Lower unit from 75 8500 inline 4 cyl will it fit an 86 115 inline 6 cyl?

Broke something inside my lower unit on my 86 mariner inline 6 115. Got a trip to canada coming up in 12 days and funds aren't really plentiful so I'm a scroungin! Picked up a 1976 merc 850 4 cylinder real cheap from a local guy whom I trust. He said it ran well and had good compression when he last used it 2 years ago. When I dropped the lower unit on the 850 to do an impeller change, it looks identical where it matters so I started checking and it appears that most of the parts in the lower unit assembly carry the same part number and externally, evertyhthing I could measure seemed to be a good match. I realize the gear ratio may be different but the lower unit that just broke was from a 90 according to the guy who sold me the boat. Instead of hanging the 85 on my boat, I'm seriously thinking of changing an impeller, giving it fresh gear oil and just swapping the lower unit.

Do I have any surprises i'm not seeing?

PS: The driveshaft on the 115 had no spring pin on top. The 850 has it. Can I just pull it or should I leave it there? Will the LU with the spring pin go in properly into a motor that had no spring pin?

Re: Lower unit from 75 8500 inline 4 cyl will it fit an 86 115 inline 6 cyl?

Don't pull the preload pin! The pin is for the type of bearing on the driveshaft, some bearings need the preload and others don't. The crankshaft don't care whether the driveshaft has a pin or not.

The 85hp L/U will have a 2.3:1 gear ratio instead of the 2:1 on your inline's busted L/U (even a 90hp-Six would have the 2:1 ratio).

So, it'll be like having a prop with approx 2" less pitch. You'll have better hole shot but less top end (unless it was badly overpropped in the first place).

Other than that, it should interchange just fine.

HTH & have a nice trip...........ed

Re: Lower unit from 75 8500 inline 4 cyl will it fit an 86 115 inline 6 cyl?

emckelvy said:

Don't pull the preload pin! The pin is for the type of bearing on the driveshaft, some bearings need the preload and others don't. The crankshaft don't care whether the driveshaft has a pin or not.

The 85hp L/U will have a 2.3:1 gear ratio instead of the 2:1 on your inline's busted L/U (even a 90hp-Six would have the 2:1 ratio).

So, it'll be like having a prop with approx 2" less pitch. You'll have better hole shot but less top end (unless it was badly overpropped in the first place).

Other than that, it should interchange just fine.

HTH & have a nice trip...........ed

Click to expand...

Many thanks!!!

The preload pin was my biggest unanswered question. I'm fairly certain that the LU I presently have has the lower gear ratio already. Although the previous owner told me it was from a 90, the boat performs like it's got the lower gear ratio already. It's pushing an older 17' Glasstream Bass Hunter and I have decent holeshot with a 24 pitch prop now.

Thanks again. You just saved me untold time searching for that info!! :hail: