never pass up an opportunity to learn from the mistakes of others.....
i have an ultrasonic cleaner that i filled with simple green soap. works great to clean out crud from corroded reels, but the water in the simple green evaporates over time. out of laziness, i've been topping the tank off with arm & hammer liquid laundry detergent. it contains mildly alkaline sodium carbonate. well, i tossed in a slightly corroded "a" series calcutta 400. um, and i forgot about it until the next morning. here are the results. so from now on i stick with soap alone and i use the timer function and not just the "on" switch.
so i went out to buy my friend a new reel to replace the old one. in the process of changing out the drag washers, i found myself looking at the two reels, side by side, for the first time. yup, the older "a" series calcutta 400 right next to the newer "b" series calcutta. so what's the difference? well, for one, a bearing replaces a bushing on the right side of the newer calcutta "b."
another difference is the set plate (part #bnt 3362) and an anti-reverse pawl (part #bnt 3361) in the newer reel.
the older "a" series calcutta 400 does not have this back up anti-reverse mechanism. so when the anti-reverse roller bearing fails, the reel goes "knuckle buster" on you.
but it does have the hole. a pin with a 3mm OD would fit just fine. it's also possible that you could hold the anti-reverse pawl in place with a screw. it might only have to fit the 3mm hole of the pawl. i'll have to check this out some more. wouldn't that be interesting, being able to add a back up anti-reverse pawl to all of the old "a" series calcutta 400's out there.
two calcutta 400 discoveries.... 
