How to Seal a Cracked Block With JB Weld

by Kevin Mclain

If a engine block develops a crack, JB Weld can be used to seal up the crack and prevent the crack from sucking in air or leaking coolant. JB Weld is a type of epoxy that seals and hardens to close cracks and holes. JB Weld is made up of a resin and a hardener. When the resin and the hardener are mixed together, they form a sealant that can be used on the damaged area. Once the hardener begins to harden inside of the resin, the epoxy forms into a solid weld inside and around the damaged area.

Step 1

Park the vehicle on a flat surface and engage the parking brake. Turn the engine off and wait and hour or two for the engine to cool off.

Step 2

Raise the hod and lock it in place. Locate the cracked area of the block. Use the wire brush to brush the paint or rust off of the area around the crack. Be sure to brush at least one to two inches away from the crack, all the way around the crack. When finished, there should be a one- to two-inch clean area all the way around the crack.

Step 3

Wipe away all of the paint, rust and dust off of the crack and the surrounding surface. There should be no paint or rust left on the area around the crack. Once the area has been properly brushed and wiped clean, apply a generous amount of rubbing alcohol to a clean rag. Wipe down the entire area around the crack and inside the crack. Wait a couple of minutes for the alcohol to dry.

Step 4

Open the JB Weld Epoxy kit and remove the hardener tube and the resin tube. Turn the empty box over and squirt half the tube of resin onto the box. Then squirt about half the tube of hardener on top of the resin. Use a stir stick to stir the hardener and the resin together until the entire mixed epoxy is the same color.

Step 5

Slide the flat scraper under the mixed epoxy and spread the epoxy into the crack first. Press the metal scraper downward to force the epoxy into the crack. Apply the remaining mixed epoxy to the one- to two-inch area around the crack. Be sure to press the epoxy into the crack and around the area of the crack with the scraper. Leave the JB Weld on the cracked area over night to ensure it will be hardened.

Step 6

Inspect the hardened epoxy the next day to ensure it has hardened around the crack. Crank the engine and let it heat up to operating temperature. Inspect the epoxy to ensure it stays welded to the block. Turn the engine off.

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