First off, this is a great post by squelchtone about how to pick one of these locks:
viewtopic.php?f=9&t=47655&p=359269&#p359269
now here are some visuals of the lock.
once the lock is open use a 7/64" hex key to take off the plate holding the guts inside
with the plate off:
as mentioned in other threads, the 6121 presents a problem in that the spring is difficult to overcome; often causing your torque key to be bent. I found the best trick is using a screwdriver once you've got it on the shear line.
here's the spring mechanism that causes the troubles:
this is the heart of the lock, containing the lock cylinder and pins.
finally the meat and potatoes. the lock has 5 pins, 4 of them spooled in this particular lock. although there is room for a 6th pin if you noticed in the picture above, just this lock only had 5.
hopefully this helps anyone trying to pick a ProSeries 6121.
Cheers!
