Merccruiser sheared drive???

I have a 1974 140 hp merc crusier ib/ob. I wrapped a cable around my prop as I was shifting in to reverse and have no gears anymore. I assume there is a shear key or some disigned weak point in the system to avoid major damage. I don't have the specialty tools to dig too deep into the drive and was wondering if anyone has any recomendations for me before I have to take it in and pay $100/hr for someone to tell me I was an easy fix.

Any help would be great

Thank

Re: Merccruiser sheared drive???

well you got your rubber hub that could have spun inside the prob. i'm new to boats myself but as i understand it the hub is pressed into the prop and that is what mounts on the drive from the lower unit. if you get a spun hub the shaft will be spinning the hub but not the prop. not really sure the best way to check for a spun hub but i would suppose there are many ways you can test for that.

Re: Merccruiser sheared drive???

500dollar744ti said:

well you got your rubber hub that could have spun inside the prob. i'm new to boats myself but as i understand it the hub is pressed into the prop and that is what mounts on the drive from the lower unit. if you get a spun hub the shaft will be spinning the hub but not the prop. not really sure the best way to check for a spun hub but i would suppose there are many ways you can test for that.

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Or, OH NO, it could be the engine coupler. The engine coupler and the prop hub ARE the shear points, per se, that protect the engine, drive unit. Prop hub can be replaced by a prop shop. Coupler is a whole new ball game and can only be replaced by lifting engine out, as it's bolted to the flywheel and couples the engine to the drive unit.. With engine off and drive in gear, can you turn the prop in both directions (will turn in 1 direction with a noticable clunking of the drive clutch dog is normal) or just one noisy direction?

Re: Merccruiser sheared drive???

i would hope the propeller hub is designed to fail with a lot less force than the engine coupler. i'd think if you wrapped a cable around the prop and sheared the coupler you've got other issues to deal with.

to the OP: did you have any issues with engine temp after this happened? if the engine coupler failed it would cause you to overheat since the driveshaft also turns the raw water pump.

Re: Merccruiser sheared drive???

A simple test to check for the spun hub syndrome is to place a mark on the prophub nut/lockwasher, and a corresponding mark on the prop. Then take the boat into the water and run under load (if possible...may just get past idle to do this check if the boat doesn't move in either gear). Then take out of water, or trim up to trailer position, and check for difference in the marks.

Drawback is that you have to do this in water, as you cant turn the prop by hand to check for this condition.

What did your engine water temp do after you lost your gears? If it rose, then youre looking at something like a blown coupling or splines, stripped upper gears, broken vert shaft (the older drives have a weak spot there), broken u-joint(s). Sure doesnt sound like this so far, so I'd do the prop test.