There seems to be a lot of you on the forum that are looking at damper plate (also know as a flex plate) replacement this spring so I thought I would share my experience since I have some extra time right now. First of all, the signs that a damper plate is going bad will be a "knocking" noise coming from the bell housing of the transmission. The plate in question is bolted to the flywheel and transfers power to the input shaft of the transmission via a splined coupling. The coupling is an integral part of the flex plate that is mounted to a set of springs that "soften" or "dampen" the jolt that you get when shifting from neutral to forward or reverse. The flex plate is there to protect the transmission, and PCM will not warrant a new transmission that is installed without also installing a new flex plate!
So if you are having this knocking sound coming from the bell housing, and it's there at idle and then tends to become faint or go away after engaging the transmission and throttling up, then chances are you will be needing to replace the flex plate soon. A flex plate is either going to work, or it's not going to work - there's no in-between. A failure will be catastrophic and leave you stranded - as it did me!
So you need to order a flex plate from a reputable supplier. There are differing opinions on this, but my experience is that you get what you pay for so I ordered mine from www.skidim.com. Regardless of where you buy it, just be sure that it is made for a marine application. The disassembly and replacement is relatively easy and can be done in an afternoon, or mere hours for some. The transmission weighs only about 90lbs (small block Ford) and maybe 120lbs with the bell housing.
Procedure: Romove the drive shaft coupling bolts and slide the shaft toward the aft of the boat. You may need to remove your prop to have clearance to do this since your prop may hit your rudder. After draining the transmission, remove the transmission and bell housing as a single unit - on the PCM 351 you will need to unbolt the rear mounts as well as disconnect transmission hoses and shift cable. Remove the starter as well. As soon as you remove the bell housing you will see the flex plate. Unbolt it from the flywheel . . .
Now is a good time to inspect your flywheel to be sure all the teeth are there! Mine was missing teeth (good place for a West Virginia joke) so I also removed the flywheel and took it to a local machine shop to be re-ringed. Also I went ahead and replaced the rear main oil seal at this time since it had a slight drip.
Re-assembly is very simple, but be sure to use a torque wrench when fastening the damper plate to the flywheel (and the flywheel to the crank if removed). Hook everything back up, put in some new transmission fluid, and away you go. Good as new!
Old flex plate.