Mercruiser upper unit interchangeability

"I recently bought a 1979 Sea

"I recently bought a 1979 Sea Ray with a 470 Mercruiser. My first I/O. Very quickly the upper ring & pinion destroyed itself.
I have determined that the lower unit is OK, and i will try to find a used/reconditioned upper unit as I don't have the tools needed to set up the gears.
My question is about interchangeability. My tooth count was 24/24, which makes it a 1.65 ratio upper. (This unit was supposedly not original to the boat) It is the type with a lifting eye cast into the top cover, which I am reading is a pre-alpa drive. I see that there are 2 pre alphas, an R and an MR. How can I determine which i have?
Also, what will work for a replacement upper? I have read that anything pre 1983 will bolt up to my gimbal housing and the lower unit, but I need to be sure. If this is so, how can I visually determine that a unit is pre '83?
I am new to I/Os and I would like to get this repaired so I can get a little more water time this year, but as I just bought this boat, money is an issue.
Any and all help/suggestions very appreciated.
Thanks,
Greg
[email protected]"

You will have to have the lowe

You will have to have the lower unit disassembled and cleaned. The unit you have is an MR for an inline 6. You should replace it with a 1:84 unit for a v-6 / 470. The later model flat top units (not Gen II)will bolt up to your gimble and not leak but the drive shaft length must be the same. Yours most likely has a 7 inch drive shaft. By the time you have the lower cleaned out it may be cheaper to find an entire good used unit. I have had very good luck with SEI new units although I am not fond of their corrosion protection. If you use the 1:84 drive the prop will most likely have to be changed to different pitch.

"I have found a good upper uni

"I have found a good upper unit that should work.
The tooth count in the upper is 20/24, which I find is for inline 4 (120 or 140?), 1.81 or 1.98 ratio. My motor is a 470, 170 HP and my ratio is supposed to be 1.81 or 1.84.
My question is how can this tooth count be for either a 1.81 or 1.98 ratio? From everything I read the lower unit is the same for both of these drives, so how can there be such a difference in the ratios? What am I missing?
Can I use this unit?
One last question: my lower unit has a preload pin on the top of the driveshaft. Will all of the drives up to Gen II accept this type of shaft?
Thanks, GB"