The flex plate is used to join the engine flywheel to the transmission input shaft. Not a lot is written about flex plates, but I can assure you that there is a lot to be concerned about. Flex plate sales and service shops are a relatively obscure group, who mostly service auto and truck flex plates. That is where the trouble lies.
Torsional dampers (flex-plates) are designed to eliminate engine torsional or cycle vibrations affecting the transmission. This results in reduced gear friction and wear while prolonging transmission life. There are many variables in selecting the correct damper for a particular application.
The Marine Industry has used these dampers ever since the first Inboard, but the design of a marine damper has taken a different course than that of the automotive industry. While an auto needs quiet and soft operation, the marine units have to be more concerned with durability.
This difference shows up most often in High Performance applications. The damper to fit a GM truck big block ,from the auto industry, costs about $75, whereas a marine damper averages between $350 to $650. A big difference that often temps people to use the automotive damper.
I wish I could justify this extremely large price gap, but the only real difference in the two units is with the spring type and number, and the gauge of the metal used to make the plate and spring retainer. However, the use of the wrong flex plate can lead to serious problems.
Besides breakage of the damper, the transmission is relatively sensitive to the load and alignment put to it from the engine and damper. It boils down to this..., a Trans repair runs from $1000 to $5000 for the average marine Trans, and taking the chance of an incorrect damper or damper failure only increases your risk of Trans failure as well. 7 out of 10 flex plate failures usually need Trans service as well...