switching out lower unit on Mercruiser MC-1 - what to look for?

[jimmwaller]

Hello,
I have an old but decent MC-1 on a GM inline 6 / 165hp. Swapping out the lower unit and a friend has an MC-1 lower unit in good shape. Problem is, it say on there it's for 120/140 mercruisers.

What's the difference between the lower unit on a 120/140 and a 165? Could I use this lower unit, or could I use it by swapping out a few parts (u joint, etc.)? Or is is completely and totally unusable to me?

Just not sure what to look for when swapping out lower units on these old sterndrives. Anything I can measure or double-check to see which other are compatible? Like I thought maybe I've read that counting the splines on the u joint was the tell, but maybe I'm misremembering.

Thanks

 

[1960 Starflite]

Gear ratio is different I think.

 

[nola mike]

Close enough that you'd be able to reprop if necessary. What's wrong with what you have?

 

[jimmwaller]

Close enough that you'd be able to reprop if necessary. What's wrong with what you have?

Not 100% sure, something is up with the shifting. Figured I'd buy another so I can still boat around while I'm digging into this one... I'll likely rebuild the whole outdrive. Important note: I have no idea why I typed "lower unit," I think I was looking at lower units while I was typing and had them on the brain. I'm talking about the whole outdrive, will edit my original post now. Given this, can I use a 120-140 outdrive and still just reprop? Thanks

EDIT: Hm, apparently I can't edit my original post... So to clarify, I'm asking about the whole outdrive NOT just the lower unit. I'm dumb, apologies for any confusion!

 

[Bt Doctur]

the old is 1 .65 and the other is 1.98 so you will need a higher pitch and/or diameter

 

[jimmwaller]

the old is 1 .65 and the other is 1.98 so you will need a higher pitch and/or diameter

thanks. So just a few absolute basic questions here:

1) just confirming, this is true if I'm switching out the whole outdrive, right? Not just the lower unit as my post erroneously stated?
2) what does the 1.65 and 1.98 mean? Not sure what those numbers refer to. Old is 1.65... what? Is that a measurement or...?
3) Will definitely re-prop, but just so I understand, what causes the need to re-prop? I was not aware that the prop makes that big of a difference... I figured that using the wrong prop is just less efficient or whatever, but will things actually break with the wrong prop?
4) just confirming, you also think that I can use a 120-140 outdrive on a 165hp outdrive so long as I re-prop?
Thanks!!

 

[achris]

Are you switching just the gear housing (aka 'lower unit'), or the whole drive?

If it's just the gear housing, then there is no difference in ratio. If it's the whole drive (gear housing plus drive shaft housing), then an increase in prop pitch of around 20% will be needed (If you were running a 15", then you'll need a 18", and since finding an 18" would be very difficult, you need to know if you were at the top end of the range or the bottom end. If at the top, put a 17" on, at the bottom, make it a 19". The recommended rpm range is 3900-4300rpm)...

Chris.........

 

[nola mike]

thanks. So just a few absolute basic questions here:

1) just confirming, this is true if I'm switching out the whole outdrive, right? Not just the lower unit as my post erroneously stated?

Yes

2) what does the 1.65 and 1.98 mean? Not sure what those numbers refer to. Old is 1.65... what? Is that a measurement or...?

Ratio of turns of prop per engine revolution. In a 1.98 the prop turns almost 2x for every turn of the engine. Essentially like a lower gear in a car

3) Will definitely re-prop, but just so I understand, what causes the need to re-prop? I was not aware that the prop makes that big of a difference... I figured that using the wrong prop is just less efficient or whatever, but will things actually break with the wrong prop?

Your engine revs will be off, your boat will perform lousy, and you can damage it over time. Yes, you need to reprop

4) just confirming, you also think that I can use a 120-140 outdrive on a 165hp outdrive so long as I re-prop?
Thanks!!

Like achris said, it depends on what you have on there now. If you have a 23p prop, you'll have a harder time getting something bigger, but it would be easy if you have a 17p.
Also, shifting difficulties in these drives are rarely due to a drive problem, and for practical purposes they aren't rebuildable by a DIYer, as you need a bunch of expensive special tools.

 

[jimmwaller]

Also, shifting difficulties in these drives are rarely due to a drive problem, and for practical purposes they aren't rebuildable by a DIYer, as you need a bunch of expensive special tools.

Thanks for all the info! Just a quick beginner question. When you say that shifting issues are almost never due to a drive problem, what are they due to? I kind of thought that the shifting on these was pretty simple... you have a cable that goes from your shifter, to a linkage at the engine, and then passes right through to the outdrive. I thought that it was basically a cable up until the outdrive, which makes me think that there's not much that can go wrong up until the drive.
Clearly, I'm wrong! But I'm just curious about where the problem is usually found, if not the drive. This knowledge might help me pinpoint where the issue is.
Thanks again

EDIT: Also running over today to grab my prop and get the specs. Is there anything else I should check, or will the prop specs be enough to let me know which new prop I should be looking at? Thanks!

 

[nola mike]

Thanks for all the info! Just a quick beginner question. When you say that shifting issues are almost never due to a drive problem, what are they due to? I kind of thought that the shifting on these was pretty simple... you have a cable that goes from your shifter, to a linkage at the engine, and then passes right through to the outdrive. I thought that it was basically a cable up until the outdrive, which makes me think that there's not much that can go wrong up until the drive.
Clearly, I'm wrong! But I'm just curious about where the problem is usually found, if not the drive. This knowledge might help me pinpoint where the issue is.
Thanks again

Usually the issue is in the cable or adjustment, or a couple of other culprits not involving the gear case. What issues are you having?

EDIT: Also running over today to grab my prop and get the specs. Is there anything else I should check, or will the prop specs be enough to let me know which new prop I should be looking at? Thanks!

The current specs will give you a ballpark, but won't be exact. Especially if you aren't sure that you're currently propped correctly (you need to know your current RPMs at WOT).

 

[jimmwaller]

Usually the issue is in the cable or adjustment, or a couple of other culprits not involving the gear case. What issues are you having?


The current specs will give you a ballpark, but won't be exact. Especially if you aren't sure that you're currently propped correctly (you need to know your current RPMs at WOT).

ok, I see, thanks. Any downside to testing with the prop I have now (just like 30 seconds at WOT) to see where I'm at? Or is even 30 seconds at WOT damaging? It would be ~5 min at slow speed to get out of the harbor, then ~1 min at medium speed to get out a bit, and then 30 seconds at WOT, and then back. Would this cause any damage?

 

[Eric 3.7LX Alpha 1]

There's no reason to re prop. Like the others mentioned, usually shifting issues are a worn out lower shift cable or bad adjustment causing the engine to cut off and stall.

The gears in the upper half of the drive are different depending on the engine in front of it. The gears in the lower half of the drive are are for forward and reverse, in other words... shifting. If you had problems with the drive shifting, and.... it was the outdrive causing it. You could just swap the lower half of each drive and continue to use the upper half you have now and same prop you have now.

But, before you get too deep into swapping anything over.. what exactly is your shifting problem you are having?