How cold can a big block motor, thats been run in fresh water, get before its damaged? The boat is on the trailer.

I assume that you are talking about a cracked block/heads. Local boat mechanic has told me <=28* for 5 or more hours is enough to break a small block. Not sure if big block would be the same.

I would think that it is not how cold but how long it was kept in below freezing temps. without being properly winterized. The only way to tell if there was any damage is to either fabricate a way to pressurize the coolant system or get the motor running and check for chocolate milk in the oil. Personnaly, I vote for pressurizing.

The water in the block would have to freeze solid to break the block and that could happen over night but my guess is it would have to be below 15 - 20 degrees. First thing to go is the frost (freeze) plugs. I popped one of those years ago because I didnt drain the block properly by using a wire to flush out the sediment in the lower drain holes. When the motor started in the spring, water poured out of the water jacket into the bilge. Replaced the frost plug and I was back in business. Lucky that time!
QuietStorm,
Do you need some info on draining your block?

Just remember that the "frost plugs" are not really that. Those holes are actually to provide an outlet for the sand used to cast the block, hence the name "core plugs". I've seen many engines, big and small block, crack at the pan rail and leave the core plugs intact.

Sutphil

Thanks for the offer but I have the manual. I wanted to know how long I could risk waiting because yesterday I was planning to sell my boat and didn't want to winterize it too early. Just in case someone wanted to take it for a trial run. I didn't buy the boat that I was looking at so I'm not going to try and sell mine now. Its too hard at this time of year. I'm going to winterize it this weekend. Playing it safe.

If you're not ready to winterize it yet, you could just drain the water out of the block and exhaust. This will get out most of the water and prevent freezing.