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1999 25 horsepower mercury locked powerhead help

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Aaron91386

New member
I have a 1999 25 horsepower 2 stroke mercury that has a locked up powerhead. I have stripped the powerhead completely down and have found the bottom cylinder in rough shape. I was wondering if anyone knows if the block can be bored out to remove any imperfections ( i was told no by a pessimistic co worker). Also wondering if anyone knows of a machine shop to do the necessary repairs. Any info at all on this subject is appreciated.
 
You should have a Mercosil block(embossed on top of block) and its a boreable material but, it takes special tooling and honing procedures and piston and rings. Some motorcycle/watercraft shops can do this and is kind of pricey if done correctly. Overheating and exhaust leakage kill these blocks....
 
I found a machine shop in Wisconsin to do the work called MAG machining, they were recommended by Mercury themselves. Cost for parts and machining was about 400 dollars. I had to buy a few more odds and end parts that drove the grand total to about 550 dollars. Just got everything assembled and is running good with compression on both cylinders right in spect according to service manual. Very happy with my decision to rebuild powerhead myself, saved a lot of money and had fun doing it.
 
As someone previously posted, exhaust leaks are killers. I have an 88 20 hp. I posted here for advice about it not idling. One thing I found while troubleshooting was milky oil on only my bottom plug. Not even knowing the cause I started to tear into the motor. Thankfully I had no damage to the cyl yet and both cyl are at 120. Come to find out my exhaust manifold seal just started to fail.

If if it had gone on running like that I'm sure it would have had the same fate as yours. The water washes out the fuel and oil, and as most know that oil is essential in these two strokes.
 
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