One more Question aboutMercury lower unit interchangeability.

sutor623

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May 23, 2011
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Thanks to Faztbullet, I have found out the gear ratios of the lower units that I have been plagued with, as well as the part #'s to the tools that I need. I am trying to find out two things, one that has been quite hard for me to figure out. I have a 1967 4 cylinder 650 with a bad lower unit. I also have another lower unit that we have had a little bit of a time identifying. The gear ratios of the units are as follows. The spare lower unit that I have has a 2:1 ratio. So that means that if any of these lower units HOUSINGS would fit my 650, and in the 2:1 gear ratio then it is the unit that I have.

900/90/1150/115/1400/140/1500.........................2:1
40/45/50/500 4 cylinder......................................2:1
650/75/80/800/850 4 cylinder..............................2:3:1

Here is a link to the pics that I have posted of my "good" and bad lower units.

http://forums.iboats.com/showthread.php?t=524723

Here are the main questions:

1. If I do in fact have a 90-150 series bottom end (which is highly likely) is the spanner nut / cover nut tool going to be the same as for the 650? I have been told that the part number for the 650 is 91-53126 but this part is now unavailable through Mercury.Do you guys have any idea where I can get a Sierra or other aftermarket tool for it? The nuts are the same size but one has 8 tabs and the other has 4. To me it looks like a 4 tab tool would work for both cover nuts. The link underneath is to the part that I believe to be the correct spanner nut tool. Do you guys agree?

2. Once I get this lower unit apart, will the 650's gears be interchangeable with the bearing carrier and prop shaft of the 90-150 unit?

http://www.ebay.com/itm/MERCURY-LOWER-UNIT-TOOL-MARK-10-15-28-GEAR-CASE-COVER-/120823258401?pt=Boat_Parts_Accessories_Gear&vxp=mtr&hash=item1c21a09d21


I know these are some deep questions but I am giving you all the information that I can to get this issue solved. Thanks!!
 

Chris1956

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Mar 25, 2004
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27,036
Re: One more Question aboutMercury lower unit interchangeability.

I cannot answer all your questions, however, I know the 850 gearcase will fit an inline six motor, and vice versa. The gear ratios are different as you pointed out. I did not think the 850 and 650 share gearcases, however, as you have indicated.

The other question is why are you planning to disasemble the gearcase? Do you want to swap gears? Isn't it easier to use a different prop? Fazt doesn't like that approach, but I had some luck with it.
 

Faztbullet

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Mar 2, 2008
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15,589
Re: One more Question aboutMercury lower unit interchangeability.

40/45/50/500 4 cylinder......................................2:1 units will not fit at all.
91-53126 but this part is now unavailable through Mercury

I found one of my suppliers that has them, $112. The gear interchange depends on driveshaft, if its a fixed bearing or preload style as they shim different and gears are different.
 

sutor623

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May 23, 2011
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Re: One more Question aboutMercury lower unit interchangeability.

40/45/50/500 4 cylinder......................................2:1 units will not fit at all.


I found one of my suppliers that has them, $112. The gear interchange depends on driveshaft, if its a fixed bearing or preload style as they shim different and gears are different.

Thanks Fazt. I think I will go a different route. Man $112, whoa!
 

ThumbPkr

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Aug 17, 2007
Messages
371
Re: One more Question aboutMercury lower unit interchangeability.

My local dealer will rent the tool for 25 bucks a day.The 50 hp lower unit is the same as the 3 cylinder 650 I think.As I told you in your other post my 650 ran for years with the 2:1 ratio but it now has the correct 2.3:1 in it.I will know how it runs next spring.
You really should be looking for the correct Mercury manual for your engine to do this right.Ron G
 

sutor623

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Joined
May 23, 2011
Messages
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Re: One more Question aboutMercury lower unit interchangeability.

My local dealer will rent the tool for 25 bucks a day.The 50 hp lower unit is the same as the 3 cylinder 650 I think.As I told you in your other post my 650 ran for years with the 2:1 ratio but it now has the correct 2.3:1 in it.I will know how it runs next spring.
You really should be looking for the correct Mercury manual for your engine to do this right.Ron G

Yeah thanks Thumb, I actually have a Seloc on the way. I was just trying to get the tool asap. All is well though, I am going to grind the ring groove out and try the unit before I worry about getting elusive tools. Thanks for your help guys Ill let you know how things go once I get my waterpump.
 
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