Swapping out a upper/lower unit - OMC NUFR-12A - questions

Hi Guys,

I have an opportunity to pick up an OMC stringer upper and lower unit for a great price. My existing lower unit has been taken out and is in pieces at the moment. (broken spring and a couple of gears) From what I could tell from the parts diagrams found for this Engine, the upper units are identical for models rated from 90 to 235 HP, but the lower unit is a little different for the 90 HP. The one I am looking at has the lower unit from the 120 and up models.
My question, and hopefully I have expressed my self accurately, is will there be an issue using the complete package for 120 HP and up? Like I said, the upper unit appears to be the same, so I would assume that those connections to the boat are the same for all models.

I appreciate any feedback from the experts, and thank you in advance!.

The newbie boater
Stan

Re: Swapping out a upper/lower unit - OMC NUFR-12A - questions

it does appear that all 1970 drives used the same upper housing (982340), but will have different internal gears to make a gear ratio to match the different HP engines. But that upper housing commonality does look like you should be able to connect yours to basically any 1970 lower. You may need the exhaust housing with the larger case lower as it may be different at the bottom.
I'm trying to find something that tells me lower gear ratios to see if the larger case lower units had the same or similar ratio to the little 90HP bullet lower.
If the gearing ends up being the same or very similar (close enough that a propeller pitch change can bring you back) a swap would be possible

I might have something on paper at work with those ratios.

What engine is this 90HP? that Ford Cortina deal?
Do you know if this drive is high profile or low profile?

because if it's low, I'd look at a different option entirely.

Re: Swapping out a upper/lower unit - OMC NUFR-12A - questions

Thanks for the quick reply. I appreciate any info you can send my way. Yes the motor is one of the Ford Cortina's.
From the pictures you attached my engine looks like the low profile pic. Since the lower unit is in pieces, and that's how I got it, I have never seen it on the boat. Here are 2 pictures, 1st one of what's left of my boats rear end, the other is the upper/lower unit I can purchase.


Let me know if you need any other pictures, I can get them. You had mentioned if it was a lower profile to not bother, was there a specific reason?

Thanks Again!
Stan

Re: Swapping out a upper/lower unit - OMC NUFR-12A - questions

Seeing that your drive is low profile, I would recommend you very quickly rip that engine and drive out of there and replace it all with a 1982-1985 400 or 800 series sterndrive setup. i.e. look for a donor boat.
I bought a boat with rotten stringers - it had a good running 1984 2.5L chevy/with the 400 mechanical drive - whole boat was $140. That Ford thing has so many obsolete parts it's not funny.
Moving from electric shift to the 400 series mechanical would be a major step up in technology.

Re: Swapping out a upper/lower unit - OMC NUFR-12A - questions

the NUFR-12A used a 2.04:1 overall ratio sterndrive with a 12:28 upper gear set and a 20:23 lower. those upper and lower ratios for the bullet smallcase drive are miles away from any other sterndrives OMC ever made. So reusing your 12:28 upper with larger case lowers is not going to work.

If you want to replace the whole sterndrive with one from the similar era, you want to look for a 100HP, 110HP or 120HP unit right up to 1977. They used a 1.92:1 overall ratio. Then you will need a lower pitch prop to get prop speeds back to where they were with the 2.04 ratio unit. I would go down 2 inches in pitch.