Actually.. since they are press on, they can slide back off.
Seems like you did enough work that maybe it's a gasket etc, but that's not common on a 3800. These motors don't typically burn oil, my best guess is that one of the seals popped off. Kinda depends on how they are put on. I like to use the Fel Pro vitons (look exactly like the zzp ones, but always come with the right number of them in the box). Then I use a 1/4" 10mm deep socket to clear the valve stem and lightly tap the socket a couple of times with a rubber mallet. Oh.. and in the last couple of sets I got, there was a nice sleeve to go over the stem and keep the seal from being chewed by the groove for the locks.
