To finish an earlier thread I'm reporting on the install of a replacement
CHT probe in our Aztec. For those who didn't read the previous edition, the
left engine CHT probe in our Aztec failed and I started searching for a
replacement for the old AC Spark Plug Co. probe. A couple parts houses and
an instrument repair company took turn explaining my options. Since the
gauge is also an AC gauge and AC doesn't make either any more, the
replacement probe will not work with the AC gauge unless the gauge is
removed and sent to an instrument repair company to be re-calibrated for the
replacement Rochester probe.

This problem surfaced in the late 1970's for owners of early model Bonanzas,
Pipers, and Rockwell Commanders. One enterprising Bonanza owner
experimented with different size resistors wired inline with the probe. He
discovered that an 82ohm resistor in combination with the Rochester probe
would produce accurate readings on the AC gauge. So after talking with an
instrument tech who was familiar with the combination and who had sold many
of them during the past 10 years, I thought I'd give it a shot.

Long story short, it works. I found a pack of 25 new 82ohm 2W 5% flame
proof ceramic resistors on ebay for less than $5 and Aircraft Spruce had the
Rochester probe for $54. I figured that even if the combination didn't
work, I still needed the probe and would give it a chance before I spent
several hours with my upper body crammed up behind our panel pulling the 6
pack gauge cluster.

Tonight I installed the probe and attached the wire. I had already wired
the resistor and protected it with heat shrink. After installing, I checked
the amperage draw on battery power only and it measured 94.3mA, compared to
1.05mA without the probe attached. So I knew the probe was drawing current.

Taxi, runup, and test flight showed that the left gauge matched the right
gauge almost exactly. I didn't have the time or the equipment with me to
check it in an oil bath, but for now it looks like it works as advertised.

Jim Burns