Driving with cracked block

[t.ducote2015]

But what if I have a hairline crack in the block. Could I still drive it or no. And if so how long could I drive it until I have to have it fixed

 

[BFG-9000]

The fix for a cracked block is generally to throw away the engine and replace it, with a used one if on a budget. Unless your mechanic intends to pull the head(s) and only change the shortblock, you are not risking anything that wouldn't be replaced anyway.

As you can even ask this question suggests the leak isn't too severe, it may be worth trying one of those sodium silicate (water glass) chemical block sealers. The only additional things you risk when going this route is plugging the radiator and heater core. As the cost of these are much less than the engine replacement, you may consider it worth the risk.

Generally, welding a cast iron or even aluminum block is difficult and costly enough that it's usually cheaper and more reliable to simply replace it. The exception is of course if it's an irreplaceable rare vintage item.