There are plenty of instances where perfectly good—and often rare—cast iron cylinder head and engine block castings are scrapped due to the presence of a hairline crack in an external surface. Often these fractures result after a freeze. Remember, water volume expands 10 percent when frozen—the jacking force can snap even a sturdy Hemi block in the blink of an eye.
In our case, we've got a healthy 440-based Max Wedge engine that runs just fine but has developed a nasty stain running down the driver side of the block. We located the leaking area—just below the exhaust port, and decided it could be fixed. To set it straight we took it to Donald Wood and the guys at RAD Auto Machine. Repairing a block using this technique takes a while, and depending on your shop, the cost might vary substantially.
Watch as we repair the crack without welding using a technique known as cold stitching. This technique is intended for low-stress surface cracks—not high load areas like main bearing webs or moving parts like connecting rods. The best use of this repair technique is to seal leaks in low pressure (100 psi or less) wet circuits—like an automotive cooling system. Let's get started.
It might be different for you, depending on the shop and the crack.
1. Looks like a blown head...
1. Looks like a blown head gasket, right? But since the engine runs fine and the spark plugs are dry (no coolant) we pressurized the cooling system. Sure enough, water seeped from this two-inch crack. Since we’ve got time invested porting this head, scrappage isn’t an option.
2. Instead of scrapping the...
2. Instead of scrapping the head, we’ll repair it with a process called cold stitching, where tapered plugs fill in the crack. Unlike welding, no heat is used so the threats of warped metal and burned skin are avoided. Readily available in many sizes from Pioneer, we chose 3⁄16-root-diameter plugs (PN TP-235) and a matched threading bit (PN 530235).
3. After Magnafluxing highlights...
3. After Magnafluxing highlights the length of the crack, use a pointed punch to make marks along the crack. Then drill through the iron with a 3⁄16-inch bit. Make sure you’ve found the absolute end of the crack since the rounded terminus left by the drill bit stops further growth of the crack. The only cracks cold stitching cannot repair are those with limited access. That’s why it is mostly used on surface cracks.
4. Next, the threading bit...
4. Next, the threading bit cuts threads into the hole to match the threads of the tapered plug. The best technique is to turn the bit clockwise a half turn to cut the threads into the hole, then back off a quarter turn to eject trapped chips. Cutting oil helps prevent burrs and extends tap life.
5. Before installation, each...
5. Before installation, each tapered plug must have its threads coated with sealant such as Teflon based Seal-All.
6. Turn the tapered plug...
6. Turn the tapered plug into the tapped hole until you encounter moderate resistance. This comparison of an installed plug and a raw plug shows that—in our case—about 1⁄3 of the total plug length is inside the head, the rest is remnant to be removed.
7. A cut-off wheel is used...
7. A cut-off wheel is used to remove the remaining plug and allow access for installing the next plug. Don’t listen to folks who say it’s ok to whack the plug with a hammer to snap off the remnant. It might have been safe with the larger plugs used in old time steam locomotive repair shops, but for smaller plugs, lavish a few extra minutes and cut them off gently.
8. The finger points to the...
8. The finger points to the desired clean cut. Note that the plug should stand slightly taller than the surface of the head. This makes the next step easier.
9. Follow the line of the...
9. Follow the line of the crack, and use a center punch to make a dimple to help start the drilling of the next hole.
10. Again we use the 3⁄16-inch...
10. Again we use the 3⁄16-inch drill bit to penetrate the head. Make sure the drill passes directly through the centerline of the crack and drill straight to avoid unwanted angles of the plug material.
11. Screw the threading bit...
11. Screw the threading bit into the drilled hole. Use the same half turn in/quarter turn out technique for every plug.
12. Next, the tapered insert...
12. Next, the tapered insert threads into the hole with Seal-All sealant applied. As you can see, the two plugs are pressing against each other (almost over-lapping), to insure the crack is sealed.
13. As we previously mentioned,...
13. As we previously mentioned, after trimming the remnant, note how the diameter of the second tapered plug overlaps into about 25 percent of the first one. This detail is critical for a successful outcome. If the plugs were set next to each other (with no overlap) remnants of the crack would still exist between each plug and allow continued coolant leaks.
14. We repeated the plug...
14. We repeated the plug installation process a total of twenty times to completely eradicate the crack. Again, note how each plug intersects its neighbor. We can’t express enough that this detail is crucial to a successful job.
15. The stubs left after...
15. The stubs left after trimming are removed by grinding. Use straight and tapered burrs to match the repair to the head’s contours.
16. Once the area is ground...
16. Once the area is ground flush, the surface is compacted with a pneumatic peening gun. This compacts the metal, further sealing potential leak paths. At this point, it is smart to re-inspect the repair with Magnaflux.
17. An abrasive disc restores...
17. An abrasive disc restores the as-cast surface and refreshes the exhaust gasket mating face.
18. Here's the finished job—prior...
18. Here's the finished job—prior to painting. Look closely and you’ll see that our repair stands a bit taller than the adjacent metal. That’s because the area is now thicker—and stronger. Some extra time spent grinding could deliver concours results, but we’re in no rush to make this area of our 452 head casting any thinner than it already is.