CHT probes causing leakage

[jetjr]

After watching high CHT on individual cylinders and recently putting in new plugs only to see high CHT patterns move!!!!

 

Not that serious, 180 deg on haul out then maybe 20deg higher than others in cruise.

 

Using detergent I tracked down the plugs are leaking just a little. Only on ones with CHT ring though.

 

Takes ages fiddling and testing to make sure they are sealed, tried putting the crush washer under CHT ring but doesnt help much

 

CHT rings supplied by Dynon, and I have 6x

 

How do I seal these reliably?

 

JR

 

 

[Guest DonC]

Remove the ring and insert into hole between spark plugs, seal with high temp epoxy, then enjoy accurate readings

 

 

[jetjr]

Ok, sounds good, do you mean cut the ring off and insert the "stub" or roll the ring to fit the hole

 

What about the heatshrink etc, leave it there?

 

Any chance of a photo, I cant picture the hole your talking about either

 

 

[Guest DonC]

Just cut the ring off, strip the covering back 30mm, twist the wires together and insert. The hole is midway between the spark plugs, about 1/8" and 1" deep. I don't know if that applies to earlier models.

 

 

[Bruce Tuncks]

I reckon this is a great idea, but I don't see where to stick the twisted wire pair.

 

I'd be willing to drill a 1/8 hole and put the wires in, to be held with a 3/32 blind rivet, but where is a good spot?

 

I've got a CHT sender on each cylinder, so those spark plug fittings are a real pain.

 

cheers, Bruce

 

 

[icebob]

Hi,

 

What about fitting the CHT sender under a cylinder head bolt near the plug hole?

 

Bob.

 

 

[Guest DonC]

Under a head bolt is no good - you get a lower reading. If your heads are the same as mine you will see the hole midway between the spark plugs!

 

 

[jetjr]

Older heads dont have this hole

 

Headstud will be cooler

 

Not much room for another hole to be drilled

 

I spoke to Jabiru, they dont have any better ideas either

 

I have 6 CHT too and leaking is a pain - using detergent and looking for bubbles seems to be the only way to tell - other than High CHT readings

 

This is a frustrating problem as I dont know if Im chasing a real issue or poor CHT ring mounting

 

 

[icebob]

Hi donC,

 

I take your point but the motor I saw he fitted a CHT lead to the spark plug and the closest head bolt(old style head) and over two months got an "offset figure for the head conection and just used that as he now knows what the temp differances are between the two.

 

I only have experiance with this one motor so was just going by what he did.

 

Bob.

 

 

[Guest DonC]

Yes, I tried under a head bolt once but 20 ft/lb made a bit of a mess of the ring, and more importantly, makes it hard to maintain head tension. On old heads, I suppose you could drill a hole, but you would have to be carefull.

 

 

[Ross]

Thermocouples

 

It was many years ago, about 40, in the Instrument section of the Combustion department at Australian Iron & Steel Pty. Ltd. (BHP) at Port Kembla during my brief experience in that department with little experience under the direction of experienced Instrument Fitters that we made thermocouples used around the various production furnaces such as coke ovens, ingot reheating, slab reheating, blast furnaces, open hearth furnaces etc if I remember correctly. The actual very high temperatures inside the melting furnaces such as the open hearth furnace were more difficult being measured by focusing the radiation rather than the hot gases or molten steel from inside the furnace up a tube and onto a number of small thermocouples in a circle arranged in series I think to produce a bigger voltage which was compared against a standard voltage produced by a special long life battery in the instrument.

 

We used two wires, something like a bare inch or a bit less of each was twisted together then heated with an oxy acetylene torch until the end of the pair melted together in a small ball on the end of the pair. The size of the ball did not seem to be critical. The two wires were of Iron Constantan and Platinum after 40 years memory ago?? Platinum is worth more than Gold.

 

Gold about $A1,219/ounce,

 

Platinum is about $US1,600/ounce or over $US 57 million/ton

 

Nowadays there are plenty of suppliers with thermocouples made up to suit a multiplicity of uses and range of temperatures and instruments to go with them. The instrument is really a special sensitive voltmeter calibrated in degrees C or F.

 

There is plenty of information on the internet on the subject of thermocouples i.e. two dissimilar metal wires welded together exposed to continuous heat produces an electric voltage or current related to the temperature of the junction. There are various combinations of wire types types suited to different temperature ranges also they can be insulated with ceramics to be inserted into a metal hole in for example a carburetor body to check on icing or used bare in exhaust flues or cylinder heads.

 

The carburetor, CHT and EGT in our RAAus aero engines are well within the range of simple thermocouples.

 

See the following site for an introduction.

 

There are probably many small firms and large ones involved in these types of instruments in Australia.

 

Thermocouple - Wikipedia, the free encyclopedia

 

I am not an expert, in fact not even up to date on what is available.

 

Regards

 

 

[jetboy]

You get a reasonably accurate reading wedging the collar that contains the junction, after cutting off the ring and removing any plastic sleeving, between the first fins nearest the exhaust flanges.

 

It might read 75 deg F higher than the plug ring system, just do dual check on same cylinder probes first. to be sure, use heatsink paste and cover the crevice where the junction is with silicone RTV then you know you are only reading the heads, not the surrounding air or the sparkplug temps.

 

Ralph

 

 

[xair1159]

Depending what instrument you use to display the temp. the thermocouple will usually be Type K (Chromal/Alumel) or Type J (Copper/Constantan). Using the wrong type will give false readings, so check your instrument manual.

 

A few of us here have used a neater way of making a probe to fit in the small hole betwen the plugs. Cut off the ring terminal, bare about 1/2" of cable and twist together tightly, then insert into a 1" or so length of 1/8" copper or brass tube which you can get from model shops. A bit of epoxy or heat shrink sleeve on the outside end will hold the cable in the tube. Makes it easy to fit and remove if necessary and a dab of heat transfer paste in the hole could also help accuracy.

 

Typical cruise CHT's with the probe in the hole are 150-155 degC.

 

Nick

 

 

[emaroo]

Depending what instrument you use to display the temp. the thermocouple will usually be Type K (Chromal/Alumel) or Type J (Copper/Constantan). Using the wrong type will give false readings, so check your instrument manual.A few of us here have used a neater way of making a probe to fit in the small hole betwen the plugs. Cut off the ring terminal, bare about 1/2" of cable and twist together tightly, then insert into a 1" or so length of 1/8" copper or brass tube which you can get from model shops. A bit of epoxy or heat shrink sleeve on the outside end will hold the cable in the tube. Makes it easy to fit and remove if necessary and a dab of heat transfer paste in the hole could also help accuracy.

Typical cruise CHT's with the probe in the hole are 150-155 degC.

 

Nick

Nick

Just wondering what holds the probe into the hole between the plugs. Does the 1/8 copper make a tight fit or does heat paste fix it.

 

Cheers Rick

 

 

[xair1159]

Rick,

 

The copper tube is a fairly snug fit and the paste helps. I have the leads cable tied to the holes in the fins for the head bolt, then both looped down and cable tied to the oil feed pipe. Not had any problems. If I can work out how, I will post a photo.

 

Nick

 

 

[Bruce Tuncks]

Here's what I've done with an older engine (no hole between spark plugs)

 

1. removed the old tabbed washer and snipped off the wires.

 

2. cleaned off 7mm of the wire pair and twisted them together.'

 

3. cut off 20mm of 1/8 inch copper tube (model aircraft fuel tube)

 

4. put the twisted thermocouple wires in the tube with a length of safety wire and tapped the lot to flatten the tube and hold the wires

 

5.pushed the copper tube between fins about 50mm from the exhaust. tapped this down with a pin-punch, and wrapped the tie-wire around the cylinder to hold it in place.

 

6. sealed over the copper tube with high-temp silicon.

 

So the CHT lead was at the bottom of the fins, and held there.

 

I tried this first in no2 cylinder, and flew the plane. The CHT reading was the same as before , so now I've done it with cylinders 1 and 3, leaving 4 as always.

 

I'll fly with this soon.

 

cheers, Bruce

 

 

[Bruce Tuncks]

update on new cht probes

 

Now that I've flown with the new cht probes on 3 cylinders ( no. 4 has been left for now to keep a benchmark ), I can report that there is no difference I can see in the readings before and after the change from the spark-plug rings.

 

So thanks to everyone who contributed to this topic... I never would have come up with this solution without all your ideas.

 

Bruce

 

 

[emaroo]

Nick

 

Have used the 1/8 copper works well. Will be alot easier to change plugs.

 

Cheers Rick