I just finished my condition inspection and decided to relocate my CHT sensors (Grand Rapids Technologies EIS 6000) while I had the cowl off. Jabiru originally approved and recommended the typical spark plug sensor which has a 12mm ring that goes between the plug crush washer and the plug. This works OK but makes it difficult to get a plug socket onto the plug without bending or damaging the sensor. It also means you have to unthread the crush washer and take the sensor off to change plugs. All this manipulation leads to flexing of the thermocouple where it is crimped to the ring and tends to lead to early sensor failure.

Last year, Jabiru came out with an approved modification to relocate the sensors to the heads (AVDALSR085-1). There is already a 1/8" hole drilled between the two plugs on each cylinder head. Apparently Jabiru uses this hole to insert a temporary CHT sensor when they dyno each engine before shipment. The mod involves drilling the hole out to 5/64", tapping it, and removing the 12mm ring terminal from the thermocouple and crimping a new small terminal for a screw and affixing it to the hole. A few small cooling fins have to be ground down a few mm for clearance to each sensor hole.

However, several people have already accomplished the mod in a much easier manner and that's what I did. Instead of drilling the hole out larger, I tapped it as-is with a 6/32 tap, and instead of removing the ring terminal completely, I just clipped the ring off and drilled a hole in the base of the ring and screwed the base directly to the head. This prevents accidentally damaging the thermocouple junction when removing the terminal.

Additional "enhancements" I tried was to put some high-temperature thermal transfer paste under the screw head and under the base of the sensor. I also put some high-temperature Loctite on the end of the screw. Finally, since some of my sensors were already starting to show evidence of wear at the junction of the wire and the crimp, I reinforced these areas with a drop of JB Weld epoxy, which is non-conductive and good to 500+F. Once the sensors are installed, the wiring is then secured along the cylinder heads with Tefzel zip ties to prevent vibration.

The result? The sensors read about 15-20 deg F higher in their new location. In my case, this is still well below allowable CHT limits - Jabiru does not adjust those limits for the new sensor location. The sensors should last much longer as they will not be subject to movement every time the plugs are removed for compression tests or replaced.

John

N750A

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John,

Why don't you post some photos of your work?

Mack

The day I did the switch-over, Mack, I was so anxious to get everything buttoned-up and tested that after the cowl was installed, I thought, "Gee, I wish I had remembered to take some pictures!" HA!

So, I'm in the flying and not "fixing" mode now, so hopefully I don't have to pull that cowl 'til the next oil change! However, the pictures in AVDALSR085-1 that I linked in the first post are really quite similar to what I did. The biggest difference is that I did not completely remove the ring sensors, instead, I just clipped off the ring and drilled the base of the ring for the screw. Works fine that way!

If I have to pull the cowl anytime soon, I'll take some pics.

John

I did find this pic in my files:

It clearly shows the factory-drilled 1/8" hole between the plugs and the spark-plug ring sensor on the right spark plug. You can see the four ridges/apexes of the fins immediately below the hole. These have to be ground down a few mm so the sensor can be laid flat against the hole when screwed-down tight.

John

Update!

I periodically check the CAMit website to see what's the latest upgrade that Ian Bent has come up with for the 3300. They are now offering a CHT sensor made specifically for the 3300 utilizing the same factory hole I used. Looks like it will be necessary to drill the hole out slightly larger and tap for an 8-32 thread, but certainly appears to be a sturdy sensor harness! No price is listed - I suspect it's not cheap, but neither is overheating your engine!

John

Hi Guys

I did the mod a couple of years ago on advise from the Jab Aust factory. They should have been set up like this from the factory. The engine department told me that it was designed for a probe but the probe was never manufactured.

http://www.mykitlog.com/users/category.php?user=ch601xl&project...

cht probe


Category Work Time: 58 Hours
 Date Hours
Details
 03-10-2011 2.00 cht probes Details

     

 


 03-09-2011 2.00 cht gauge Details
     

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