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As per our Boat Doctor..... in 1974 (and during the 165's useage) Merc wasn't even thinking about the B drive.

The 165 was fitted with the A drive predecessor.... the old R, MR or MC1..... I believe.


BTW.... the 165 was GM's 250 cu in inline 6 cylinder, and was a Dam good engine!
Keep that engine from over-heating...... give them routine oil changes..... and tuned up.... and they would run for ever!


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The MR was the predecessor/transition drive to the alpha 1, it is virtually identical to the alpha 1 so it's not that.

To go back that far it is most likely a I drive. the predecessor to the R drive. after the R came the MR and very shortly after the MR came the full Alpha.


Almost all drives (not all but most) prior to the MR/Alpha, one of the main differences was the way the vertical drive shaft bearing was installed (below the water pump housing). The old style (I,R) the drive shaft bearing had NO retaining nut and the vertical drive shaft had a preload pin in the top of it to put downward force on the drive shaft/gear.
The MR/Alpha changed and used a retaining nut to hold down the bearing (also the bearing {a tapered bearing} the bearing and race was reversed when this occurred) and the preload pin was removed. So the MR/Alpha the pinion gear was allowed to "float" and would due to design move upward towards the water pump direction when under load. Thus the need to preload the bearing with the retaining nut.

This is demonstrated when doing gear lash checking because you must use a spring and collar to pull up on the drive shaft when checking gear lash.

Also one tell tail sign is the large needle bearings just above the pinion gear (the ones that fall out when disassembling the drive), The MR/Alpha use 18 needle bearings, I believe the R and older use 13 (larger) needle bearings.
I could be wrong about the R but I believe this is true.

There were a few different drive back then so it is a bit difficult to nail it down with out a serial number
 
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