20hp Merc crank shaft splines??

Got a motor from my dad. Serial #4713924. Best I can tell, the newest it could be is 1977. The story is that the crankshaft splines are stripped. He was told that was the issue over the phone by an old boat mechanic. Now that may be the case, but I want to be sure. His issue was that the prop will spin at low rpm, but slips when you give it gas. I'll try to attach pics. I really don't know much about the guts of a boat motor. When you turn the prop by hand, it makes kind of a clicking noise and the long skinny shaft runs through the lower unit doesn't move. Should it?

View attachment 1

Based on that serial #, the motor is a 1977. The bottom pic is too blurry to be able to tell anything, and a close up of the shaft end would help, I don't see the damage on the shaft that I would expect to see if it were slipping.

Depending on what gear you have it in, it will click and the shaft will not rotate when the prop is rotated in one direction, but rotate the prop in the opposite direction and the shaft should rotate with no clicking.

I agree the drive shaft looks ok although blurry the crank end looks chewed up. From this drawing, the larger ring is the carrier (part 3) and the female splines are actually part of the crankshaft (part 1)?
Tim

I'll try to get better pics tonight. There is quite a bit of gunk up in the crank hole/splines. the prop section of the lower unit has been removed. When you turn it forward it clicks, when you try to turn it reverse it won't move. At least I can't get it to budge.

I'll be heading home for lunch and will try to get better pics then.

Pics didn't turn out any better. I'll have to take the drive shaft housing off to see it better I think. And I got the drive shaft to turn by turning the prop so we're good there.

You probably have a spun hub. Take to a prop shop and have new bushing pressed in. Have same motor.

Sent her with my buddy today. He works at one of the boat shops near the lake. He said in 20 years of working on engines he has never seen one of these strip the splines in the crankshaft. I'll let you know what he finds.

[url=https://www.tinboats.net/forum/viewtopic.php?p=358843#p358843 said:

sacalait » 08 Jul 2014, 16:54[/url]"]You probably have a spun hub. Take to a prop shop and have new bushing pressed in. Have same motor.

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I was thinking the same thing - spun hub. I had it happen on my 18' and it played out the same - plenty of thrust at low RPM but give it any gas and it can't take it.

I have a 76, awesome motors! Good luck, i hope its simple and cheap to fix!

Spun Hub!! He's going to see if they happen to have one at the shop, if not I guess I need a new prop. He said they might have one that'll fit. Or I may need to buy a new one. I see they list two different ones , 8 spline or pin drive. I'm thinking 8 spline since I don't recall a pin anywhere when I pulled the prop off.

https://boatpropellers.iboats.com/Mercury/20_HP_(1963-1981)_11.00-Pitch_Propellers/228/?cart_id=106165860

This is the first one I found. Does this look correct?

There are a few reputable shops that do mail-out replacements for pressed-in hubs (you mail your prop to them & they send it back with a new hub.) I know a friend recommended one to me, I'll ask him tomorrow & find out who/where it was. I believe they were in Florida, but I could be mistaken.

Breaking in a pressed in hub is tricky - you have to basically use it gently (barely above idle only) for almost a day before you can start pushing it again. I didn't get that memo, tried using it for the first time in a weed-choked pond, and immediately spun out the new bushing :*( Apparently there is still lubricant between the hub and the prop that needs time to dissolve or wash out prior to applying full torque to it.

Learn from my ignorance...

Thanks onthewater. I'd have never known about the break in period. I've seen a few "new" props for sale on different websites but I'm guessing when I call they won't have them in stock. Maybe I'll get lucky.

I've got an impeller kit on order, figured we'd get that changed while it's apart. Now which prop should I order? There's a two blade and a three blade available. Both are about the same price.

Found a place to re-hub the prop for $45 but they said they some times they crack. I'm guessing they are talking cast props not the stainless ones. Am I correct in this? Or is there still a chance a stainless prop could crack under the pressure? Mine happens to be stainless.

I suppose anything is possible, but it sounds like a much more likely scenario with aluminum than stainless.

When I had mine done I told them ahead of time that it was stainless, and I don't recall them warning me about cracking - not that this anecdote is really good for much. But then again they didn't warn me about the break-in period either so they probably aren't a good example.

Yep, the nice lady I talked to said there is a much less chance of it cracking a stainless prop.

Got the prop back from the prop shop with a new hub pressed in. Need to fill the lower unit with oil and test her out.