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.