How can you successfully repair a cracked engine block by welding it—and is it actually possible?

Repairing a cracked engine block via welding first requires proper diagnosis. Given the engine block can be repaired by welding up the crack, certain precautions must be taken to ensure success. There are a variety of methods used in welding cracked engine blocks. Each process brings with it a unique set of techniques, challenges, and varying degrees of success.

If you’re curious to know more about welding cracked engine blocks, you’ve clicked on the right link. We will discuss the process involved in welding engine blocks to crank another day. Before we get into the specifics, let’s take a look at considerations you’ll need to ponder before breaking out any welding gas.

Are Repairs Even Worthwhile?  

Determining if you can weld the crack in your engine block takes a bit of detective work. In this process, you’ll decide which methods will work best and what tests you can use to check that the repairs have been successful.

But first, decide if you’re in for attempting the repair or not. Some engine blocks simply aren’t worth the time. If the block is common enough, sometimes buying a brand-new unit is the best solution. It all depends on your budget, how much time you have, and ultimately the intended use for the engine. Daily drivers won’t stress the repaired block too much necessarily. But fitting the engine into a platform that demands high output might be putting your fate into the hands of Lady Luck.

The extent of the crack you’re repairing is a huge consideration as well. While welding is generally considered a last-ditch effort, sometimes even adding metal to the mix won’t salvage the block. Heeding the rules of welding in general—as far as what can and cannot be welded together in terms of gaps—will likely steer you either towards repair or a new block entirely.

Diagnosing the Block

However, if you decide you want to go the repair route, here are some ways to diagnose the extent of the damage:

• Check areas of high stress, such as the area around the cylinder heads, valve seats, spark plugs, and generally anywhere else in the vicinity of the combustion chamber. Since the engine heats and cools in concentration in these areas, damage likely occurs here first. That’s not to say there could be parts of the block where the casting has gotten too thin to stand up to the varying temperatures and pressure.
• Magnetic particle testing allows for a quick diagnosis. When compared to other methods, it’s quite simple. As the name suggests, magnetic particle detect flaws in the object, based on the interruptions they create in the magnetic field. Because though you can potentially see the crack on the surface, it’s often harder to determine how deep that imperfection might go—or if it branches off.
• Pressure testing is one of the most common ways to determine leakage. Cranking the case up to 10-40 PSI will reveal leaks that occur at most operating temperatures. But while it can be easy to narrow down an area, cracks “may extend under the surface like the roots of a tree.”  

Most people often overlook the source of cracks, too. Determining this factor will not only help you decide on a method of repair but can also extend the life of your repaired block. As EngineBuilderMag.com advises, “it’s important to identify and eliminate anything that might have cause the engine to overheat . . . otherwise your crack repairs may be short lived.” Set up your repaired engine for success, because it beats the alternative.

Considerations in Welding Cracked Engine Blocks

Miller Welds identifies 4 common materials you’ll encounter when working on engines: aluminum, cast iron, steel, and magnesium. As with all welding, it’s important to account for the type of metal you’re working with before you begin any type of repair. And along with these metals come various considerations, pros, and cons.

Most engine blocks you’ll encounter are made from either cast iron or an aluminum alloy. Generally speaking, older blocks are often cast, while newer blocks contain more aluminum content. Let’s take a closer look at each metal.

Cast Iron

Known to be “quite strong, but also quite porous and brittle,” cast iron takes a large amount of patience and care when welded. Whereas aluminum transfers heat relatively well, cast iron retains heat and doesn’t move temperatures very quickly—at least without incurring damage. Poor castings are more porous by nature. This translates to a more difficult time welding.

When welding cast iron, it’s best to clean the block as well as you can before you even begin. This means soaking the engine block in boiling water to remove any leftover oils that may be present. Prepping the areas you’ll be welding starts here, too. After preparing the block, you’ll want to slowly bring the entire piece up to temperature. Miller Welds cautions, “Pre-heating the part to at least 500 degrees Fahrenheit is helpful—but be careful to not go higher than 1200 degrees.” Further cracking can occur if the block suddenly decreases temperature in one localized area, so it’s best to equalize the heat across the whole block.

Most cast iron engine block welding is done with a TIG. Filler rod “high in nickel content is recommended.” Miller Welds recommends using short passes, cleaning the part between passes to achieve a better result. Even though you’ll be moving inch-by-inch (literally) across the block, the idea is that you won’t have to do these repairs again. At least, for a long period of time. Another way to achieve better welds is to peen the welded area so that the hot weld gravitates towards those open pores in the casting. Just give the weld a few taps and you should be good to go. Bringing the entire engine block down to “room temperature,” as it were, in a methodical fashion will also ensure tight welds that won’t crack under strain.

Aluminum Alloy

You can find aluminum in many parts of your engine, “including oil pans, intake manifolds, engine blocks, cylinder heads, and valve covers.” And though aluminum and cast iron do behave differently when it comes to temperature regulation, there are common themes between these two metals. Let’s take a closer look.

Since aluminum is used as an alloy in most engine components, these castings “tend to be more porous, which impacts how long the material can be welded before oils and contaminate material is pulled into the heat zone around the weld.” Just like with cast iron engine blocks, you’ll want to start out slow and small with your welds. You’ll likely have to weld a short bead, stop, clean your weld area, and then start again. Temperature control is the name of the game with aluminum especially, since it conducts heat so well. And because you are working in a high-heat application, your choice of filler rod makes a huge difference. Purchase those in the 4000 series but avoid any with high magnesium or manganese content.

If you’re interested in seeing step-by-step instructions on how to weld a cast aluminum crank case, check out this article on ItStillRuns.com. There are plenty of YouTube videos to reference as well.

Steel and Magnesium Repair

The heart-stopping “ping” noise you hear when working on a cast iron engine is enough to crush anyone’s hope. However, with mild steel, the metal itself is more flexible. Casting brings with it inherent downfalls. Heat does not transfer well between areas and as such, MIG or TIG welding is the best route. In fact, the same routine you use to weld cast iron will work for mild steel, too.

In regards to magnesium, this metal is “lightweight, very strong, and less prone to cracking.” TIG welding magnesium is similar to welding aluminum. The metal will respond more quickly to temperature changes, which you can use to your advantage. At the same time, be aware that magnesium is highly flammable. And since filler rods for this type of metal aren’t as common as others, welding your magnesium engine block could be more expensive than performing the same restoration to a cast iron, aluminum, or steel counterpart.

These are just a few things to keep in mind as you weld your engine block. Before we delve deeper into the techniques you’re able to choose from, have you considered epoxy?

Epoxy as an Alternative Stitching Method

You’ve probably heard the saying that it’s held together by bubble gum and shoe strings—but have you heard of a cracked engine block held together by good old epoxy?

Believe it or not, it is possible to repair your cracked engine block with purpose-developed epoxies. These JB Weld-style compounds mimic the expanding and contracting that happens as the engine heats and cools during normal operation. Though they may not necessarily be comprised of metal as the block itself is, metal can be inserted. Repairs done with high-heat application epoxy are often referred to as “cold repair techniques,” since no heat is necessary as part of the fix.

The best part about epoxies—beyond the gnarly smell they put off—is that they cure within a day or so. That means you could set your engine out to cure overnight, go to work the next day, and come home to a usable block. However, epoxies will only work if they are used exclusively on non-combustion chamber areas. Cracks in the combustion chamber require a different technique.

Speaking of which, here is a short guide to the techniques you can use to repair your cracked engine block.

Welding Techniques Used in Repairing Engine Blocks

It may seem straightforward, but there are actually quite a few approaches to welding an engine block. Choosing between methods depends on your skill level, the extent of the crack(s), and the materials at-hand. As HotRod.com advises, “While the explanation of the operation is pretty simple to understand, it’s a delicate operation and is definitely best left to those with healthy amounts of welding experience.”

Metal Stitching

It’s hardly possible to run a needle and thread through an engine block, but what if the equivalents were metal studs? That’s about the idea with metal stitching.

One of the most well-known approaches to metal stitching is called Lock-N-Stitch. The process begins with diagnosing the extent of the crack and then using metal pins to “stitch” the opposing sides together. In fact, it’s a process Jay Leno has used on one of his engine blocks.

Begin by locating the end of the crack and drilling a hole. This hole is crucial to “prevent it [the crack] from spreading further when welding.” Lock-N-Stitch features a variety of dowel pin sizes, so you’ll need to choose the one that best fits your application. Then, use a supplied jig to drill similar-size holes across the length of the crack, at a preset distance. You’ll want to alternate along the sides of the crack for that “stitched” appearance—and for a better hold in the end.

After that, you’ll thread the holes for the specific pins you’ve chosen, and then insert them. Alternate between them and cinch down the pins before moving onto the spaces left. Once you set the pins in place, grind off the excess part of the head.

Repeat this process again, drilling holes between the already-installed pins. After you’ve completed this second round of pinning, the stitches you’ve created should be level with the face of the block itself. Peening the dowels will ensure the metal fills what used to be a crack.

Pros and Cons of Metal Stitching

Metal stitching, pinning, and/or studding refer to the same process. Stitching can be an easy way to repair your engine block without having to use welding gas or a welder. Plus, as long as you have a power screwdriver, drill, and a few other basic tools, you should be able to complete repairs on your engine block via this method in just a few hours, tops. “You can even repair the cylinder walls using this technique,” says StreetTechMag.com. And unlike when welding, you won’t have to clean or prepare the metal before you begin repairs.

However, one of the downfalls of this technique is that you might be introducing a different metal type into the equation. This is especially an issue in situations where the metal experiences drastic changes in temperature. “The problem in most cases is the same,” notes BlueDevil Products, “as with cold metal stitching . . . the wide range of operating temperatures in your vehicle’s motor along with the thermal expansion of the metal cause most repairs to only last for a short time.”

For these reasons—and many more—cold metal stitching often comes in handy when large, sometimes irreplaceable machines require repair. There are cases in which welding is not a viable solution. And in those instances, cold metal stitching can be a wonderful alternative.

Good Old-Fashioned Welding

Now we get to the good stuff, the part of the article you’ve been waiting for. Crack open those gas bottles because here we go!

Reference back to our discussion about the types of metal used in engine blocks. Remember, cast iron and aluminum alloy are the two most common types. When welding cast iron, you’re able to choose from furnace welding, flame spray welding, or brazing. Aluminum is simpler, since you can TIG weld it.

When furnace welding, the block itself heats up to 1,000-1,400 degrees Fahrenheit. That little hunk of metal should look red with anger once you pull it out. It’s best to use a filler rod that is similar to the engine block alloy to achieve a stronger weld. Bring cast iron down to temperature gradually, to avoid stress cracks and/or hard spots.

Flame spray welding uses some of the same techniques as furnace welding. However, it’s much “faster and easier than furnace welding, and can create strong and long-lasting repairs the same as furnace welding.”

For aluminum, TIG welding holds the key to successful revival. Whereas cast iron blocks need super high heat, aluminum only needs to preheat to 350-450 degrees Fahrenheit. Still pretty hot for aluminum but nothing like the 1,000-degree temperatures we saw earlier. As long as you can fill in the cracks with an alloy similar to what’s found in the engine block already, repairs should be more about adding metal than anything else.

Pros and Cons of Welding

Yes, welding up the cracks in an engine block can be possible. Well, as long as the cracks are located in a weldable area. Welding, when compared to alternative methods, makes a much stronger bond. Since part of the cast material melts with new blood (or metal, we should say), strength increases as you weld. Welding typically allows you to penetrate deeper beneath the surface as well.

That being said, there are some downsides to welding an engine block. Depending on where you do end up welding, you may need to check clearances once the block has cooled. Take the cylinder for example. If you weld near that, you may need to take action to square those measurements up again. It’s all about understanding where the heat of your weld goes and what distortions are there, if any.

Weld On and Drive Off

That’s about it for this article on welding your cracked engine block. We hope that you’ve enjoyed our discussion and urge you to click around. There’s always something new to learn when it comes to welding, so stay tuned for more! Plus, we appreciate you sharing your thoughts as well. Have you ever successfully welded up your cracked engine block?