Can A Jb Weld Hold On A Crack On My Engine Block?and If Does How Long Can It Hold?

Screw it why not.  If JB don't do it throw it away. You really don't have much to lose trying it. I've used it for a lot of stuff like cracked headers and stuff I'd like to know if it works for engines, it would only have to hold against like 60-100 PSI so I say GO FOR IT!!!!

I have used jb on 2 engines and still going. [ I live in ohio and buy vintage cars to resale. I get a few with water instead of antifreeze and have had 2 cracked blocks. ] fixed both and also fixed one head I still drive the car today 4 years later!

Jb weld well hold if put on right,!Drill two hold one on both end of the crack an clean all the paint off around the crack .O by the way not big hole just a little biger then the with of the crack  and push it in and let it set for a last 24 hrs that it.

What usually cracks a block is either a defect or overheating due to lack of oil or water/coolant. Depending on the cause of the crack and the severity of it will decide if the engine will hold together or not. As far as the spark plugs that will only help in the starting of the motor.  jb weld makes a machinable epoxy that can be used in fixing engine parts so jb weld is some pretty strong epoxy and may last for a long time before it breaks again.

Sonny, why would you weld a block!? Anyway, it isn't going to blow up on you, but it WILL die! Changing the spark plugs isn't going to do anything for it at this point in time. Drive it till you kill it I guess. Heat, abuse, and age will kill a block, or a defect in the manufacturing process.
Either way, start the process for it's funeral o.k.?! Haha. Hope this helps, good luck.

Jim is right. While JB weld is wonderful stuff (you read this on the package) it does have its limits. If your crack is on a major and load bearing part of the block JB weld will at best work for only a short time.

Engines and block at junk yards are pretty cheap?