Im in need of a new thermostat for my gen 5 502. There is a couple of options, a 140, or a 160 degree..

Closed or open cooling system? Water pump or bypass?

You need the 160 degree to keep your stock EFI engine from running to rich and fouling out the spark plugs.

All the Merc black EFI engines use a 160* TStat.

160 just removed mine last month. Make sure you get a new cork gasket up top too, they turn into a bock of rust, cant even tell there one in there!!