Hi,
I have a 1977 Johnson 70 hp (70773S) motor that is running well but has about a 4" crack in the lower unit from the prop forward. The crack doesn't enter the oil reservoir, but I'm concerned about the crack size and the strength of the area. It was a barn find that I bought for cheap money and got running. The PO stated that it was on a pontoon boat, and by the looks of it, the leg was never removed from the water. The upper half of the motor is in good condition, but the lower unit has significant corrosion. I'd like to replace the lower unit. I've found a number of lowers for on Ebay, but I'm concerned about compatibility.
I have found a lower unit from a 1975 75hp for reasonable money, but I don't know if it will work with my motor. The gearcase numbers are the same, but the are a few small different part numbers. Shift rod and driveshaft are the same between both models. Also, in research I've found that around the mid-70's (mid '75 is what I read) they changed the shift rod direction, so that when the rod is pushed down, it goes into reverse instead of forward.
So I have a few questions:
1) Beyond shifting the rod, and turning the shaft, is there any way to discern visually if the unit is compatible or not?
2) Is it possible to swap a couple of components from my existing lower to change the shift direction on one of the pre-'75 lower units??
3) What year/hp combinations would be compatible with my motor??
Lastly, one other option is to buy a used gearcase and swap my components over. I've never done a swap myself, and while I'm certainly capable, I don't have any lower unit specific tools. How difficult to swap components into a new gearcase if I have to use an above average set of homeowners tools???
Thanks for the help!
I have a 1977 Johnson 70 hp (70773S) motor that is running well but has about a 4" crack in the lower unit from the prop forward. The crack doesn't enter the oil reservoir, but I'm concerned about the crack size and the strength of the area. It was a barn find that I bought for cheap money and got running. The PO stated that it was on a pontoon boat, and by the looks of it, the leg was never removed from the water. The upper half of the motor is in good condition, but the lower unit has significant corrosion. I'd like to replace the lower unit. I've found a number of lowers for on Ebay, but I'm concerned about compatibility.
I have found a lower unit from a 1975 75hp for reasonable money, but I don't know if it will work with my motor. The gearcase numbers are the same, but the are a few small different part numbers. Shift rod and driveshaft are the same between both models. Also, in research I've found that around the mid-70's (mid '75 is what I read) they changed the shift rod direction, so that when the rod is pushed down, it goes into reverse instead of forward.
So I have a few questions:
1) Beyond shifting the rod, and turning the shaft, is there any way to discern visually if the unit is compatible or not?
2) Is it possible to swap a couple of components from my existing lower to change the shift direction on one of the pre-'75 lower units??
3) What year/hp combinations would be compatible with my motor??
Lastly, one other option is to buy a used gearcase and swap my components over. I've never done a swap myself, and while I'm certainly capable, I don't have any lower unit specific tools. How difficult to swap components into a new gearcase if I have to use an above average set of homeowners tools???
Thanks for the help!
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