1987 120hp Broken driveshaft, again

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Just working on an engine that I put a replacement lower unit on a few weeks ago because it was cheaper than a new drive-shaft. The original broke on the fusion weld just below the water pump. The engine's back, with another broken shaft, exactly the same position. I have checked the power head bolts and they are all tight. I checked that the drive shaft is centred in the housing and turn ed engine to make sure it wasn't wandering around, all ok (did that even make sense?).....

I'm puzzled... Any suggestions? All, no matter how crazy will be looked into..

One more thing... Props fine, nothing was hit.

Chris........
 

Joe Reeves

Supreme Mariner
Joined
Feb 24, 2002
Messages
13,262
Re: 1987 120hp Broken driveshaft, again

Unfortunately you'll need either another lower unit complete or a driveshaft installed to do the following..........

First, with the water pump removed and with the lower unit in neutral, check to make sure that the driveshaft turns freely with absolutely no binding encountered.

This will be a test, no need to have the water pump installed or the exhaust housing extension is the engine is so equipped.

Now, with the engine of course in a tilted position, gently slide the lower unit upwards into the engine until the top surface of the lower unit touches the surface of the long exhaust housing.... DO NOT FORCE the lower unit up tightly! You can insert some retaining bolts to keep the lower unit from falling back off.

Now, holding the lower unit up (surface to surface touching), look closely to see if both sides are touching evenly. If only one side touches and it is necessary to force the lower unit up tightly to a normal installed position with the bolts, then that long exhaust housing is bent/warped out of line which results in a constant side pressure on the driveshaft.

The manufactuer allows an out of line variation of 10%. Anything over that is a problem. Unfortunately to find the out of line variation..... the housing must be removed from the engine and situated on a solid workbench in order to use the proper gauge to find the variation. Hellava lotta work!

The above condition normally only takes place if the engine has been hit from behind by some vehicle when trailering OR by hitting a huge underwater stump or floating railroad tie.

If no problem is found (both surfaces meet at the same time, driveshaft twirls freely), the driveshaft breakage must be considered a coincidence, a flaw within the metal, a shaft that simply could not withstand the sudden jolt of constant shifting, that sort of thing.

This is why the high horsepower driveshaft design was upgraded to a two piece shaft design, to get rid of that unforgiving rigid one piece problem shaft.
 

F_R

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Jul 7, 2006
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28,195
Re: 1987 120hp Broken driveshaft, again

Agreeing with everything Joe just wrote, but we should point out that the spacing difference from side to side will be very small, and you may eliminate it by pulling it up with the bolts. So don't be fooled, sometimes you can't see the forest for all the trees. The accurate way of course is to disassemble it and check it on a surface plate with a dial indicator---as mentioned, not an easy task.
 

Joe Reeves

Supreme Mariner
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Feb 24, 2002
Messages
13,262
Re: 1987 120hp Broken driveshaft, again

F_R.... Appreciate the back up. Just put that writeup into my database..... wouldn't want to have to type that again from memory.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: 1987 120hp Broken driveshaft, again

Thanks guys,

That was one of the things I checked when I put the replacement gearbox on the first time. When I slid the lower unit in I made sure the gearbox touched the exhaust housing evenly. I did... I think we just have a couple of rotten drive-shafts. I am putting a 2 piece in, should solve the problem...

Chris...
 

Joe Reeves

Supreme Mariner
Joined
Feb 24, 2002
Messages
13,262
Re: 1987 120hp Broken driveshaft, again

Yep, two piece shaft..... that'll do it.
 
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