View Full Version : X-115 GT-115 gear case guts

08-07-2011, 07:20 PM
Crazy Dave (who owns a highly profitable tick farm somewhere in the south) has been at it again ... he always wants to know what is inside things so he took this apart and figured out how it works.


08-07-2011, 07:28 PM
The double sided prop shaft gear that the red arrow points to has splines inside it and can slide front and back on the tube it is mounted on. That's right, its on a hollow tube, not directly on the shaft with the prop. The green arrows point to the tube. The prop shaft passes all the way thru it and is located and supported at the front by the tapered roller bearing.

08-07-2011, 07:58 PM
Now the reverse gear gets some arrows. It is always engaged to the back of the rear driveshaft gear. It is mounted on a bearing spinning on the tube ... a tube outside of a tube, so it does not cause the tube with the double sided gear to turn ... in fact, it turns the opposite direction of the tube, because its on the other side of that back gear.

08-07-2011, 08:04 PM
Next we revisit the first hollow tube that the double sided gear runs on. All 4 arrows point to different sections of one piece of metal ... the very long arrow points to "dogs" on it that will engage the shifter sleeve in a couple more posts

Lars Strom
08-07-2011, 09:09 PM
Hmm..interesting..I did not know how a two pinion gearcase with reverse worked..and I raced that gear case 1973.. the Evinrude 65SS used the old X-115 lower unit.

Thank you..now I can sleep good..

08-08-2011, 04:04 AM
Thank you for the lecture, Sam. I have never seen the inside of one of these, nor seen any catastrophic failure. Great OMC engineering! rgds.

08-08-2011, 09:24 AM
2 views of the shift slider that engages one set of shaft dogs or the other.

That this shift slider is hollow and cup shaped is very important, as you will see later.

It has internal splines that are always engaged to the actual center solid prop shaft and always turns with it. As we see it here the prop would be to the far left and the rear bearing and seal areas are visible

In the bottom picture, Dave is holding on to the slider; with it pulled farther forward than it naturally goes ... just so we can see the splines on the prop shaft while they are engaged to the slider.

08-08-2011, 09:31 AM
The hole in the prop shaft spline area is where a spring loaded ball goes to provide a neutral detent position. You can see the corresponding space inside the slider where there is a gap in the internal splines.

08-08-2011, 09:39 AM
Now we will put the shifter slider on the prop shaft in position to run the boat forward.

The pink arrow is pointing to the splines on the prop shaft where they are slightly visible.

The white arrows point to the tangs or "dogs" that engage to some other dogs for the actual shifting.

The slider is moved all the way to our left, in its closest to the prop position, that's why we can see a little bit of the prop shaft splines down in the cup where the pink arrow is pointing. You can also see the empty space behind the bottom dog inside the cup.

This is the solid prop shaft at the center of everything; the area at the right will slide into the hollow shaft where the gears are.

08-08-2011, 09:56 AM
Now we'll look at the hollow shaft again. Here is the end with the dogs for forward, all the way at the left side of the shaft where the black arrows are pointing. Pink arrow points to the hollow center of this shaft, that's where the solid center prop shaft goes .

08-08-2011, 10:04 AM
The reverse gear and its outer support bearing ... and the dogs made onto the back of the gear. The reverse gear is not attached to the outer bearing or the hollow tube inside the reverse gear, it floats in both.

08-08-2011, 10:29 AM
Here are the positions of the dogs on the shifter slider cup for each shift function.

When the cup is in the middle position being held by the detent spring ... the dogs inside the cup touch neither of the other shift dogs.

When the cup is pulled back, its dogs engage to the dogs on the hollow shaft and the main double sided gear transmits power.

When the cup is full forward, its dogs engage the dogs on the back of the reverse gear ... in that case, power comes down the back double shaft to the back gear and power transmits off the back of the rear gear to the reverse gear.

08-08-2011, 10:32 AM
The double sided gear, the reverse gear and the pinion gears are always turning. The reverse gear is always turning opposite to the hollow shaft.

08-08-2011, 10:49 AM

Forward: Power comes down from the motor and splits to the 2 driveshafts just below the water pump. From each driveshaft power goes to a pinion gear driving the floating double sided gear mounted on the hollow shaft. Power automatically positions the double sided gear so 50% goes thru each gear. Power transmits along the hollow tube, thru the slightly larger hollow reverse gear to the dogs at the back of the hollow shaft. The shifter slider cup is at the full back position and the cup is engaged to the dogs at the back of the hollow shaft. Power transmits from the slider to the solid center main prop shaft. The solid main prop shaft and the hollow tube are turing together.

Reverse: Power comes down from the motor, transmits to the rear lower driveshaft, to its pinion gear and off the back of that rear pinion gear to the reverse gear. Then off the dogs made onto the reverse gear - to the dogs on the shifter cup (which is now in its full forward position). Again, from the shifter cup to the solid center prop shaft. Now the hollow shaft is not connected to the prop shaft and they are turning opposite directions, one inside the other.

DO NOT COPY PICTURES OR TEXT FROM THIS THREAD TO ANY OTHER WEBSITES You may download and save for your own personal use.

Lars Strom
08-08-2011, 11:47 AM
My gear case from 1973..

08-08-2011, 05:51 PM
Thank you for the lecture, Sam. I have never seen the inside of one of these, nor seen any catastrophic failure. Great OMC engineering! rgds.

These are good units for racing but shift parts aren't great as you can clearly see from these pictures. I don't recommend using them for fishing or skiing.
My dad and other guys that raced these always locked them in forward which was allowed after maybe 71 or 72.

I heard from one of the OMC race team guys that this unit was a joint venture project of Charlie Strang and Doc Jones.

Lars Strom
08-08-2011, 06:07 PM
..not very good for shifting..but a very strong gear case..
Over in Sweden we took the Super Strangler V-4 without shifting..
put this gearcase on instead..installed the shift level on the mid section and raced local offshore racing..The role said..must have neutral/reverse..

Yes..this is the latest strongest ON V-4 race engine OMC raced before going V-6..

08-08-2011, 06:09 PM
I wonder how the shimming was done?
OK pinion height by tool and feeler gauge but how do you set the back lash let alone the load per shaft, just with the timing gears?
How do you hold the double gear still?

08-08-2011, 06:27 PM
The double gear is not held still, it slides on the shaft and self centers, automatically equalizing the load on each input gear and shaft.

08-08-2011, 06:33 PM
thats my point, maybe lock the prop shaft load up main drive shaft and set the 2nd timing gear to a known backlash?

08-08-2011, 06:39 PM
I don't understand what you are trying to adjust

08-08-2011, 09:18 PM
so how did they set them up?
Just raise lower pinions, run and check contact patch except you still need to do the set the timing gears or the 2nd shaft will do nothing and the first will force the driven gear into it?
Any other twin box you lock the propshaft and hence hold the driven gear(S) to do your timing gear backlash.

Saying that I have never worked on any OMC twin boxes only Merc

08-09-2011, 04:42 AM
Whichever gear leads forces the driven gear to the second gear. All the set up has to do is not bind and be close. All the gears and splines are straight, there is no critical timing like a Super Speedmaster

08-09-2011, 05:44 AM
Most, if not all, of the OMC V4 and early V6 twin pinion gearcases were built by Ward Cox who was a member of the race team. The pinion gear vertical location was shimmed based on the distance from the propshaft centerline to the upper bearing carrier. The transfer gears, at the top of the driveshafts, were used to split the load equally between the pinions and into the floating bull gear on the propshaft. Transfer gear lash would be checked with the propshaft held solid and a load on the driveshaft. The transfer gears were then rotated until lash was 0.001" or less. Each transfer gear rode on a different spline which made this possible. If the transfer gears were not properly positioned the pinions would get uneven loads resulting in early failure. Ward built so many of these he was able to do all of this by feel. All OMC racing gearcases used this basic design, GT, 1:1, CC, CCC etc.

08-09-2011, 05:17 PM
Jim, you mean the lash is checked with the shafts loaded vertically away from the propshaft, correct?
That way the pinions are against the thrust bearings which is the natural position when the unit is under load.
I made a tool out of a couple of valve springs and set screw collars for the purpose.

You can really tell when one of these units are not set up right, they will feel rough when you turn them almost like the gears are shot, then you pull the case apart and the gears look fine if not run too long that way.

I have 14 V4 race lowers that I have been tinkering with and still trying to get all the details.

08-09-2011, 05:29 PM
how many different twin pinion boxes were made?
V8 (F1 and offshore)
or did they make new ones each time a new engine came out?


Dave S.
08-11-2011, 07:49 PM
The back shaft has flats that you can hold to set the lash.....Who called me Krazzy......Ya be lucky to drop the top gears in place.... then try again and set them so the load is even...... SSM is harder as the top gears are hypoid,,,,, Nice pixs

08-11-2011, 07:55 PM
Why did you let me put the shifter arm on backwards? Tick farmer

Anyone else notice that? Go back to posts 3 & 4 and look :o

Dave S.
08-11-2011, 08:09 PM
Thats a right hand case.............

08-12-2011, 08:44 AM
Thats a right hand case.............
drive gear infront of the driven..
IMHO its a left case

08-12-2011, 08:50 AM
Dave knows its a left, he's joking that putting the shifter arm on backwards changes the rotation. He got you.

08-12-2011, 08:51 AM
got me!

08-13-2011, 09:08 PM
Here is a cutaway view of the assembly and a picture of a complete GT115 motor.

Dave S.
08-14-2011, 05:26 PM
I am at a mess when it comes to putting that shifter back in. Was hard comming out any one know any tricks.???? Thanks Dave S.

08-16-2011, 04:49 AM
Here is a cutaway view of the assembly and a picture of a complete GT115 motor.
The GT115 and the 55hp SE engine made their appearance with exactly that lu in `67/`68. The fact that OMC served the teams running the two different motors with props out of the same box, leads to the assumption that the lus were fitted with different ratios. However, our `69 SE motor really got going when fitted with ss aftermarket props (made in Sweden).

08-16-2011, 04:55 AM
Inspection sheets show the same 14:23 ratio for both motors

08-16-2011, 04:56 AM
Here is a cutaway view of the assembly and a picture of a complete GT115 motor.

PS: in retrospect still amazing that with underside of propshaft in line with underside of keel on both deep Vee boats (Grupstra and Dalla Pieta`), the motor still could pick up sufficient cooling water through the gills above the torpedo. We never experienced a cooling system related failure, and in 2 1/2 years only had one upper crankcase seal and one rectifier failure.

08-16-2011, 06:17 AM
Inspection sheets show the same 14:23 ratio for both motors

I am well aware of that, but that does not explain my conundrum, The Swedish aftermarket props turned out some 3/4" smaller in diameter than the works props. They spooled up better and seemed to track better. Only the experimental 4-blader might have required more development time, with potentially a reduction in diameter.

08-16-2011, 06:30 AM
I don't understand the puzzle ... assumed different ratios, but were not ... smaller props worked better ... agrees with having the same ratio

I see agreement, not puzzlement