300 Merc rebuild

[Skeeter Ty]

I have recently had to rebuild my 1998 300 Merc 2-stroke. I now have 2 cylinders bored over and sleeved. What is the likely hood that I will have issues with this in the near future? Before the rebuild the motor only had 165 hours on it.

[Nikon Man]

Need allot more info. Which 2 cylinders failed, what was the cause of the failure, how we're you running when the failure happened etc, etc, etc...... Pics of the cylinders, pistons, heads etc, etc, would also be very helpful.

[TheRickster]

If the issue that caused the failure was not found and resolved it will repeat.

Rick

[Skeeter Ty]

I purchased the boat in July and had it out on the water for about an hour when the trouble started. I was running around 4k to 5k rpm and it began running rough cutting out ect. Took it to mechanic and the spark plug electrode in #6 cylinder had broken off and scarred up the cylinder wall and washed out the piston. He rebuilt the motor and I put the boat in the water and was starting the break in period but it only ran for ten minutes and the motor LOCKED UP. He said he ran it in the shop for 30 min, so in less than hour it was locked up. When I took it back we opened her up and in the #2 cylinder one of the bearings had fallen out while he was putting it back together. Of course he is standing behind his work and is currently rebuilding it again, but my concern is if properly rebuilt how long will it last? Is it definitely going to cause me problems?

[EuropeanAM]

Need allot more info. Which 2 cylinders failed, what was the cause of the failure, how we're you running when the failure happened etc, etc, etc...... Pics of the cylinders, pistons, heads etc, etc, would also be very helpful.

+1... the cause of the ORIGINAL failure must be located and addressed.

STRONGLY recommend that the entire fuel system be gone through (pump tank, start with only FRESH, high-test fuel), lines, pumps, VST and injectors, etc. ALL be serviced.

Oil injection system cleaned out, filled with fresh oil, tested and confirmed functional.

Failure to address the potential causes (or determined causes) will result in a repeat of previous history.

[ChrisCarsonMarine]

The motor should perform well and live a normal service life as long as the cause of the first failure is corrected,and no mistakes are added.I believe the question you're asking is wether or not it's ok to have two cylinders oversized,while the other four remain standard.Although not my preferance,this is an acceptable practice for a repair,and is permissable by Mercury,and in fact is often seen in their reman motors.If the rest of the job is done correctly you should be fine,Chris

[Skeeter Ty]

Thanks to all! I guess I'll give a shot.

[Nikon Man]

There is no crystal ball that can tell you the hours you'll get as service life. Properly maintained and in good rebuilt condition to start with can net you in excess of 800 hours. I say that because I know of 2 300x motors that have lived a good life that long. But anything can happen to change that, bad load of fuel as an example will do you in quickly, I speak from experience here............