Mercury 40, 50, 60, 70 HP Drive Shaft Compatibility 1977 and 1989 Models

[rustnrot]

I have two lower units. One a 1977 on the left and a 1989 on the right. The 1989 has a short shaft and the 1977 is long shaft. The 1977 is off a 70 HP 3 cyl but would also be the lower unit for 3 cylinders and 4 cylinders of many years. The 1989 is on the right and is off a 60 HP 3 cylinder but again used on many 3 and 4 cylinders of many years.

1. Assuming the drive shafts are the same length, will the later model lower unit bolt over to an earlier model engine and vice versa? It sure seems so.

2. Will the short drive shaft from the 1989 work in the 1977 lower unit? The part numbers are different -- but has anyone actually installed a later model drive shaft into an earlier lower unit? The various bearings, etc are the same part numbers that go with it, however the lower water pump housing is different due to a different bottom seal.

[mercmack]

mesure the distance between the from two studs by dirve shaft..if there the same they will bolt up to any of those models mentioned..blue band motors and newer..
if you need an extra long shaft dive shaft will will trade you (2) long shafts for the short shaft one..Mercmack..:):):)

[rustnrot]

Not familiar with the "blue band" moniker. What years does this represent?

I measured the studs -- they are the same distance apart.

Now, does anyone know about parts interchangeability -- to get a "better" ratio I want to remove the short shaft drive shaft from the 1989 and install it into the long shaft 1977 housing. Will that work??

thanks!

[Roy Hodges]

I am pretty sure that both units has the SAME gear ratio.

[rustnrot]

I carefully marked the shafts and checked. The 1989s (actually have two) both measure for the 1.83 ratio, the 1977 is a 1.64. I did see a mention of both these ratios in a Merc book. And there is a definite difference -- very easy to spot. The 1.64 makes 1 and two thirds rotation of the drive shaft to one rotation of the prop shaft. The 1.83 as expected turns much further on the drive shaft (closer to two turns) for one rotation of the prop shaft.

I want a 1.64 ratio short shaft.

[Popa Sam]

I carefully marked the shafts and checked. The 1989s (actually have two) both measure for the 1.83 ratio, the 1977 is a 1.64. I did see a mention of both these ratios in a Merc book. And there is a definite difference -- very easy to spot. The 1.64 makes 1 and two thirds rotation of the drive shaft to one rotation of the prop shaft. The 1.83 as expected turns much further on the drive shaft (closer to two turns) for one rotation of the prop shaft.

I want a 1.64 ratio short shaft.

You are correct on the gear ratio's. The 1.64 was on the early 3 cylinder 65 hp motors that came out in 1972. They were poor performing motors. By switching to a 2:1 unit from a 50 hp of the same time period the overall performance was improved. The later 1.83 was better but still performed poorly. The gears will interchange from one unit to the other. If my old and over used memory is correct the driveshafts will not interchange. I think the shaft is a different length on the internal part. You should be able to determine if this is true by measuring from top of gearhousing to propshaft centerline on the two cases. None of this is offered as being correct, just my opinion.