Kevin Stohlhammer wrote:
> I am trying to find cht probes that will fit under the plug on my
VW. So
> far everything I have found has an offset connector that will not
fit into
> the spark plug well.
Steve Bennett was a fan of mounting CHT probes under the head studs,
but I wouldn't recommend it. I put one there on my VW, carefully
torqued the head nut back like it was, and the head stud broke a week or
two later, requiring a complete tear down of the engine, since it broke
off flush with the case, as I recall. Below is something I wrote in
2017...how to make your own CHT probes. You would only have to swap out
the ring terminal, not actually weld the thermocouple together, but the
ring terminal part number is in the message below. They are still
working fine, despite having had the plugs out several times since then....
Subject: home-brew thermocouples...for cheap!
Date: Thu, 8 Jun 2017 20:46:31 -0500
From: Mark Langford <m...@n56ml.com>
I finally got my Revmaster heads last week, and have installed them and
have put five hours on the engine so far. With new-found compression
and big-valve heads, it's back to the ~170mph top speed and nice climb
rate it had after the first rebuild.
But the Revmaster heads come with 12mm spark plugs, and at $35 each, I
really didn't want to shell out $140 for four CHT probes, so I bought
some K-type thermocouple wire for $9.50 off ebay (free shipping),
12mm-#8 ring terminals for $0.55 each from Digikey, and built all four
CHT probes for $18. Having seen thermocouple welders on youtube, I used
my TIG welder set on 10A (the lowest setting) and just barely struck an
arc with the pedal to weld the twisted thermocouple wires together in
about 1.5 seconds, crimped the thermocouple to the ring terminal, and
double heat shrank the terminal unions, with some superglue added
underneath the shrink tube for good measure. Now I have home-brewed
custom thermocouples for cheap, that are exactly the right length for
each spark plug. And the fact that some are as much as 20% longer made
no measurable difference. I tested all four (plus a spare OAT sensor
and a Fluke temp probe) and they match within .6 degree F of each other
using a six channel Omega tester at room temperature, so I'd call them
"close enough for KR work"!
Something else I learned was the effect of tinning the instrument end of
the thermocouple wire. My thermocouple wire is stranded, so when I plug
into the little slots on the tester the strands separate and make a
mess. [And yes, know there's a connector for that job.] So I wondered
what the accuracy difference would be if I tinned the end of the
thermocouple wires with some solder. The answer, despite what I've read
over the years, was "no measurable difference at all", at least not at
ambient temperature. I flew the plane today, and cylinder head temps
were amazingly uniform for a change. Of course a new pair of Revmaster
heads didn't hurt either.
So neither length nor material compatibility at the connections make
enough difference to accurately measure, and the difference may just be
the way I welded them or some other tiny factor. And given the low
accuracy required for CHT duty, the tiny differences in accuracy are "in
the noise" anyway. So there's another thing I can quit fretting over,
and so can you. See enclosed photo...
Mark Langford
m...@n56ml.com
http://www.n56ml.com
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