Mercer Pre-Alpha Lower Unit Replacement?

[auscarter12]

I have an early 70's model Aristocraft 19 with a Mercruiser 165 and a Pre-Alpha outdrive. This past season, the reverse gear went out. We tried cable adjustment, cable replacement, etc. to no avail. Finally, I decided to have it looked at by a mechanic and it was determined that the clutch in the lower unit is worn down to the point that it won't engage reverse gear. I have been quoted some pretty ridiculous prices to have this repair done. To me, it seems simpler (and cheaper) to buy a used lower unit with a good clutch in it, install it, and just chunk the old lower unit. Has anyone done this before? And if so, do you have any pointers? Also, does it need to be a Pre-Alpha lower unit, or can it be an Alpha One/Gen 1 lower unit?

 

[achris]

You do not have a 'pre-Alpha' unit, because Merc never anything called 'Pre-alpha'. What was being made in the 70's was the Mercruiser 1-drive (MC-1 for short)...

Any lower unit from the early 70's to 1990 will fit directly onto a MC-1 upper unit with no issues (very common to see different upper/lower pairs). They are (in chronological order), MC-1, 1-R, MR and Alpha One....

All complete upper and lower assemblies between 1970 and 1990 are completely swappable, plug and play. The only issue to watch for is the ratio. Mostly determined by the tooth count in the upper box, but there are some higher reduction lower units around too. See DonS wonderful post on ratios, it's all explained. (In the 'stickies' section)

Chris......

 

[Bt Doctur]

achris, maybe depends on the lower driveshaft spline count

 

[achris]

achris, maybe depends on the lower driveshaft spline count

From 1970-on the count is the same.

 

[auscarter12]

achris,

Does the fact that the driveshaft on my current lower unit has a preload pin, and some driveshafts for units I'm seeing for sale do not have a preload pin? I'm not sure what the pin is for.

 

[Scott Danforth]

iBoats sells em

http://www.boatmotors.com/motorparts/index.cgi?flag=1&engine_type=sterndrive&part=202&manufacturer=2

 

[auscarter12]

Mercruiser "R" drive shaft versus Alpha drive shaft

I currently have a Mercruiser "R" outdrive and I am looking to replace the lower unit. The current lower unit has a drive shaft with the pre-load pin attached at the top. I have read that you can put an Alpha lower onto an "R" upper unit no problem, but the Alpha's lower unit has a drive shaft without the pre-load pin. Will this pose any issue?

 

[Bt Doctur]

No issues with the lower units. the preload pin was omitted when the gears were recut to a different design.

 

[achris]

I currently have a Mercruiser "R" outdrive and I am looking to replace the lower unit. The current lower unit has a drive shaft with the pre-load pin attached at the top. I have read that you can put an Alpha lower onto an "R" upper unit no problem, but the Alpha's lower unit has a drive shaft without the pre-load pin. Will this pose any issue?

The upper drive shaft (the one in the top box) has been the same from 1970 all the way to today. The drive shaft in the MC-1 and R lower units has a preload pin to push the drive shaft down to keep the lower gears at the correct mesh depth. The MR and Alpha One lower units allow the lower gear to 'seperate' under load to achieve the correct mesh depth, hence no pre-load pin.

As I said before, all MC-1 (from 1970), R, MR and Alpha One uppers and lowers are completely interchange compatible.

Chris.........

 

[auscarter12]

achris,

Awesome, thank you! Also, the gear ratio for my current drive is 1.62/1.65. Is it possible to get a lower unit with a different gear ratio? And if so, does the prop need adjustment? I looked for DonS post on gear ratios but was unable to find it.

 

[achris]

I looked for DonS post on gear ratios but was unable to find it.

 

[achris]

Awesome, thank you! Also, the gear ratio for my current drive is 1.62/1.65. Is it possible to get a lower unit with a different gear ratio? And if so, does the prop need adjustment?

Yes, 2 different lower ratios are available. The 'standard' is a 13:21 or 17:28 which gives us the 1.62 or 1.65 overall with a 24:24 upper. There is also a 14:28 gearset which was usually coupled to a '1.94' upper giving an overall ratio of 2.40:1, usually put behind heavier boats with a 3 litre engine, but if you were to put that with the 24:24 upper, you'd have an overall ratio of 2.00:1....

Props can only be 'adjusted' by about an inch either way by changing the trailing edge cupping... If you do change the ratio, then the formula for finding the replacement prop is (New ratio/old ratio times old prop pitch).....

Chris........

 

[auscarter12]

Thanks achris!

You've been more than helpful. Much appreciated!