Sheared the driveshaft on my Merc 50

Yesterday, I hit an underwater obstruction out in the flats. Been through there a hundred times, and never knew that pipe was there. Found it yesterday, the hard way.

Was running along on plane, and BAM!!! I got towed back in, as the boat wasn't going anywhere. Upon getting it home, I did not see any external damage to the lower unit. So, I pulled the lower unit......found the problem. There was only about 1/2 of the splines left, and I could tell it had wrung off, leaving the nub from the end of the driveshaft up in the recess of the power head.

I checked it over by engaging the shifter lever in either direction, and it engaged both ways, with the prop clicking one direction, and turning the shaft in the other. So, as far as the lower unit goes, I just need to take it up to the shop and have them install a new drive shaft (hopefully)

Now the question is: how do I get that nub out of the power head? Someone suggested tapping directly on the center of the flywheel shaft with a hard block of wood, in the hopes of the vibration from that causing the piece to fall out of that recess. Any ideas?

Strong magnet? Unfortunately it is likely to be jammed in there pretty bad due to the damage. If you can get the powerhead off, maybe drilling a hole in the center of broken shaft and using an easy out to pull it? or drill and tap and use the bolt to pull on.

Good luck with that one. I hope it isn't stuck hard.

Well, I went up to the boat repair shop, they said a new driveshaft is about 600 dollars. Plus, there could be internal damage in the gear case as well. New gear case is 1800 dollars. Not to mention the problem of extracting the broken-off driveshaft stub that still remains in the PTO of the power head. They told me not to beat on the top of the crankshaft, and said it may be likely that the powerhead would have to be unbolted, then try to drill and extract the driveshaft stub. More $$$$

I was about ready to give up hope, and just try to find a used motor, but, I decided to at least try to see if I could extract the nub of the driveshaft. Looking at the end of the sheared driveshaft I could see that it had sheared about 3/8" up from where the splines started, and the little bit of remaining splines were twisted, looking like the end of a Twizzler candy.

But since there was so little of the splines, my hope was that when the shaft snapped, the engine was still spinning and it simply chewed the tips off the splines, but maybe the part remaining was a clean break. So, I looked in there with a lighted borescope, sure enough, it looked like the ends of the splines were clean, no burring or mangling. I proceeded to stick a telescoping magnet up there and let it make contact with the end of the shaft. It wouldn't move. Then, I took a short piece of brass rod, and a hammer, then smacked the top of the crankshaft while my girlfriend put a little downward pressure on the magnet.

Guess what? Out fell a nice, clean piece of driveshaft with intact splines. It worked! That's half the battle.

Since I'm a cheap SOB, I refuse to spend 600 dollars on a driveshaft.....my plan is to remove the driveshaft, put it on the lathe, bevel the broken edge, then use a dial indicator to square up/align the piece with the splines and TIG weld it back together. Then re-assemble, and hopefully keep on rolling! Next step is to tear into the gear case, and make sure everything looks OK. Gotta do that anyhow to pull the driveshaft.

I'll let y'all know how it works out.

Which boat is this one? Hope you find no problems in lower end.

This is on my Triton 1650SC johnboat.

Hopefully, there won't be anything bad in the lower unit. My girlfriend's brother is pretty good with outboards, and I have a repair manual for this engine, so, between the 2 of us we should be able to get the lower unit apart. And truthfully, this will be the hardest part. TIG welding the shaft back together won't take very long at all, just gotta make sure it's squared up by running with the dial indicator.

Anyhow, here she is:



Ain't that cute?

I believe this ought to fix it......



broken shaft now beveled, squared, and tack welded.


Meanwhile, a look in the gear case......


No nasty stuff going on in there, oil was clean, no shavings or missing gear teeth. [-o<






TIG welded with ER308 stainless wire. This done after being tack welded at 12, 6, 9, and 3, o'clock, and first checked on the lathe with a dial indicator, where I found it to be no more than .007" out-of-round with the other section of the shaft, at least as best as it could be read, given the surface of the splines.





Even tried to line up the splines as best as possible. However, note that the weld is higher than the groove of the spline. This would be a problem when trying to press the driveshaft back into the PTO. So, one more step......





Machine the weld level with the depth of the splines, so that it doesn't protrude and interfere with anything. Not very much material removed, and because the weld was beveled, the penetration is enough that there is plenty of weld remaining, even after turning on the lathe.


My water pump impeller is a bit chewed up, so, I need to replace that before trying to run again. Boat dealer closes at 12 tomorrow, not sure if I'll be done with work before they close. If not, I'll get it first thing Monday AM, and get my lower unit back together, and see how it does.

It's pretty great to have those kind of skills and saves some $ to boot. Way to go.

Very impressive work.

[url=https://www.tinboats.net/forum/viewtopic.php?p=357475#p357475 said:

bobberboy » Today, 08:09[/url]"]It's pretty great to have those kind of skills and saves some $ to boot. Way to go.

Click to expand...

Thanks! Indeed, I probably just saved myself about a thousand dollars, maybe more.

As soon as that little nub of driveshaft fell on the ground and I saw the splines were nice and clean, it was like an instant brainstorm....I said "TIG weld it back together!" Hey, this wouldn't be the first time I've modified or welded a driveshaft. My jet boat's driveshaft is heavily modified from its original configuration, too.

[url=https://www.tinboats.net/forum/viewtopic.php?p=357476#p357476 said:

lckstckn2smknbrls » Today, 08:46[/url]"]Very impressive work.

Click to expand...

Thanks!

Just a thought and remember I have no experience in machine work or welding. Shouldn't you have machined the shaft diameter then welded the nub on.

In one of the pics, it shows where I beveled one side of the break, so the weld would be of sufficient depth with enough root, and not just a surface weld. :wink:

Went up to West Marine and found an impeller this afternoon, and got the water pump back together. Got the lower unit bolted back on, and tested it on the hose. It immediately began peeing, so, that meant the driveshaft was turning. Tried forward and reverse....both directions worked.

So, I launched the boat back in the water, and tried a test run. All good. Ran it at WOT briefly, and ran around for about 10 minutes. No excessive vibration, no strange sounds. Everything appears to be as normal as it can be, considering the serious nature of what happened. (Kinda like me, with the encephalitis that nearly took me out in February.)

Skills and tools...nice job

Looks like some nice repair and welding work there. I arc welded a small shaft and pulley back together once, and by using the 12, 9, 3 and 6 tacks it stayed mostly in line too. Since most of the torque on a drive shaft is concentrated on the outer edge it looks like you got plenty of weld in the most stressed area. How did you keep the heat off of the splines and avoid getting them too brittle? Did you do any post weld heat treatment on the shaft? The ER308 wire should keep the weld from getting too brittle anyway.
Congrats on a great repair. Catch any fish while you were out?

I didn't do any heat treatment, just welded it and let it cool down. Stainless is a good filler wire for brittle metals, such as cast iron. I've used stainless arc welding rods to repair cast iron well pump housings, electric motor mounts, etc.