Merc 850 lower unit

[joeveebee]

Hello,
I am looking for a lower unit for my 1973 Merc 850 85 HP outboard. The part number on my lower unit is 1623-4021. I am trying to find out what other lower unit part numbers if any will fit this motor. I have found one with the part number 1623-5243.
Does anyone know if this will work for me? Thanks for any help.
jVB

 

[Chris1956]

Re: Merc 850 lower unit

JVB, Any lower unit from a 4 or 6 cylinder inline merc from the late 60s until 1980 or more will fit the motor. Your gear ratio was 1.85::1 on your 850. Most of the other LUs have a 2::1 gear ratio. You may need to use a prop with 2" less pitch than usual. I used to put the LU from my '77 1500 on my '73 850. I dropped the pitch from 19 to 17, after the LU from the 850 bit the dust.

 

[joeveebee]

Re: Merc 850 lower unit

Chris,
Thanks a lot for your reply. Right now I am watching a few lower units on Ebay.
One is from a 1972 50hp, part # 1643-6037 I assume that is a 4 cyl?
One is from a 90 HP inline 6 (unknown year)
and one is from a 80 hp 4 cyl, (unknown year) part number 1623-5243

So are you saying that any of these would bolt up okay?

The one that I have on my 73 850 is part 1623-4021 and makes a horrible grinding noise in forward and reverse. After looking at prices for the innards of one of these units, I've started looking for a whole new LU. Is it usually cheaper to find a used one than to rebuild the one I have?
Understanding of course that buying used is more of a gamble than rebuilding.
Thanks again.
j-

 

[emckelvy]

Re: Merc 850 lower unit

Your 850 has a 2.3:1 gear ratio. You'll have to change prop pitch if you get a lower unit with a different ratio (such as 2:1 which is found in all Inline Sixes '72 and up).

The 66 C.I. 4-cyl "800", "850", 80HP, and 75HP models up to their end run (sometime in the late 80's) all have the same 2.3:1 gear ratio. You can't use a 4-cyl 50 HP lower unit as that's a smaller gear housing and won't bolt up.

Nor could you use a newer "Big Four" gearcase (the 100, 115, 125HP variations of the late-80's motor which has a fiberglass, wraparound cowling).

Here's a link to a previous post with more info:

http://forums.iboats.com/showthread.php?t=196092

HTH........ed

 

[joeveebee]

Re: Merc 850 lower unit

Thanks for the great info, my head is spinning a bit trying to tell all these LU's apart. The one on ebay that I think I might get a reasonable price on is from a 90hp inline 6,, unknown year. Will that one bolt up or do I need to know the year.? I'm going to email the guy and ask him if he knows the year and see if he can give me the part number from his unit.
Thanks again.
j-

 

[emckelvy]

Re: Merc 850 lower unit

Is this the 90HP you're looking at?

http://cgi.ebay.com/ebaymotors/Merc...1QQihZ015QQcategoryZ50439QQrdZ1QQcmdZViewItem

It would definitely fit if the same shaft length as yours (assume longshaft?). It would have a 2:1 gear ratio, so if your present prop is "perfect" for the boat you'd have to go to a prop with at least 2" less pitch. It has the nice stainless driveshaft, which is very resistant to any corrosion around the driveshaft seal area. That preload pin he mentioned is important if the lower unit came with a tapered bearing. The pin & spring put pressure on the driveshaft bearing to keep it seated in the race. If you were to pull up on the driveshaft with the water pump top removed, a tapered-bearing driveshaft would have noticeable "slop". A straight-race bearing type wouldn't move much if any. Bottom line, if you were to get this one and it had the tapered driveshaft bearing, you'd have to get a replacement for the pin & spring. Not a big deal, just something to factor into the price.

Here's another one:

http://cgi.ebay.com/ebaymotors/MERC...5QQihZ004QQcategoryZ50439QQrdZ1QQcmdZViewItem

It ain't purty but the price is cheap so far. If you do get it, definitely have the guy weld up that chip as he offers. It's not in a pressure-containing area but if left untouched it could lead to a crack.

Just so you know, here's one of the newer-style Big 3 and Big 4 lower units that WON'T work on your motor. The difference should be obvious in the pictures:

http://cgi.ebay.com/ebaymotors/Merc...7QQihZ018QQcategoryZ50439QQrdZ1QQcmdZViewItem

Cheers............ed

 

[joeveebee]

Re: Merc 850 lower unit

Hey thanks for that info Ed,
It has been awhile and I'm still looking for a lower unit. I am looking at this one from a 650. Any chance it would work for my 850?

http://dwarfurl.com/ca9c3

Thanks again.
j-

 

[emckelvy]

Re: Merc 850 lower unit

Not a very clear pic but it looks like it would've worked. It sold cheap enough!

 

[mrlassi1]

Re: Merc 850 lower unit

You are getting good advice. One of my past project boats had a Merc 850 with a bad lower unit. I found an in-line 6 150hp with a blown power head and bought it for $500.00 It included a power trim and tilt, good lower unit, and a 19" SS prop that I had repitched to 17" due to the difference in gear ratio. All were removed and installed on my 850. I ran it for six years after the retrofit with no issues then sold the rig.

 

[stormmn]

Re: Merc 850 lower unit

I just took the lower unit off of my 1973 850 and put a "outboard jet G" jet pump on it, the lower unit has the prop and all parts with it and was serviced every two years befire storage as it did not get a lot of use.

I have had the boat sice 1975 and it has sat for the last 15 years with less than 50 hrs. on it, my brother banged the bottom scag and put a ding in it and the prop.

I will guarentee the unit, $400 plus shipping.

will send pictures if you are interested....


Norm

stormmn@rconnects.com

 

[emckelvy]

Re: Merc 850 lower unit

Do you guys realize you are replying to a thread that's over 3 years old???????