Will this lower unit work on a 1980 Merc 70?

[YN2(SS)]

In an attempt to remotor my friends rebuilt boat, we bought a 1980 Merc 70 with Power Trim. I tried to get some parts for the motor and found that the lower unit isnt exactly the right one for this motor. The lower unit that is on it has a flush plug and no place to screw in the fitting for the speedo pitot tube (the one for the 70 doesnt have a flush plug and does have a fitting for the speedo pitot tube). I called my marine salvage yard guy and he believes this lower unit is for a 76 to 78 Merc 850. My first thought was that this was a good thing, cause it should be beefier. Then I got to digging in the manuals.

A 1978 merc 850 has a WOT of 4800-5500 rpm and the lower unit reduction is 2.3:1
A 1980 merc 70 has a WOT of 5000-5500 rpm and the lower unit reduction is 1.64:1

So, I guess the prop would turn at a lower RPM (with the 2.3:1 ratio) given the same engine RPM whether its a 70 or 85 hp merc. But the 85 would have more power to turn that prop. Will this put to much strain on the 70 powerhead? It would have more torque, right?

My biggest question is, can I use this lower unit and jus choose a prop that will get the WOT rpm in the middle of the range (5000-5500)? Should I consider changing the diameter of the prop too (to smaller) and then dial in the pitch? Or do I need to get a lower unit that is the right one for the motor. Kinda dont want to spend the money. But will if we need to.

 

[Faztbullet]

Re: Will this lower unit work on a 1980 Merc 70?

The lower unit that is on it has a flush plug and no place to screw in the fitting for the speedo pitot tube

That because its a pre 1980 lower unit.

I called my marine salvage yard guy and he believes this lower unit is for a 76 to 78 Merc 850.

Wont even fit...housing, stud spacing, driveshaft all different. :facepalm:

My biggest question is, can I use this lower unit

The one thats on engine now...Yes if it is the correct gear ratio. As you found out the "small bore" 3 cylinders(50/60/65/70 hp) use a 1.64 ratio and it could have a 2:1 ratio used on the 2 cylinder 35-40hp or the 40-50hp 4 cylinders. Mark the propshaft and place in gear, rotate and count how many turns the flywheel make to make 1 prop revolution and that will be your ratio.

 

[YN2(SS)]

Re: Will this lower unit work on a 1980 Merc 70?

If the ratio is different, should I not use it?

 

[emckelvy]

Re: Will this lower unit work on a 1980 Merc 70?

The 4-cyl 85 hp lower unit will not bolt up to a 70 hp 3-cyl motor. The 850's gearcase is not only wider & longer, but the driveshaft is larger diameter with different spline count as well.

You might find that an earlier 650-3 L/U will fit but there were small differences in those too.

Later model 60 hp Threes (non-Bigfoot models) were merely a prop-rated 70 so should have the same lower unit as yours.

Some Merc 40 & 50 HP models may work but be careful to match up all features such as shift shaft style (splined or square), driveshaft spline length, and the spacing between the (2) stud holes in the upper fwd section of the gearcase, as this spacing varies also.

You may have luck finding a parts motor on your local Craigslist classifieds. Or check eBay Motors.

Here's one on eBay that looks to have the correct spline seal setup on the driveshaft:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=140500679917

Here's a few more likely suspects:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=140499465873

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=220668990039

Note that if you were to find a 40 or 50hp lower unit, the gear ratio on those is usually 2:1 which will allow the motor to spin up higher for any given prop size. Which is a good thing for this motor, they don't last long if overpropped. So when you do get it running, check wide open throttle rpm and prop accordingly, shoot for somewhere around 5500 rpm with light load and no pssgrs in the boat.

HTH & G'luck.......ed

p.s. don't forget to match up the correct driveshaft length, likely you'll have a Longshaft motor which measures 20" from where the motor sits on the transom, to the anti-ventilation plate around the circumference of the lower unit.

 

[YN2(SS)]

Re: Will this lower unit work on a 1980 Merc 70?

First off, Thanks Faztbullet and emckelvy for your help.

The unit bolts up perfect, but it does have the flush hole and not place to attach the speedo tube. I marked the driveshaft and propshaft and the driveshaft turns 1 2/3 turns for 1 turn of the propshaft. Thats what the ratio is for the correct lower unit, 1.64:1. Lookin at a manual, I'm seein that pre-80 500's, 650's or 700's had the same specs. In any event, it bolts up, the ratio is right and the water pump is already rebuilt. Its gonna get paint soon. Should be on the lake by tomorrow (Thats what Ive been sayin for a year, its almost done though!)

 

[Chris1956]

Re: Will this lower unit work on a 1980 Merc 70?

Install the Pitot (speedo tube) on the hull of the boat. Use a standard flusher instead of the flush plug.

 

[YN2(SS)]

Re: Will this lower unit work on a 1980 Merc 70?

Ive got a set of muffs for when I run it. Is that what you are referring to, or what is a standard flusher. I liked the idea that we could bring it home and jus run some fresh water through it and was out any sediment. Fresh water only.