So I recently tried to replace my holding tank and made a mistake. Catalina Direct offers 2 models for the Catalina 36. Neither of them fit the 1983, but one is within a 1/4" when measured from the top and the port side looking starboard. It's off by more than 6 inches if you can measure the part near the centerline of the boat, but that part is inaccessible until the tank a out. So this is a bit of a warning. There are more than two sizes of tanks used on the 36 over the years. The new tank is a custom part, so now I own it. It's a Ronco B167 (18 gallon). I paid $400 for it. I'd be happy to get $100 if anyone wants it.
David Hickman
1983 Catalina 36 #154
"Beach Belle"
David
Mobile, AL
S/V Beach Belle
1983 Catalina 36 Mk I #154
Std. Rig, Fin Keel, M25