Also the TN-28 of 1952 / 53 had the OMC Universal points rather than the old guillotine style. In engineering that change they made subtle changes to the flywheel & starter.
Although the displacement, bore & stroke are the same as the later 5½ horse AD motors, they are completely different animals. The TN & predecessors are rotary valve 2-strokes, while the AD is a reed valve 2-stroke. Carburation is different. Pistons, block & mounting are different.
The older TD & TS models had their impeller down in the foot of the casting on the prop shaft. Because of the neutral clutch on the TN they had to move the impeller above the neutral hub on the drive shaft so that it would keep spinning & pumping water while in neutral.
The old TN and TD 5s are great little motors but are ancient, having been out of production for over 60 years. They are actually a restyle of a late 1930s design. Because of that, plus the fact they have bronze bushings for bearings & use leather seals in the carb make these engines a bit challenging to restore compared to the next generation. That, plus low compression courtesy of leaking through the journal / bushing clearance means that despite new piston rings you may still have low compression, courtesy of worn out bearings. There is not a practical way to restore the bushings / journals unless you hard chrome the journals. Usually when the block is worn beyond spec you simply find another block.
When they are running right they can idle down low to barely a tickover.
