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The manual description of where to drill the EGT probe
holes is vague to say the least. The result is that in a lot of
circumstances the readings tend to be on the high side even though the
engine running condition is correct.
Rotax simply define the location of the sensor's probe
as being 100 mm from the piston
 skirt
which is fine so long as you can relate that information to the exhaust
tract as a whole.
The first illustration shows the distance from the
piston skirt to the manifold mounting face including the compressed gasket
which is 59.50mm for a 582 engine. Jhe remaining 41.50mm is within the
manifold itself so therefore the positioning of the probe hole is
important to ensure reliable indications.
Too far downstream of the
correct location will result in higher readings and likewise too close to
the piston will yield lower readings. Think of it in terms of a welders
gas flame where the hottest part just beyond the blue tip and the coolest
inside the cone.
Lay the manifold so that the welded flange is parallel
to the work surface. Cut two pieces of thin aluminium cut to the
dimensions of the flange faces and mark their exact centers in each plane.
Tape these to each mounting flange. Mark a line on each of the manifold
branches exactly 38.00mm back toward the ball junction and parallel to the
welded flanges. Next mark a line exactly half way across each of the
branches. Where the lines intersect is the positions of the probe tips.
Because the flange faces are welded onto the branches at
an angle the position will be different dependent on which face the probes
will be mounted. It is important to remember therefore that the spots
indicate where the tip should lie and it will be necessary to place
packing washers under the mounting bands to adjust the insertion of each
probe.
If
the tips lie closer to one wall then again there will be errors in
readings.
The manifold is now predrilled and has two threaded
bosses to accept the probes which eliminates entirely the problem but the
582 manifold is shared by other engine types and therefore it is up to the
installer to position the probes correctly.
After the band clips have been formed and tightened then
wrap a length of safety wire over the bands just in case they break
allowing the probe to exit the holes from vibration.
Finally because the probes are hot junction types ensure
the braided cables are kept out of direct airflow and are not secured to
"cold" attachments because again readings will be affected.
Where it is necessary to to make an attachment to a cold
surface insulate the cables with plastic sleeving.
Graham Laught, UKAvid
http://www.sportflight.com/kfb/howto.htm
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