Merc 3.7 drive swap for Merc 120 ?

[mr 88]

Front of bullet is starting to weap oil on 1979 Mercruiser "3.7" and I have 2 good "120" drives taking up space in the garage.Will try some JB weld in slightly pitted area.If that does not work we go to my question. I know I can swap out the different gears,just do not want to pay for it.I was wondering if I could make up the small ratio difference in the gears with the prop.

 

[stonyloam]

Re: Merc 3.7 drive swap for Merc 120 ?

You might just be able to swap the gear housing (lower) and keep the upper with the correct ratio. If they are similar model, should work. Serial numbers?? However I repaired a truck oil pan with JB and it lasted for years LOL.

 

[mr 88]

Re: Merc 3.7 drive swap for Merc 120 ?

I guess no one else has had this issue and thanks for the idea of just switching out the lower gears. The ratio is so close,the 120 is 1.98 upper and lower. The 3.7 is 1.84 upper and lower as well. So if i swapped out the lower I would think the differnce would be minimal,but I am not a engineer so I guess it might be crap shoot.Run the 1.84 up top and 1.98 in the bullet vs stock 1.84. I just rebuilt her and bumped her up with a 4 barrel so I think with the extra hp the prop on it may just work out fine.Or I find someone to swap out the lower gears that is reasonable and be done with it.

 

[stonyloam]

Re: Merc 3.7 drive swap for Merc 120 ?

OK just to clear this up a bit, the gear ratio is determined by the gears in the driveshaft housing (upper) the gear housing (lower) gears are all the same. So with an Alpha I outdrive for example all of the gear cases are interchangable no matter what the driveshaft housing gear ratio is. Here is an example: parts # 25 deternine the gear ratio https://www.mercruiserparts.com/Sho...r=70&bdesc=DRIVESHAFT+HOUSING+AND+DRIVE+GEARS the gears in the gear housing transfer the power from the vertical driveshaft (from the driveshaft housing) to the horizontal prop shaft, either forward or reverse. To sum it up, gear ratio is determined only by the gears in the driveshaft housing, and shifting (F,N,R) is done by gears in the gear housing.

 

[mr 88]

Re: Merc 3.7 drive swap for Merc 120 ?

I think you have confused me. Are you saying that by keeping the original upper and mating it with the 120 lower that the gear ratio will be what is called for in the manual. When I look at the specs it makes me think that they use matching gears in the bullet and upper because of the ratio they list. Your suggesting that the lowers are pretty much all the same and the upper determines the ratio,correct?

 

[Eric 3.7LX Alpha 1]

Re: Merc 3.7 drive swap for Merc 120 ?

I think you have confused me. Are you saying that by keeping the original upper and mating it with the 120 lower that the gear ratio will be what is called for in the manual. When I look at the specs it makes me think that they use matching gears in the bullet and upper because of the ratio they list. Your suggesting that the lowers are pretty much all the same and the upper determines the ratio,correct?

Correct sir!!

The lowers will be the same

 

[mr 88]

Re: Merc 3.7 drive swap for Merc 120 ?

Thanks guys,out to the garage to drain unit,and swap out,saved me some money and more importantly a headache wondering if the patchwork would hold up over the season.Yep, a new impeller will be in place as well.

 

[stonyloam]

Re: Merc 3.7 drive swap for Merc 120 ?

+1 Eric, just make sure outdrives are the same generation. Good luck!

 

[achris]

Re: Merc 3.7 drive swap for Merc 120 ?

If they have '120' and '3.7' labels in the drives, they will be..

 

[mr 88]

Re: Merc 3.7 drive swap for Merc 120 ?

So I inspect water pump on weeping drive and there is a big piece of what appear to be foam with the density of a key float with out the outside shell only bigger as it almost filled the cavity from the bottom up into one blade of the impeller.It was starting to fall apart from age. The impeller/gasket/base all looked like it had no time on them as well. I bought it as is from a dealer with there work order from the original owner which had stated "severely overheated" etc. that was dated a couple years ago and there it sat,not repaired, until I came along. My big question is how in the world did something that big [smaller sink sponge] seemingly in one piece make it into there through the strainer holes. Has nothing to do with my org post.