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  Gear ratios 115 Commercial Lower Unit vs Standard

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Author Topic:   Gear ratios 115 Commercial Lower Unit vs Standard
Gene in NC posted 08-17-2003 12:07 PM ET (US)   Profile for Gene in NC   Send Email to Gene in NC  
What is the difference between gear ratios in 115 commercial lower unit vs standard 115? Will the commercial fit the standard 115? Will I need special prop?
Jamie 20 outrage posted 08-17-2003 02:08 PM ET (US)     Profile for Jamie 20 outrage  Send Email to Jamie 20 outrage     
what brand and year?
Gene in NC posted 08-17-2003 03:11 PM ET (US)     Profile for Gene in NC  Send Email to Gene in NC     

Whoops! '89 OMC 115 commercial lower unit and '87 OMC 115 standard 20" engine on a Montauk..

Bigshot posted 08-18-2003 10:52 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
The commercial lower unit is a V6 lower I believe.....don't do it because you will need a new prop,etc.
brisboats posted 08-20-2003 02:36 PM ET (US)     Profile for brisboats  Send Email to brisboats     
20" commercial (v-6) lowers for a V-4 are rare. Measure the bullet, a 4 1/8" is the typical 2:1 gear ratio (13:26) that all V-4's have had since 1978. The large case V-4 lowers that I am familar with came with the larger V-6 case, which had a 4 3/4" bullet and had a gear ratio of 2:20:1 as opposed to the V-6 gear ratio of 1:86:1. But these were all 25" shaft and came on the V-4 140hp. Hope this helps. BTW the V-4 and V-6 cases bolt up the same.

Brian

Bigshot posted 08-20-2003 02:40 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I thought the 100 commercial used a V6 lower.

I know the 140 Looper does and maybe even the 130's.

Bigshot posted 08-20-2003 02:43 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
PS...They are not all 25" shaft and do have a 1.86 or close. My neighbor has a 140 Looper 20" shaft with a V6 foot on his 16' Checkmate swinging a 24" wheel, definately not a 2:20 raio.
brisboats posted 08-20-2003 05:18 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Doubt if it is swinging a 24" prop that it is a 1:86.

Brian

brisboats posted 08-20-2003 05:37 PM ET (US)     Profile for brisboats  Send Email to brisboats     
PS No V-4 lower ever came with a set of 1:86:1's closest you could get would be pre 1978 135hp with a set of 1:92:1. From 1978 on all V-4's are generally 2:1 with some oddball 2:20:1's.

Brian

Bigshot posted 08-21-2003 10:39 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Brian he has a 140 looper(1987) with a factory 20" V6 lower swing a 24" OMC prop. Boat runs 60+mph.....aint no 2.20:1.
brisboats posted 08-21-2003 02:09 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Bigshot.. My guess is your buddy has a large case v-4 lower with the most common 2:1 gears. A factory V-6 lower on a 140hp with the 1:86:1 would be a poor performance choice and doubtful could swing a 24" prop effectively. He would have had to knowingly mount a facory V-6 lower as big case V-4 lowers never came with 1:86:1. The fact that you say the boat does somewhere in the 60mph range confirms a 2:1 gearset to me.

I never said anything about all V-6 lowers having 2:20:1 ratio my point was these are the one's I have seen on 25" shaft commercial motors. The idea was these motors were bought for grudge duty on bigger heavier boats. Putting 1:86:1 ratio gears in the commercial lowers that would be pushing these kind of loads would be reverse engineering.

Plain and simple a big case (V-6) lower mounted by OMC on a V-4 motor from 1978 on either had 2:1(most common) or a 2:20:1 gearset. Ain't no way they had 1:86:1 or close.

Gene in NC posted 09-04-2003 11:30 AM ET (US)     Profile for Gene in NC  Send Email to Gene in NC     
Now have the suspect lower unit in hand. Checks out 20", 4 3/4 gearcase and 2.2/1 gearing. Clean shifts, and smooth, quiet gears.

Now that I have it, what do I do with it?
brisboats posted 09-04-2003 11:16 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Gene, if you need a lower I would bolt it up and use it. If you are pushing a heavier boat you will like the 2:20:1's.

Brian

brisboats posted 09-04-2003 11:19 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Oops just saw that you have a montauk, I would still use it you will just need to prop up.

Brian

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