Interesting lower unit fact

[SkiDad]

Been thinking of picking up a 85 hp lower unit and I was reading my Clymers manual this weekend and I ran across this info. Maybe you already knew it, but i though it was cool.

The 85 hp and 125 hp force engines lower units (1 piece shaft) are stamped on the lower unit near fill hole with a 3 or a 4 - see picture attached

3 = 2.00:1 ratio
4 = 1.73:1 ratio

 

[Jiggz]

Re: Interesting lower unit fact

I checked on my '89-125 HP and the stamp is 4A, I understand the 4 is 1.73:1 gear ratio. I wonder what the "A" stands for. I've attached a gear ratio table for all 84-99 Force engines showing the 125's having two versions for gear ratio, i.e. 1.73 and 1.93. I wonder is that "A" refers to the 1.93. I guess I would just have to verify with the flywheel and prop. Thanks for the info though.

 

[Frank Acampora]

Re: Interesting lower unit fact

If you have a later model 125 with the Mercury water pump, the gear ratio is 1.93. This usually is dual exhaust, has a two piece drive shaft and uses Mercury gears.

 

[MickLovin]

Re: Interesting lower unit fact

A stands for Aussie :facepalm:, but seriously then what would my 100HP chrysler be if you don't mind me smuggling in on the convo.
Might check to see if it is stamped, think it would be stamped PBG, Pretty Bloody Good Cause it's a Chrysler

 

[SkiDad]

Re: Interesting lower unit fact

I checked on my '89-125 HP and the stamp is 4A, I understand the 4 is 1.73:1 gear ratio. I wonder what the "A" stands for. I've attached a gear ratio table for all 84-99 Force engines showing the 125's having two versions for gear ratio, i.e. 1.73 and 1.93. I wonder is that "A" refers to the 1.93. I guess I would just have to verify with the flywheel and prop. Thanks for the info though.
View attachment 222555

interesting - you must have one of the last 125s made I see in my manual it says type A are a two piece shaft but are single exhaust (water ports on bullet). It also says that type A is 1.93 - it says it was used on 90 and 120 and 85 l-drives and 125 l-drives - but no mention of regular 125s. Also says Type B is dual exhaust and 1.93. Not sure how accurate all that is in this book.

that might be a nice ratio to have too. That might get me right where I need to be and still keep my 15 pitch props (would be quite a hole shot)

 

[Jiggz]

Re: Interesting lower unit fact

If you have a later model 125 with the Mercury water pump, the gear ratio is 1.93. This usually is dual exhaust, has a two piece drive shaft and uses Mercury gears.

My LU is single shaft with dual exhaust which would mean it is a 1.73. I will verify it sometime today after installing my local T&T trim switch on the lower cowling. I already have a working T&T switch on the throttle handle but wants a local one so I don't have to go back & forth onboard the boat when flushing or launching and recovery.

 

[pnwboat]

Re: Interesting lower unit fact

The SELOC manual indicates 1.93:1 for 1989 125HP motor. The CLYMER manual shows 1.73:1 for the 1989 125HP motor. Only way to know for sure is to check it yourself.

 

[MickLovin]

Re: Interesting lower unit fact

Woohoo I have a 3 stamped on mine so that's 2:1 ay is that what I should be running?

 

[pnwboat]

Re: Interesting lower unit fact

MickLovin, it's easy enough to check to be 100% sure. Make sure the ignition key is in the OFF position. Put the motor in gear and rotate the flywheel clockwise (looking down at the flywheel) by hand....or wrench. If it takes two rotations of the flywheel to turn the propeller once, then it's a 2:1 ratio lower.

If it takes about 1 3/4 turns of the flywheel to turn the propeller once, then it's a 1.73:1 lower.

My 1988 125HP motor is much happier with the 2:1 lower, and so am I.

 

[oldboat1]

Re: Interesting lower unit fact

interesting info. Is there a practical application? That is, are there different lower units (w/different ratios), for the same engine -- so one could look for a replacement unit with a different ratio? thinking the ratio info. is important for prop selection? Curious.

 

[pnwboat]

Re: Interesting lower unit fact

Changing the gear ratio can change the performance characteristics of the motor. For my set up, there was a significant improvement in performance. The increase in performance was much more pronounced than simply switching props. And yes, you have to match the propeller pitch to the gear ratio so that you achieve your performance objective and don't exceed maximum RPMs. There are many variables that come into play as every boat is not exactly the same, however, generally speaking the 4 cylinder Force motors that originally came with the 1.73:1 ratio lowers will see an increase in performance when switching the the lower units that are in the 1.93:1 and 2:1 ratio. All the 4 cylinder lower units starting in around 1990 and later came from the factory with 1.93 or higher gear ratios.

 

[oldboat1]

Re: Interesting lower unit fact

thanks, pnw. appreciate it.

 

[SkiDad]

Re: Interesting lower unit fact

I checked on my '89-125 HP and the stamp is 4A, I understand the 4 is 1.73:1 gear ratio. I wonder what the "A" stands for. I've attached a gear ratio table for all 84-99 Force engines showing the 125's having two versions for gear ratio, i.e. 1.73 and 1.93. I wonder is that "A" refers to the 1.93. I guess I would just have to verify with the flywheel and prop. Thanks for the info though.
]

I'm the original poster. My dad's water pump went out a few weeks ago (1988 Force 125) Anyhow his lower is marked 4A. I checked the ratio and it's 1.73. Thought it might help someone b/c that was the odd ball stamp.