Mercury 4hp with stripped crankshaft splines

Hi All,

I've been working on an older Mercury 4 hp Gnat motor. It's running really well and has good compression. My issue is that the splines in the crankshaft have stripped out. Now I know that this is normally a death sentence for most motors, and this being a Gnat, it already had two strikes against it. Earlier today, I welded up the end of the driveshaft and recut the splines, hoping that there would be enough meat left on the crank to spin the prop. Unfortunately, it seems that the crank is completely stripped.

Now I'd hate to toss this motor because it runs so well, but I'm not going to toss the stupid money that would be needed to replace the crank. I was driving and I had a crazy thought this afternoon:

What if I weld a 6 point socket to the end of the crankshaft? Then I could drill and weld a nut onto the driveshaft, slide the lower in place and use that assembly to replace the splines.

Am I way out in left field with this one?? I know that I'd have to pay close attention to the alignment of the socket to the crank. I thought that maybe even using a deep well socket with two or three nuts aligned and welded together to increase the bearing surface.

I wouldn't be considering this if I was talking about a 150hp, or even a 50... but for my lowly little 4, I don't imagine that there would be enough torque to break that apart.

Now, I know that once the nut was welded to the driveshaft, I would have to grind it off to replace the waterpump, but I just replaced it this year, and it should have some significant life left to it.

If this doesn't work, I'm going to have to part it out, which will be a challenge of it's own as not many people still have these motors.

Any thoughts??? Suggestions??? Alternatives????

Thanks!

Interesting situation you are in as I'm rebuilding a 4.5 single at this moment. The model series I have has the drive splines open to the crankcase to keep the splines lubed with premix. And is a great place to let in unwanted air. My pwc is a greased spline with a seal at the impeller. Wonder about that 2cyl Gnat? I'm also curious how much shaft whip might be controlled by the amount of interface between the male and female spline. I would maybe look for a tighter interface spline assy and find a way to keep things lubed.

Someone here had been given a motor, I think single cyl 4hp, that acted to them as a stripped spline. It turned out to be the prop hub slipping.

I only wish that it was the prop hub. When I spin the flywheel, I can hear and feel the clunking in the bottom of the powerhead as it skips splines. It won't even turn the prop when it is out of water and turning by hand. I can spin the driveshaft easily with the prop, but that doesn't turn the flywheel.

It's possible to build up the end of the shaft even further, or even maybe use some aluminum foil wrapped around the end of the shaft to thicken it a bit further instead, but I'd be concerned about the strength of that when I open the motor up. Maybe use the foil to center the shaft and the socket/hex to drive the prop...

I have to pull the powerhead tonight and look at the clearances before I go much further. From what I recall, there's a good bit of empty space below the powerhead, but I don't remember exactly how much.

Thanks!

kofkorn said:

I only wish that it was the prop hub. When I spin the flywheel, I can hear and feel the clunking in the bottom of the powerhead as it skips splines. It won't even turn the prop when it is out of water and turning by hand. I can spin the driveshaft easily with the prop, but that doesn't turn the flywheel.

It's possible to build up the end of the shaft even further, or even maybe use some aluminum foil wrapped around the end of the shaft to thicken it a bit further instead, but I'd be concerned about the strength of that when I open the motor up. Maybe use the foil to center the shaft and the socket/hex to drive the prop...

I have to pull the powerhead tonight and look at the clearances before I go much further. From what I recall, there's a good bit of empty space below the powerhead, but I don't remember exactly how much.

Thanks!

Click to expand...

You might be able to use a spider type coupling. There should be space enough. It might be easier to fabricate a mounting strategy with available spider couplings with the hardest elastomer center piece you can get. Give shaft whip a thought though. At 5000rpm and the shaft not well controlled at one end...but at this point I'd go for it.

So I took a closer look at the bottom of the crank, and I was a bit surprised by what I found. Looking at the bottom of the crank, it looks like the driveshaft was only engaged about 1/3 of the way and stripped out all of those splines:


I figured that I might be able to do something a bit different, by lengthening the driveshaft a bit I could at least engage the remainder of the splines for some use of the motor. So I did a few measurements and figured that I could safely lengthen the driveshaft by just under 1/2" and still leave some space between the crank and driveshaft. I cut the shaft, extended it to the new length and then re-welded it back together. When I was done, I managed to have no visible runout on the shaft. I've seen many shafts with a 0.050" or 0.060" runout with no problems, so I'm assuming that this will be ok.

So I went and re-assembled the lower unit on, and there still is a grinding sound and no engagement with the splines. I'm totally stumped.

Additionally, upon closer inspection, the splines on the crankshaft aren't female, they're male. So the part engaging with the driveshaft is a coupling #20 in the dwg:


Additionally, as noted in the image of my motor, there is a harmonic balancer that I can't find anywhere in the exploded views. Is the outer ring on the coupler supposed to be this balancer? It's just hard to tell from the image.

So now I'm concerned that if the interface between the crankshaft and coupler is ruined, then welding anything on to the coupler would be pointless. I'll have to scratch a set of marks between the crank and the coupler and see if this is turning.

I'm the kind of person who can't just let something go. I'll get it to work by hook or by crook....

Thanks for the help!

That is a coupling assembly 70786A 1. Those Gnat 4hp had some issues I've read somewhere here...looks like a piece of it is still there? That would suck if that was the issue now with the shaft modded.

I'm not sold on the gear case not being the issue but you say its okay..

See here https://www.marineengine.com/parts/mercury-outboard-parts/40-4-gnat-2-cyl/7085673-thru-7146487-can/crankshaft-piston-and-connecting-rod

So, in the end, I figured out what the issue was... I removed the powerhead again to check the splines on the coupler and to make a mark across the driveshaft and coupler to see if it was slipping. After making the mark, I aligned the coupler with the end of the driveshaft and dropped it on. Once I did this, I noticed that the powerhead wasn't sitting flush on the midsection. It was pushed up by about 3/8" which was coincidentally the approximate amount that I had lengthened the driveshaft... #-o

So, lather, rinse, repeat. I pulled the lower, cut and re-welded the driveshaft back to the original length (Luckily, I had marked it off before lengthening it). I re-assembled the lower unit, and it took me about 5 tries to get it alinged properly where the shaft fit into the coupler. Surprisingly enough, there is enough room in the midsection around the coupler to fit the drive shaft without going into the coupler. This to me is poor design, there should be some amount of alignment that prevents this from occuring.

Anyway, lesson learned. Pay attention to the alignment on the Mercs...