A cracked engine block can be devastating to your car--not to mention your wallet. Depending on the extent of the damage, there are several repair options that range in cost and difficulty. However, welding or rebuilding an engine block should only be done by qualified personnel.
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There are a number of sealants on the market. K&W Engine Block Sealer and SteelSeal are among the choices. Although directions will vary a little with each sealant, you basically need to drain your radiator and then pour in the sealant. Afterward, you need to run the car for the prescribed amount of time, and then drain the excess sealant and refill the radiator. The SteelSeal website says that for its sealant you only need to run your car at 1000 RPMs for 30 minutes, while according to My Honest Mechanic, you have to run K&W in your car for 500 miles. The problem with that is that for 500 miles you don't have any over boil or antifreeze protection.
According to the website Old Marine Engine, metal epoxy can be used depending on the size of the crack. Two of the most common epoxies are JB Weld and Marine Weld. The epoxy can be used as a putty to fill the crack. Most epoxies have a 24-hour cure time. You also need to be sure that the epoxy you use is suitable for the high temperatures in your engine.
Although many people think that cracked metal cans simply be welded, that isn't always the case. With a cast iron engine block, welding is difficult because the whole engine block has to be heated to about 1000F before welding can even begin. Often, cracks will return after the block has been repaired.
Depending on the location, size and severity of a crack, the engine block can be rebuilt. But, this should not be attempted by the average do-it-yourself mechanic. Furthermore, rebuilding an engine block is one of the more expensive options.
There comes a time when the car owner must decide whether or not the car is worth repairing. It is always wise to get several estimates before having major work done. But, depending on the age, value and condition of your car, rebuilding an engine (or even replacing just the block) may cost more than your car is actually worth.