If you do some research and listen to some of the old-timers, repairing cast iron can be done, but it is not easy. Welding cast iron takes serious skill, tools and long-forgotten techniques. The part to be welded must be preheated before welding and allowed to cool very slowly, otherwise the repair will just crack out again or worse, shatter. Stick Welding cast iron hardens the metal, so the surrounding metal remains soft. This creates a whole other host of problems, as the soft metal will fatigue where the harder metal is, failing again. There is a technique called Flame Spray welding which literally uses powdered metal and a blowtorch to melt the powdered metal to the cast iron. It is a tricky technique to learn and not many facilities offer this service anymore, but when done right it works.
You could always use an epoxy like JB Weld, as the package certainly says it is capable. In our experience, the stuff really does work, but would you want to trust the repair of a rare engine block with a $5 epoxy? There is another solution and the original principle has been around since cast iron itself. It is called pinning and it works.
There are several versions of pinning; smooth dowel-style pins (the original version), the home-brewed bolts and screws method, and the modern version called Lock-n-Stitch. The smooth dowel pins require drilling holes in the crack, tapping in brass or steel pins and then peening the ends to seal them. This is tricky to accomplish and takes some practice to master. This is the way it was done from the very beginning. Since pinning is so tricky, many of the old-timers modified the method by using bolts and screws to do the same thing, except the pins were held in with threads. The problem with this method, as well as the dowel method, is that this puts outward stress on the metal, which can further spread the crack, and if you have lost a section of block, forget about replacing it. Using these repair methods on anything other than a relative low-stress area will likely result in further failure.
Gary Reed, owner of Lock n Stitch, developed a new, modern approach to cast iron repairs. Based on the traditional method of pinning and stitching, the Lock n Stitch (or LNS for short) uses specially designed threaded pins to seal cracks and breaks in cast iron. The patented thread design actually pulls the metal together rather than spread it apart. This allows the LNS method to do things never before thought possible. LNS pins are commonly used to replace entire sections of engine block, piecing in plate steel to cast iron. In fact, the LNS method was used to repair the cast-iron dome on the US Capital building, which weighs in at over 9 million pounds. Had Pass and Stow had LNS pins, the Liberty Bell would still be ringing.
What this means for you is that should the motor in your 396 SS Camaro Pace Car convertible crack, you can fix it. High-stress areas like a broken main web can be repaired; the LNS pins will not fail if properly installed. The LNS website has examples of LNS pins in use in massive machinery that must withstand hundreds of thousands of pounds of force. The best part is that you can do it yourself.
We discovered this product after bringing home a ’69 383 Chrysler High-Performance coded motor for a street-strip project. Once we got it in the shop, we found a section inside the lifter valley that had been treated to a grinder. Upon further inspection, we found a 4-inch long crack. The block was purchased at a good price and since wasting money is never good, we looked for a repair solution. After we regained consciousness from our sunken hearts, the research began and we happened upon LNS. A few calls were made and we had our plan- fix it with Lock N Stitch pins in our shop.
The process sounds harder than it is- locate the crack and mark it (We used the SpotCheck kit from Maganflux), drill a hole in the block, countersink the hole, tap the hole with a special tap, insert the pin, and then drill another hole right next to the first, while overlapping the first pin, and repeat. In other words, this seems like it is going to take forever, cause a real headache and be very frustrating. Once we got into it, it was really easy. LNS provides a spacing jig that allows you to set a series of holes that are fully prepped and pins inserted, then you drill the holes that run between them to complete the task. In three hours, we repaired a 4-inch hole using 26 pins, from start to finish. Once the drilling and pinning is completed, the repair is ground flush and you even use a needle scaler to replicate the original finish so it looks like there was never even a repair made. If the repair is to be made in an oil or water galley, it should be pressure tested before being put back into service. You can even repair the cylinder walls using this technique, as these repairs can be machine bored and honed. LNS stitches are the best way to repair a cracked cylinder head or valve seat.
The possibilities with LNS are almost limitless. There are several types of LNS pins, and they can be used on any type of metal, including aluminum. This is a real solution to a problem that many restorers and hobbyists have faced. It really works and is much less expensive than the less-reliable Flame-Spray method. Lock n Stitch is available in separate components or in kit form, which is what we used. The kit provided the drill bits, taps, pins and even the liquid sealant (similar to thread lock) and tapping fluid. For about what the scrap yard would pay for a junk block, we fixed our 383 in just a few hours. Just think of the possibilities. How many times have you passed on a rare motor or cylinder heads at a swap meet because of a crack?