Several years ago I had a heating problem with one of my new ECI
cylinders on my newly Lycon rebuilt O-340.  After modifying the
baffling many times with little success I examined the hole for the
temperature probe and noticed it was formed by the casting and not
machined as were other cylinders manufactured by Lycoming. In fact it
extended almost a quarter inch further in the cylinder than the
standard Lycoming probe hole and the other three ECI cylinders. I then
decided to connect ring thermocouples to both the upper and lower plugs
(VM 800 allowed these to be connected to cyls 5 & 6 on the CPU). Lycon
informed me that a ring thermocouple would read about 50 degrees F low
on the upper plug and about 50 degrees high on the lower plug. The
difference between the two rings was 110 degrees in my case and the
average was 50 degrees below the probe reading on that cylinder.