Broken output shaft & coupler
 

Hello again everyone. I having a problem with breaking the Mercruiser output shaft 93906A 2. I need something
stronger. The first one I replaced was twisted and the 2nd was just a few days ago and it completely broke and
spun the coupler. So my question is does anyone know of a stronger shaft for sale or a shop that can make one thatI'm not going to snap off?

Thanks

That shaft is not usually a weak link. I would think you'd be breaking vertical shafts first.
I think you might have an alignment issue.

I bet that your engine is moving around - That is the only way I see you breaking a shaft and blowing the coupling.

I've had a prop shaft and a vertical shaft break....never have broke an input shaft....neither of these are near a robust as the input shaft.....like what everyone else is saying, you have something wrong alignment wise

I have IMCO SCX 4's on the boat I don't think they would break before that stock output shaft. I also have front and rear motor plates so I can't imagine it's moving around but I had not thought to check that. As far as alignment I wouldn't think it could be off that much, the output shaft bolts to a flange with a universal joint. If the alignment was off that bad I would think the universal joint on either end of the driveshaft would break. I really think I'm just over powering the stock shaft.

How much power?

It right at 1,000hp per engine.

Where is the shaft breaking? I also believe it is an alignment issue. If it was mine that is what I would check first.

With a 1000hp and running SCX's, the input shaft may have well become the weak link.

The shaft is breaking in the splines outside the coupler.

This is the reason I don't think it's an alignment issue, and please let me know if I'm not looking at this the right way. On the back of the bell housing is a bearing that this shaft rides in, that is bolted to the engine. The driveshaft hooks to that output shaft with a universal joint. It is not mounted to the boat but to the motor so if the motor moves it will move with the motors. On the other end of the driveshaft is another universal joint and a bearing that the input shaft from the drive goes into. So my thoughts are with the 2 universal joints in between the motor and the drive even if the alignment was off a little it wouldn't matter.
Thank you for your input.

I would be surprised to hear a drive input shaft sheared off, it sees much less torque than the vert or prop shaft and has sorta a torsional action to it because of length, how much hp /tq do you have, what props?

I see you have 1000 hp, my 272 Baja had over 1000 hp and ft lbs of tq, one drive had stock std input shaft and ujoints, my other drive had xr input shaft and u joints, sheared plenty of vert shafts and prop shafts but input shafts still looked like new, I guess it could happen but again, I would be surprised, wondering if your running tall 5 blades that really stress it or something? Smitty

Am I correct to assume that this is the forward engine in a staggered application ?

Triple boat ? If so , same pitch on all props or higher on center eng ?

Yes you are correct.

1,000hp per motor, props are labed Herings 17/38. It's not the drive input shaft that I'm having a problem with my SCX drives are fine. It's staggered engine boat and it's the forward engine and it has a output shaft that goes in the coupler to a drive shaft then back to the input shaft from the drive ( SCX 4's )

Not a triple boat.

Props are 17/38 5 blade Herings that have been labed. I know I'm putting a lot of load on the shaft that's why I'm looking for a stranger one.

Try Denny's Drivshaft in Buffalo NY
He should be able to eliminate the weak link and help you find the next one 😉

Thank you I will look them up. I'm afraid of what the next weak link may be.

Maybe a good test for one of the drive guardians?

In the forward engine when your running scx drives ,the shaft you are talking in the week link , l had two of them go, they where twisted ,check the pilot bushing in the end of the crank, Brian forehand says they go all the time when running scx on a high horpower motor.

I didn't even know what a Drive Guardian was, I had to look it up. That's a great idea. I just bought a new coupler and bearing today if I can't find a better shaft soon I'll get one more stock one just because I'm going to Key West poker run with this boat and I'm running out of time. I'll be looking into a drive guardian for next years boating season. Thanks for the idea.

I'm glad I'm not the only person having this problem. I'll have to look to be sure but I don't think the shaft goes through the coupler far enough to reach the crank. I'm not even sure it's pilot bushing in the crank but I'll look to be sure. Thank you

simple...heavy boat, twins, tons of load...how hard are you hammering it getting on plane ? rule of weight/load/power here...

Good luck, totally different that i was thinking. When i bought my steelings box it came with a staggered boat drive shaft, it used no coupler or splines, instead ot had a h coupler that bolted directly to flywheel with a crank button, it used 1450 series u joints and a super heavy duty driveshaft which are the baddest as thing out there, maybe you could convert?

I don't know if this what your set up looks like, but this is the arrangement used on a 525 drivelive
https://www.mercruiserparts.com/Show...ecting+Rods%29

they list a pilot bushing as being used for a drive line

this is a breakdown of the shaft arrangement
https://www.mercruiserparts.com/Show...nts%28Bravo%29

I don't have to hammer it to get on plane the drive hight is so high that hammering it wouldn't work. 2,000-2,300 RPM's will get on plane

Yes that is the same set up I have in my boat. The shaft that goes in the coupler is the one I keep breaking.

Mine also has a coupler that bolts to the flywheel and a splined shaft the goes in the coupler the other end of that shaft hooks to a driveshaft and the driveshaft runs aft to the input shaft of the drive.

Ah yes, now I see what shaft you are describing.

Any ideas on where I can get a better one?

didn't you say you didn't have a pilot bearing on yours .If you don't ?

Here's one place. Send them one of your old ones and they'll build one that'll take whatever power you put to it. They do steel, aluminium, carbon fiber. Fast turn around times too. Wiles Driveshafts 3775 US-79 Scenic, Paris, TN 38242
(731) 333-3264

maybe ck with some custom drive shaft shops that build race car shafts, they may be able to come up with something..

I can't say that I've ever looked to see, if it's one in the crank I'm going to put one in.

Thank you, I'll call them today!

That's what the place above does. I've had several done by them. They do great work.

You need to talk to Ralph and he may or may not be there today but will be there Monday for sure.

It's no pilot bushing in my crankshaft. So I'm going to instal one maybe that will help.