TOPIC: Tower of Power lower unit questions

I have a 66 950ss. I am looking to pick up a rebuild able lower unit. What I am wondering is what year span and models will interchange. It is a long shaft, so does that make a difference? I want to rebuild one this year to have ready because I dont think mine will last much longer than this year, the rest of my motor is in great shape!!!

65 and up. if it looks the same it will fit but check the gear ratio.there are different ones

dave bernard wrote:

65 and up. if it looks the same it will fit but check the gear ratio.there are different ones

Yes, and if you can find a lower unit off one of the 4-cyl 66 c.i. models (800, 850, 85hp, 80hp, 75hp), it'll have a 2.3:1 gear ratio instead of 2:1. Later-model 650-4's also came with 2.3:1 gears.

The older motors like this gear ratio and generally you'll get better performance swinging a higher-pitched prop at slower propshaft rpm.

'79 and newer lower units have an improved gearcase with swept-back fin, and around the mid-70's Merc went with stainless-clad or a composite-type of stainless shaft, which pretty much solves the problem of the shaft rotting away in the oil seal area.

You'll find that the newer driveshafts may lack an O-ring groove to seal the crankshaft/driveshaft splines, but no worry; you can just grind a groove in the correct position.

HTH.........ed

Thanks to both of you....Ed, my question for you is this.... I live in Utah and use my boat most often at approx 7800 ft. I have wanted to go to a 15 pitch prop, however I wonder if I do go to a lower unit with a 2.3:1 ratio, will this change help me to achieve better out of the hole, and or top end? I do still plan on picking up a 15 pitch prop because I do fish at 1 lake that is at almost 10,000 ft, however I am assuming that there is probably not much I can do for that altitude outside of re-jetting...Your thoughts?

Yes, going to the 2.3:1 gear ratio (while keeping the same size prop) would help hole shot. It might reduce top speed but that depends on how well it was propped in the first place.

Sometimes, if you're a bit over-propped, the change in gear ratio lets the motor spin up into its power band and you'll get a better turn of speed as well as a better hole shot.

You might still need a re-jet at 10,000 feet, but it just depends on the motor. A lower-pitched prop does help in that respect.

HTH.......ed

I suppose now its just a matter of finding one. Don't suppose you have one in great shape just sitting around do ya? Seriously though, are the enough interchangeable parts between the 85 hp and the 950 ss to make it worthwhile to pick up an 85 hp of the same year range for parts and use the lower unit to get the gear ratio?