Yamaha 200A (crack in block?)

[bmtsa2003]

My mechanic (my employee as well) just rebuilt a Yamaha 200 hp 2 strk power head. New crank shaft, shaft bearings, new pistons, new pins, new rods with new bearings, new gaskets new everything. Yesterday 8 days later my operator is limping back to the dock with an overheating engine (also put new impeller during overhaul). My mechanic determined the problem was the new head gasket and claimed that I had purchased an inferior product from a manufacturer called MP (Marine Parts). Guess he believes that Yamaha actually makes gaskets. I aslo noticed that he had put silicone on the brand new gasket when he installed it. I am not a mechanic but this seems to be the wrong thing to do.

Anyway, he installed another gasket and 2.5 hrs. later the operator called that the engine was overheating again. Today back to the mechanic and now I am told the the block is cracked. I have just spent over $ 3,000 and now I have a cracked block?

Anyone have any suggestions what causes a block to crack? I know it was not from freezing since we are in the tropics.

[gonzo]

Overheating and then getting water to cool suddenly will crack a block.

[Bergalia]

Sounds like Gonzo's got it... Check coolling inlet and outlets. Could be inlet is semi-blocked and allowing only intermittent flow through the system. Two-strokes heat up very quickly - and perhaps a mere minute delay in coollant flow can cause that critical change in temperature.

[bmtsa2003]

Thank you Gonzo and Bergalia. I finally looked at the block myself and there is no crack. There is a small chip on top of the block exactly where the new head gasket had failed. Seems that I have to get the block refinished and put one new cylinder sleeve in. I gues I will have the heads done at the same time. I clearly can see that the mechanic should have noticed the problem when he reassembled the motor with all new guts inside. If not then at least at the time the first head gasket blew.

Another habit that this mechanic "had" was to remove the thermostats altogether (on more then one engine). Since we are in the Caribic and are running from the warm lagoon into a river that has much colder water this seems to be just asking for problems. Another bad habit was to disconnect the heat sensor. I finally fired him.

Karl