Installing the Gasket and Hard Stops
Whether you install the mounting gaskets or not, you should apply marine sealant between the device and the mounting surface to help prevent water intrusion behind the mounting surface.
Mounting gaskets are provided with the device, but are not required for installation. If you install the gaskets, you must also install the included hard stops to prevent over-compression of the gaskets, unless you are replacing a previous model of chartplotter installed in the same location.
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If you are not replacing a previous chartplotter in the same flush-mount opening, install the hard stops
on the back of the chartplotter by removing the adhesive backing and pressing them into place 
in the designated recess near the mounting holes.
You should install a hard stop near every nut plate location. Be sure to install the hard stops in every designated recess along the top and bottom of the chartplotter.
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Place the gasket pieces
along all four edges on the back of the chartplotter sure they align with the mounting holes and hard stops.
The GPSMAP® 9x27 models have split gasket pieces along the top and the bottom due to their size.
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Remove the adhesive backing from the gasket pieces and press them into place
along the edges on the back of the chartplotter.
The shorter vertical gaskets may overlap the top and bottom gaskets. You may need to trim the vertical gaskets for the best fit.
When applying the split gaskets along the top and bottom of a GPSMAP 9x27 model, you should align and secure the outside edge first, and then move toward the middle. You may need to trim any overlap in the gasket pieces where they meet in the middle.