Identification of lower unit, will it fit my Merc 275?

[Stilldouble]

Does anyone know what kind of lower unit this is? (See attached pic) The only thing I know is;

- The Mercury part number is 1623-859399 R13
- This is a Mercury (V6) lower unit
- long (20") shaft
- 13 spline prop shaft, 4.75 diam

The question(s);
1) what kind of lower unit is this?
2) Will it fit my 1992 Merc 275 (3.4 L) (engine number OC249166)?
3) Would it be a stronger lower unit than I already have?

I have had some commets from the forums that the lower unit I have (original) is not that strong and I have to be careful surficing this engine.
I can see that this is different from what I have, the fin is bigger...

Best regards

/Stilldouble

[Riverratt]

If it is only 13 spline then it is not for a V6. It should be a 15 spline.

[Stilldouble]

Hi,
Interesting, has it always been like that the V6´s are 15 splines? In this case we are talking of engines from the beginning of the 90´s... Or is this (15 splines) a modern phenomenon?

Best regards,

/Stilldouble

[chris_lacey]

Inline 6's as well. 13 spline is usually only OMC V4.

That's prop shaft. Drive shafts are 13 spline. ;)


3.0 litre have had 8 and 13 spline drive shafts at the top and 13 at the bottom of the intermediate shafts.
13 on the stub shaft.

Sorry double, I don't know about 3.4's

[Dave S]

The Gearcase in the photo is a 3.0 case..... you can't use a 3.0 case on a 3.4.... the drive shaft is different as is the 4 bolts placement is sligltly off.:eek: Are you sure you need a 20 or 25 inch lower unit?

[Raceman]

I have had some commets from the forums that the lower unit I have (original) is not that strong and I have to be careful surficing this engine.
I can see that this is different from what I have, the fin is bigger...

Best regards

/Stilldouble

I can't tell from the pictures what the gearcase is, but 3.0's and 3.4's have different driveshaft splines from all the other Merc V6's and the inlines listed.

BUT............. the thing is, you ain't got any business "surfacing" a 3.4 anyway unless you want to have a large pile of scrap aluminum on your hands. The 3.4's were very durable engines when used within their limitations, but they won't live in high perf applications. Surface it and twist it and you break it.......... it's just that simple. You won't ever see a plow mule or an ox entered in the Kentucky derby.

[Dave S]

RM ya need to Give me your 3.4 Race stuff cause' the factory didn't surface T4s.:D

[Stilldouble]

In your opinion, would running this engine with Tempest Plus 26-27P (for keeping the rev below the allowed 5500), 2" below the pad, be within this engines limitation?

Best regards,

/Stilldouble

[Raceman]

In your opinion, would running this engine with Tempest Plus 26-27P (for keeping the rev below the allowed 5500), 2" below the pad, be within this engines limitation?

Best regards,

/Stilldouble

I would say yes, but I'm not even sure you need to turn it that hard. My friend who worked in Warranty at Merc's Atlanta Branch when the engines were in production said that they were among the lowest warranty incidence of anything Merc built and it seemed like most of the guys that broke em had em on center console multi engine offshore boats that they'd bungie the throttles wide open on and go launch em off the rollers. I've got one that's got a zillion hours on it and has never been down. They're durable/tough old motors if you take care of em and don't turn em hard or unload/reload em suddenly. There's just too much rotation mass to treat em rough.


RM ya need to Give me your 3.4 Race stuff cause' the factory didn't surface T4s.:D

Dave, back when I bought my first one, Tricheal told me (he worked in the program) that the problem even with the T4's was when they put em on sprint boats for shorter courses and tried to turn em hard. He said the race version was bullet proof set up for the longer courses where they turned em around 7K max.............. any more and the upper and crank bearings were the weak link. (I think he said upper and lower only, but that coversation was like 12 or 15 years ago)

[Dave S]

Gotta GIVE me ya 3.4 stuff.:D They seem to have a bunch of weak links, the early ones hab small rod bolts-_-_ they were SPS but old 225 size. The later ones had big bolts like the 3.0s. Big heavy crank and not enuff bolts holding the case halves together. Just what I have seen.:cool: