The Cylinder Head Temperature (CHT) - Gasket Type (Washer) - probe, replaces the standard copper sparkplug gasket on the sparkplug. You may install the probe on the upper or lower spark plug (the one that provides the best correlation with the other temperature probe). Due to the sparkplug location, the gasket probe may read 25'F higher (generally the sparkplug is the hottest part of the cylinder, which will cause this discrepancy).
The JPI EDM series CHT 18mm probes can be used on the EDM 700.
Q: Does this probe piggyback on the spark plug gasket or the stock CHT probe?
Per JPI, you may install this style probe on the upper or lower spark plugs
Q: What is the overall length of the JPI EDM Series CHT Probe, part # 10-27093?
Approx 19 inches.
Q: The JPI EDM series CHT 18mm probes can be used on the EDM 830?
Yes, these are compatible.
Q: For clarity about the question on piggybacking the existing sparkplug gasket, I believe the existing sparkplug gasket should be removed when installing the JPI sensor. If both are installed, the sparkplug will not be installed deep enough in the spark plug hole leaving threads exposed. This could cause pre-ignition leading to possible damage to the cylinder or piston.
Thank you for providing this follow up, we will post it on our site for other customers reference.