1.* It just seems that from the start in the 1960s to the time my engine was built 1982, they had 20 plus years and lots of engine hours to know they had a problem.
2.* You think somewhere along the line they would have made a better pump drive tang.
1.* Not necessarily.* Given the average annual use of recreaetional boats which seems to vary from 50 to 100 hours a year, it could take well over 20 years for the problem to become frequent enough to be noiticeable.* Like I said, we had a cracking drive tang on one coupler but the other coupler was fine.* I have no idea what contributes to the cracking.* Harmonic vibration, perhaps?* This could explain why it happens on some engines and not on others* with the same amount of time on them.* Every engine even of the same type is a bit different in some way or another.
I don't hear of these things breaking left and right.* But it's something that occurs just often enough for our diesel shop to be well aware of it (which is why when we needed the pump overhauled they removed the coupler too because they wanted to check it's drive tang) and Bob Smith is certainly aware of it since he's the one who told me about the less-than-ideal design of the coupler (which he designed).* He said it was a logical design at the time but time has shown it to be not as good as it could have been.
2.* Maybe they did.* I have no idea if this problem can surface only in older FL120s or if it occurs across all the years of manufacture.* In any event, it appears to be random and totally unpredictable.* Had our water pump not needed a new shaft seal when it did we would have continued operating the engine with no idea that the coupler was getting close to failing.
The good news is that there is a great cure which is actually better than the original setup.* The Johnson pump is smaller yet moves more water and so far as I know has an utterly reliable drive coupler.