1977 Mercury 850 lower unit compatibility

O

Outboard_Tinkering

Recruit
Joined
Jun 28, 2011
Messages
1
I need to know if the lower unit from a 1973 Merc 85hp will work on my 77 merc 850. I have serial numbers for both if needed. Thanks
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,877
Re: 1977 Mercury 850 lower unit compatibility

It will fit. You might check the gear ratios. The '73 gearcase is 2.3::1. The '77 gearcase may be 2.3::1 also or it may be 2::1. You made need a prop change if they are different, but only trial and error will determine if that is necessary.
 
Faztbullet

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,938
Re: 1977 Mercury 850 lower unit compatibility

It will work as all the 4 cylinder engines of 75/80/85 hp use a 2:3:1 ratio....
 
micky d

micky d

Cadet
Joined
Jan 8, 2011
Messages
10
Re: 1977 Mercury 850 lower unit compatibility

hi guys, sorry to open this a year later, but i was wondering if a 90 hp blueband gearbox will fit an 850 thunderbolt head?

thanks in advance for any info..

cheers Mick
 
Faztbullet

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,938
Re: 1977 Mercury 850 lower unit compatibility

It will bolt up but the gear ratio is different....it will be slow out of hole as engine cant spool up even if you drop pitch. Been there done that...
 
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