Tinguy your not quite right the 53/54 super fast-twins were similar.  They all used the same power head. ’55 was the first year for the five bolt lower unit.  Five bolt LU was used on 15/18/20/25 for a sack of years and it held up well.  Somebody will correct me if I am wrong but I think that is correct.  The five bolt was also used on the ten horse 55/56/57.

dale

Once I thought I was wrong….but I was mistaken!

There was the good and the bad and the ugly. As already mentioned, 1956 saw many improvements for the good side. However one of them turned out to be a new problem. The new exhaust housing put the drive shaft and crankshaft inside the exhaust system where it was exposed to all the nasty combustion byproducts and spent cooling water resulting in failed crankshaft seals, water in the powerhead, rusted and striped splines etc. It took years of re-ngineering to deal with it

In all fairness this problem was not unique to this motor. It appeared in many other makes and models also.

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Tinman