor of six, there were still false warnings that would prompt an unwarranted engine shut-down. Thus the system was removed in favor of pilot reference to engine and EGT gauge indications of power failure.

You kind of hit the nail on the head. The only thing this gauge is supposed to do, the why behind its installation, is indicate that the engine is hot or cold. We are flying, the only, that I can think of, twin-engine piston airplane where the AFM or POH requires EGT to be used to identify an engine failure.

• The three identifiers
• RPMFuel Flow
• EGT

Give the location of the single probe, after all the exhausts stacks join, I think it is a real mistake to use this instrument to define anything related to peak EGT operations. I know Cessna covered its use for leaning with a paragraph, but the single probe has a lot of promise for compromising a cylinder.

The adjustable knob on my left EGT managed to stop having any effect. I suspect that the gearing inside is plastic, so it just shows full scale hot or cold. The Insight G4T is Primary EGT replacement, and I'm kind of considering pulling the gauge completely. I notice that passengers tend to look at it with that front side full "hot."

I started flying the P337G in 1975 out of Harlingen for Air Central, a 135 Operator, with a majority of our flights into Mexico. My training and advice were to use Fuel Flow for engine management and keep the EGT pointers "near" together for aesthetics.

[Ernie Martin]

For PatrolPilot: the key question is whether the Aircraft Spruce type K probe works with the ORIGINAL factory gauge? let us know when you find out.

I understand the concern about using EGT for leaning, since you are getting an average of all cylinders and one of them may be running too lean and cause damage. That's all I've got and I use it for that (running ~ 75 degrees ROP), but I also watch other parameters like cylinder head temperature.

Ernie

[patrolpilot]

Originally Posted by Ernie Martin

For PatrolPilot: the key question is whether the Aircraft Spruce type K probe works with the ORIGINAL factory gauge? let us know when you find out.

Okay, Ernie, the short answer is yes it works.

In order to understand why it will work, I asked my mechanic at Center Line Aero (PKV) and very quickly got squared away on EGT probes. This is specifically Alcor style (Cessna, Alcor, Alcor-Tempest and generic) E & K probes. Almost all other EGT manufacturers use proprietary leads.

Alcor, one lead is shorter than the other, and leads are color-coded on the PMA FAA products.

• The short probe lead matches the long lead on the aircraft.
• The long probe lead matches the short lead on the aircraft.

Color, the PMA-FAA probes and leads match color as well.

• Type E - Black and Red
• Type K - Yellow and Red

Generic leads, know your "Type" and match leads short to long.

Okay, what types? I pulled out the parts manual when I got home, see the attached images. I hope this helps. I think I get probes now!

I wanted to point out that my image shows threaded PMA probes vs. the clamped probes. That is simply the first image that came up. Coding is exactly the same no matter how they are attached to the pipe.

[Ernie Martin]

Thank you, PatrolPilot, but I'm still VERY surprised by this. I get the lead length, but I've been under the impression that it's much more, that a gauge which uses an E probe will NOT work satisfactorily with a K probe, that you might get a reading on the gauge but perhaps not the full range of temperature readings. This may be reflected on the right of your last figure, where it says "Be certain of TYPE". When you next fly, please let me know if you seem to have good EGT readings with the K probe and the original gauges. Thanks.

Ernie

[patrolpilot]

Ernie, airplanes with a serial number 1726 or less, use an E Type. The only way it wouldn't is if a later model indicator was installed at some point in its life. In that regard, the part numbers for the "Indicator, Power Economy" are:

• Through SN 1726 - C668502-0107
• SN 1727 and up - C668503-0101

Serial numbers 1727 or greater, use a K Type. My airplane is #1795, so it uses a "K". Center Line Aero used a Alcor-Tempest 86255 from Aircraft Spruce as a replacement and yes, it responds appropriately within the range markings of the gauge. I've attached a snip from my invoice.

As a note, the indicator was an option on SN 1671 or less. It was not required equipment to identify an engine failure.

I cannot comment on the generic leads other than it should meet the resistance/ohms of the appropriate type, E or K.

[Ernie Martin]

Good to know. Thanks.

Ernie

[edasmus]

First paragraph cut and pasted from Ernie's original post:

1. I'm having trouble finding the EGT probe that is clamped on the exhaust pipe of the engine, feeding temperature to the EGT gauge. Looks as shown in the picture below. Cessna PN is C668501-0110 (the PMA version is Alcor 86156 but it's no longer made). Any info would be appreciated. Aircraft Spruce sells an MM-112 but not sure it works. Thanks.




My results:

So I flew my airplane yesterday for the first time with my Poplar Grove O/H front engine. All went very well.

I replaced the Cessna EGT probe C668501-0110 yesterday. I'm almost certain it was the original probe from Cessna and was in pretty bad shape though it was working when removed for overhaul of the front engine but it did not survive the O/H process. My rear engine had that probe replaced years ago with the above mentioned PMA version Alcor 86156 and as Ernie mentioned it is no longer available from Alcor. I spoke with them myself and they confirmed this.

This probe in question is a "Type E" probe as opposed to the more common "Type K" probes which my JPI engine monitor utilizes.

Universal (universal-co.com) makes both types of probes. The Universal MM-112 Type E probe sold by Aircraft Spruce worked in my application yesterday. There are 2 versions of the MM-112 sold. The other version is the MM-112-1. The only difference of the 2 versions is the connectors. The "-1" which is the version I used has eyelet connectors versus plug connectors on the other. I spoke to Universal (super helpful) before I purchased and they told me they were 99% certain their probe would work. The only concern was the scale may have been different. By that I mean the range the needle would display on the EGT instrument. Mine is adjustable and EGT temps being relative and specific numbers not mattering, a simple quarter turn of the adjusting knob on my instrument brought the needle movement inline with the rear engine.

So there ya go. My airplane is a 1973 C337G.

By the way, the Universal "Type K" probe works fine with my JPI monitor. I suspect it is simply a matter of making sure whether your application is a Type E or Type K probe. If you have that correct, I suspect no problems should occur!