[A/S] Leak at shower drain fitting

["Christopher45103" <blickcd@xxxxxxx>]

Started up the water system in my '79 International center bath 
tonight.  Found that the shower stall is leaking where the drain 
connects to the floor pan.

It took a while to find the source of this leak, and even longer to 
figure out how to access the bottom of the shower pan.  I drilled out 
the pop rivets that hold in the heat vent below the shower door, 
removed the flex duct from the vent and pushed it out of the way, 
which leaves about a 4" diameter hole to work thru.

By feeling the drain pipe, I am pretty sure that the leak is a bad 
seal between the flange on the drain fitting where it's flange seats 
on the floor of the shower.  No leaks from above running down the 
underside of the pan because when running the shower I held the head 
close to the drain.

I can't figure out how to get the drain fitting loose to re-seal it.  
On the bottom there is a black rubber fitting which slips over the 
top of the drain pipe, and it has a hex shaped collar at the top, 
evedently where it screws around the drain fitting.

I tried to unscrew it by hand but it wouldn't budge.  I was leary of 
putting channel locks on it and breaking the shower pan so I thought 
I'd post a message about it first to see if there is any trick to 
it.  I assume that after removing and cleaning the fitting it is just 
bedded in with plumber's putty.

Thanks in advance,
Christopher