I encountered a Mamiya RB67 lens that would not mount to a camera (lens and body both cocked as required). RB67 lenses mount with a breech lock ring that rotates to secure the lens - the lens body does not rotate. Looking closely at the rear of the lens, it was apparent that the ring was dented inward in one spot, as if the lens took a small whack on the rear edge.
The dent is a lot easier to take out if you can remove the lock ring, but how wasn't totally obvious. With hints from some old posts from Mamiya_Repair and paul ron, I got it off.
1. On the ring, about 180 degrees from the red mounting indicator dot (so at 6 o'clock when the lens is mounted), there is a single small slotted screw. Remove this. You will need a fairly thin-bladed screwdriver. This is just a stop screw that stops the mount ring at one end of its rotation.
2. Now turn the ring, as if (un)mounting the lens. It will look like nothing is happening and you can't pull the ring off. This is because the ring is actually threaded onto the lens body with a fine pitch left-hand thread. Once you see how the whole thing is put together, this makes sense, but at first it feels like it isn't loosening. You just have to keep turning (past the former stop-screw stop, and several turns more) and it will eventually be apparent that it's coming off. This left-hand thread is what I couldn't figure out from the internet or the repair manual, hence this post for some future reference.
3. There's probably 40-50 years of dirt between the mount ring and the lens, you can clean that off now.
4. Straighten the dent. I do not think you should use pliers or similar naughty instruments to bend the ring. Instead, use a heavy plastic or rubber hammer and a block of wood, and carefully hammer the dent out. It would be ideal to have a concave curved wood surface to hold the ring, but you can get away with a flat wood block and offsetting the block and hammer position, I think.
5. Make sure there aren't any rough edges left on the inner bayonet surfaces of the ring. Re-install the ring by screwing it back on and refitting the stop screw. You may have to iterate a few times to get the lens to fit on easily.
The dent is a lot easier to take out if you can remove the lock ring, but how wasn't totally obvious. With hints from some old posts from Mamiya_Repair and paul ron, I got it off.
1. On the ring, about 180 degrees from the red mounting indicator dot (so at 6 o'clock when the lens is mounted), there is a single small slotted screw. Remove this. You will need a fairly thin-bladed screwdriver. This is just a stop screw that stops the mount ring at one end of its rotation.
2. Now turn the ring, as if (un)mounting the lens. It will look like nothing is happening and you can't pull the ring off. This is because the ring is actually threaded onto the lens body with a fine pitch left-hand thread. Once you see how the whole thing is put together, this makes sense, but at first it feels like it isn't loosening. You just have to keep turning (past the former stop-screw stop, and several turns more) and it will eventually be apparent that it's coming off. This left-hand thread is what I couldn't figure out from the internet or the repair manual, hence this post for some future reference.
3. There's probably 40-50 years of dirt between the mount ring and the lens, you can clean that off now.
4. Straighten the dent. I do not think you should use pliers or similar naughty instruments to bend the ring. Instead, use a heavy plastic or rubber hammer and a block of wood, and carefully hammer the dent out. It would be ideal to have a concave curved wood surface to hold the ring, but you can get away with a flat wood block and offsetting the block and hammer position, I think.
5. Make sure there aren't any rough edges left on the inner bayonet surfaces of the ring. Re-install the ring by screwing it back on and refitting the stop screw. You may have to iterate a few times to get the lens to fit on easily.
