Lower unit compatibility

[Surfrunner]

I have a 2000 Tracker (Mercury) 90hp motor that I blew the lower unit on last fall. Since that time, I have been looking for a used lower unit that will be compatible, and I have been bombarded with differing information about what will fit. I have heard that only a 90hp unit will work, I have heard that anything from 75-115 is the same, and I have heard that 75-115 will fit, but that the gear ratios are different. I am wondering if anyone can conclusively tell me what hp rating and what years will fit my motor. Specifically, I have found a guy who wants to sell me a 1996 115 unit. Will this work? I appreciate your patience if this seems like a dumb question, but I am out of my element when it comes to outboards. Thanks for your help.

 

[RLT III]

Re: Lower unit compatibility

serial # of motor... typical if they are both V6 2 stroke then yes it will fit

 

[Surfrunner]

Re: Lower unit compatibility

Boat is in storage right now, and I can't find where I have the numbers written down. I'll keep looking though. I can tell you that my motor is not a V6, it is an inline 3. I was under the impression that the 115, at least 2000 and up, is an inline 4.

 

[RLT III]

Re: Lower unit compatibility

In that case you'll need a 3 cyl lower unit to match if your buddy's is a 3 your good if not keep looking

 

[Faztbullet]

Re: Lower unit compatibility

The only units that will fit your motor are the 3 cylinder models from a 75 and a 90 horsepower only in the 1998-2006 years. This is due to Mercury changed to a 6 jaw clutch in 98, the 97 and earlier use a 3 jaw clutch. You can put a newer 6 jaw on a earlier 3 jaw motor but you have to change out the shift rod for it to work. I looked up a new and refurbished one and there is not but $400 difference between them..

 

[Surfrunner]

Re: Lower unit compatibility

Thank you. This is exactly what I needed to know. Is the gear ratio the same in all of these units (75 and 90 1998-2006)? Also, curious as to what kind of prices you were finding on new/refurbished and where? I have found nothing reasonable local.

 

[Texasmark]

Re: Lower unit compatibility

serial # of motor... typical if they are both V6 2 stroke then yes it will fit

The 90 is a three banger with the lower unit design of and after '89.

Mark

 

[Texasmark]

Re: Lower unit compatibility

From 75 hp thru 125 hp in 2 cycle, you have the same lower unit FIT from probably the 1989 changeover thru at least 2003. Don't know about 4 cycles. Difference in the lower units, besides FIT is gear ratio.

The larger 4 cyl engines of the series, like 100 hp up, use a 2:1 ratio. The 75 and 90 use 2.3. Not a biggie as far as I am concerned on a light boat, but you need to know what you have as the prop pitch to do the same amount of work will be a lot higher (about 15%) on the 2.3. I have one.

Mark

 

[Faztbullet]

Re: Lower unit compatibility

Is the gear ratio the same in all of these units (75 and 90 1998-2006)? Also, curious as to what kind of prices you were finding on new/refurbished and where? I have found nothing reasonable local.

Gear ratio is the same on all the 3 cylinder motors, the difference in the 6 jaw lower units are the gears, prop shaft, clutch and shift rod. I looked up a reman unit with 1 year warranty and its $1812.50 and a new one is $2062.50( if in stock). Is yours not repairable? If so just repair it..a gear set, bearing kit and seals will put you back about $600.

ME-G3-07-N Mercury New Lower 75/90HP 1998-2006 6 Jaw $1,862.50 $200.00 core
ME-G3-07-R Mercury Reman Lower 75/90HP 1998-2006 6 Jaw $1,612.50 $200.00 core

 

[Texasmark]

Re: Lower unit compatibility

Note he said "the same on the 3 cyl", did not include the 4 as I said, it's different.

The manual shows how to distinguish between the 3 and 6 prong reverse shifter. The casting on the top rear has a mark that is perpendicular for one and at a 45 degree angle for the other.

If you are getting the whole lower unit you don't need to be picky as the mating surfaces are along the prop shaft, internal to the unit. However, Merc apparently doubled the slots to help in preventing wear out (in later designs) which allows the shifter to slip out of R gear when worn.

Mark

 

[Faztbullet]

Re: Lower unit compatibility

A 3 jaw unit will not work on a 6 jaw motor as shift rod (throw) is different, you can update a 3 jaw to a 6 jaw by installing newer rod.

 

[Surfrunner]

Re: Lower unit compatibility

Thanks guys, I really appreciate the help. Faztbullet, it's unfortunately not repairable, or so my own eyes and those of two different mechanics tell me. Unit got so hot it burned up/melted everything in it. It's a mess. I've actually been looking around for a good used unit, but have been confused by all the people trying to sell me different ones that they claim will fit. I appreciate you guys setting me straight. Now I know exactly what I'm looking for, and in fact I may have one lined up. Need to call the guy today.

 

[Texasmark]

Re: Lower unit compatibility

A 3 jaw unit will not work on a 6 jaw motor as shift rod (throw) is different, you can update a 3 jaw to a 6 jaw by installing newer rod.

I havent been in there physically, just on paper. I guess the 6 slot are exactly the same interface dimensions as the 3 just 60 degrees apart rather than 120. Seems if I were going to that trouble, I would change out both sides and get the full benefit of the 6 interfaces.

Make sense?

Mark

 

[Texasmark]

I have a 2007 mercury 115 lower unit that took a crap. Will a 2001 mercury 115 fit it?

2:1 gear ratio on 115. 90 has 2.3. Depending on the boat, if you backed off your pitch with another prop having the same characteristics, the same percentage (15%) that may give you the same results. Faztbullet should know the answer...... is there more to it than that.

Never quite figured the reason for the lower ratio on the 90 unless it's used in heavy load applications and either props aren't available in shallow enough pitch, or there is some magic I missed. It's all about gear ratio. A given prop of a given dimension on an given boat turning at a given rpm (the prop) gives you a given result.
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You're talking about a gear train and propeller torque/thrust requirement. The gears in the system are drive shaft, prop shaft, and propeller pitch/slip. A x B x C = the desired result. If I change the value of A and B but the mathematical "product" remains the same, what difference does it make if I swap their values by 2.3/2 = 1.15 or 15%. I never could figure that one out.

It did matter to me as on my last boat, 2002 90 hp, I was running a 13 1/4x24 Ballistic XL 3 blade at 5600-5800 rpms. There were no higher pitched props available from them and to change prop mfgrs. or types would have thrown in a bunch of variables. Had I had the 2:1 gearbox like is on the higher 4 cyl engines of the series, I could have had my engine loaded a little more and used my 24P getting a few more mph.....which I would have done and put me over the 50 mph mark rather than kissing the underside.