Mercury (Force) Sport Jet 90 Cracked Cylinder Block Causes

[lilngineer]

Hi,

I have recently purchased a 1994 Sea Rayder with a Mercury [Force] Sport Jet 90. The history is that the previous owner was running the boat on the lake one day and suddenly had a loss of power and it was brought in. The engine was found to have low compression in the bottom cylinder and needs to be rebuilt due to a damaged piston [see pic] and a crack in the cylinder block near a port.

I plan on performing the rebuild shortly, but my question is what likely caused the crack? I spoke with the tech who examined the engine and he stated that the bolts for a cover 90 degrees to the cylinders (transfer port cover?) were loose [see pic]. If this is true, what likely caused this condition? Whatever the true problem was, I would like to ensure that the problem does not return with the rebuilt engine.

Any opinions/help is greatly appreciated!

Thanks,

Jason

 

[lilngineer]

Re: Mercury (Force) Sport Jet 90 Cracked Cylinder Block Causes

Also, one other thing...

If a suitable replacement block cannot be easily found as Mercury no longer supports the Force engines, can this block be welded? Isn't it cast aluminum?

Recommendations?

 

[Frank Acampora]

Re: Mercury (Force) Sport Jet 90 Cracked Cylinder Block Causes

The bolts are on the transfer cover. this covers the intake side of the block. Loose bolts may cause a loss of power depending on how loose. If it is the original engine with factory gaskets, those gaskets are self gluing and tend to seal even with loose cover bolts. This would not have caused a cracked block. When you dis-assemble toe block, show some photos. That may be a stock 85-90 sitting on a jet adapter. If that's the case, you could replace the block with a regular outboard block.
I couldn't see the crack you were writing about, however, if it is cracked at the port and the crack goes through the aluminum and the steel liner, forget about it. It will be more trouble than it's worth to repair it.

 

[lilngineer]

Re: Mercury (Force) Sport Jet 90 Cracked Cylinder Block Causes

I will post some additional pics once I have the engine torn down. Any additional thoughts on what might have actually caused the crack in the block if its not the transfer ports cover bolts?