When is a battery dead?

Wednesday I drove the '66 W108 250S to work
and back no problems. Thursday A.M. it won't
start. It cranks, but won't fire.

So I...

(*) put new plugs in.
(*) put new cap in (not rotor).
(*) charged battery overnight inside.
(*) checked to see if the automatic choke on
the back of the Zeniths was working okay.
The spring seems to be working fine (spray
cold silicone lube on it and it moves in the
right direction) and it actuates the small
lever for the choke okay.
(*) verified that it is getting gas.
(*) verified that it is getting spark.
(*) coil is about 2 years old.

I took the battery to a shop and they said
there were no bad cells. Measured 12.3 V off
the posts. They put an (8A?, can't remember)
load on it and it went to 9V. He said this
was okay but I tried it again when I got home
and now it cranks slow enough to stop every
once in a while with that classic "dead battery"
sound.

So my question is this:
Is 9V considered acceptable for a 12V battery
to dip to when cranking? He said that if it
had gone to 7V, then he would say there is a
problem. Where do you draw the line?

The battery is a 5 year old (Interstate)
# 27F (675 CCA).

- Jeff

__________________
- Jeff

1957 W121 190 (history)
1966 W108 250S
1967 W108 250S (parts)
1982 W123 240D (history)
1989 W124 260E