Question about 1958 Mercury Mark 78

rebluz4

Recruit
Joined
Nov 10, 2011
Messages
1
I have a situation that has plagued me for almost 50 years. My family owned a Mark 78 and it was a
tremendous engine. We had it for four years and only had one problem with. It is this problem that I am hoping you can solve for me.

As I remember it, the Mark 78 did not have a shear pin. I think it had a slip clutch. Was that in the propeller or the propeller shaft?

One afternoon I hit a stump running about 8 - 10 mph. It didn't appear to do any damage to the prop or the lower unit. It still had its usual top end. The next week I went to start the motor and it was frozen. Naturally, I was accused of tearing up the lower unit.

I was never told what caused the motor to freeze. Could hitting an underwater obstruction cause the engine to
freeze?

I hope someone on this forum can help me finally put this mystery to bed.


Thanks,
 

Chinewalker

Fleet Admiral
Joined
Aug 19, 2001
Messages
8,902
Re: Question about 1958 Mercury Mark 78

If hitting an underwater obstruction cracked the housing in some way, allowing water in, then yes, it could cause the motor to appear locked up. It would take awhile for rust to set in (like the week you were away from it). Easiest check would be to drop the unit and see if the motor turns over then.

There wasn't a slip clutch, per se, rather it had a rubber hub in the propeller that was intended to either absorb the shock for a small bump, and slip if you really clobbered it. Once the hub slipped the prop needed to be either replaced or re-hubbed.
 
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