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Pinning Down The Cold Repair Process
By Brendan Baker
Cracks in cylinder heads and blocks present various challenges to rebuilders. When a crack is found a rebuilder must decide what the best method of repair will be.
Many machine shops don’t have the equipment and training to weld cast iron. One repair process, though, has been around for years and has proven successful, especially for cast iron.
Cold repair, also known as pinning or stitching, has been around the automotive world for quite some time. Cold repair was first used in the ’20s to repair industrial equipment, and by 1933 metal stitching had evolved in the automotive industry to using tapered cast iron pins to repair cracks in cast iron. In the years since, it has gained a less-than-positive reputation due more to poor training and lack of understanding than poor technology. However, if armed with the knowledge to properly repair these otherwise scrap-destined pieces, a cold repaired crack can last forever.
Ralph Picariello, technical director of Dresher, PA-based Seal-Lock International, agrees that pinning is, indeed, a permanent repair. "People who tell you that pinning doesn’t work say that because they’ve seen a lot of poorly pinned jobs. A good pin job will last just as long as any weld-repaired head or block."
A basic style of pin, or plug if you prefer, is a threaded tapered screw that comes in various sizes. The most common sizes, for automotive purposes, range from .150˝ to .240˝ with about 27 threads per inch and .025˝ taper. In addition, the diameter of tapered pins can range in size from .150˝ to 2˝.
Pinning has made a big difference in our shop," says Scott Wichlacz, owner of Manitowoc Motor Machining & Parts in Manitowoc, WI. "We have used the hook type thread for a long time as well as the tapered plug and it has helped us to repair things we would normally have had to weld."
With the cold repair process, it’s much quicker than welding and there are less inherent stresses on the casting. Welding cast iron is time consuming and often a frustrating experience for rebuilders. "You find that, especially with older castings, when you heat them up and weld the cracks and it cools off, there were more cracks that just weren’t opened up yet. Therefore, you end up welding them again and again. We try not to do that if we don’t have to," says Wichlacz.
Types Of Repairs
Cracks form all different shapes and sizes. However, there are really just two types of crack repairs that are of concern. The first type is a crack found in a part of the casting that is structural or requires strength across the repair to prevent the repair from separating. An example of this is a crack near a head bolt hole in a block or one extending from a bolt hole or freeze plug hole. Cracks found on the top side of a cylinder head that run crosswise can also be considered structural because of the load placed on the repair when the head bolts are torqued. Items to consider to help determine if a repair will be structural in nature include bolt torque, thermal expansion, function and the shape of the casting. One of the most common errors is to underestimate the spreading force caused by installing stitching pins on long cracks. According to crack repair experts, a crack over two inches in length when repaired with standard threaded pins can induce enough spreading force to cause a crack to extend especially on outside and inside corners and near edges.
The second type of crack is a seal-only crack. This type of crack repair has no structural role. It can be sealed, simply, using standard threaded pins. In order to meet this scenario the surrounding structure must have enough integrity and strength to contain the spreading force of the pins. Most short cracks in the combustion chambers of cylinder heads meet this requirement. Just remember that long cracks will change from being simple seal only cracks to structural simply by the total number of pins used.
According to Gary Reed of Lock-N-Stitch (LNS) in Turlock, CA, there are essentially only two types of pins that are used to address the multitude of cracks rebuilders may encounter: pins that spread and pins that pull. "When you look at doing crack repair there are all kinds of things that come into play – especially with cylinder heads. These are the most common cold repairs we see. Engine blocks are the next most common," says Reed.
Often times, when repairing a cylinder head, engine builders find that cracks run across the valve seats. When this happens: "if you use a tapered pin, you’ll have to install a new valve seat insert," Reed says, although he points out that with stitching pins that have a shoulder it’s recommended that you don’t install a new seat insert unless the old seat is worn too deep. "That’s a significant difference as far as time involved with the repair and it makes a better repair."
The biggest challenge that rebuilders have today in repairing cracks across seats in cylinder heads is that they were not designed to accept new valve seats, according to Reed. "Unless they came with replaceable seats from the factory, the casting is different on the inside," he explains. "Subsequently, when you bore out the valve pocket to accept the new seat, the corner of the pocket will be very thin. And when you go to drive in the seat the spreading force can be enough to open up the repair and cause a leak."
Not only are the castings thinner making it difficult to install a new seat without a leak, but a tapered pin that is threaded all the way tends to leave a line around the pin where it leaves a little thin flake of iron right where the thread comes to the surface. LNS approached this issue by adding a shouldered area to its stitching pins. This allows solid metal to sink into the seat surface, eliminating the line around the pin. Lock-N-Stitch provides a special countersink tool that matches the angle of the shoulder of the pin to assure that the threads sink below the surface. Reed also adds that with this type of pin there is no need to install a new seat in most cases, because the new seat can be machined right on the pin. It is impossible for the pin to burn out or wear at a different rate than the seat, even the exhaust seat.
Reed, who also operates a fully functional machine shop out of his California facility, says that one of the things that motivated him to come up with a different type of pin was trying to improve the old style tapered pin. "With a tapered pin you drill a hole and tap it (some ream the hole before tapping) and then you screw the pin in. If you tighten it too much and it breaks below the surface, it leaves a low spot, which isn’t pretty," says Reed. "But to avoid that, you have to cut the excess pin off with a die grinder or saw, which is awkward down in a port and wastes time. All of our pins have a predetermined break-off groove that assures the head of the pin will twist off leaving a small amount above the surface."
Though cracks in engine blocks are not as common as head cracks, they’re still prevalent enough to pay attention to. There are block cracks caused by freezing, normal operation, and heat and cracks caused by accidents.
Freeze cracks: These usually appear in the weakest places. These types of cracks occur most often in corners and in the middle of big flat spaces. Rarely do you see a freeze-cracked cylinder bore. These cracks tend to be long. If you use standard threaded pins, a cumulative spreading force occurs as you overlap the pins over the length of the crack. Lock-N-Stitch’s CastMaster pin pulls together instead of spreading. Therefore, no spreading pressure is applied and crack extension does not occur.
Heat cracks: The cylinder head is most susceptible to heat cracking. However, blocks do crack due to heat. Fire can be a cause; also, some older engines have exhaust manifolds that run very close to the edge of the block, which can overheat the block.
A difficult issue with heat cracked blocks is that if it was hot enough to crack, it probably hardened the cast iron as well. Hardened cast iron is a particular challenge because you can’t drill it and tap it. "One of the causes of heat cracks could be a previous weld," says Reed. "If you get heat related cracks that are too hard to drill and tap, you’ve got a problem. A remedy is to cut out the damaged area and stitch in a new patch. You have to pay attention to hardening because it affects stitchability."
Normal operation cracks: This is becoming more common in late model engines because the castings are thinner than they used to be. "We’re seeing lots of late model engines getting rebuilt at 100,000 miles that have cracks in them and they were never overheated," Reed points out. "This is indicative of a design flaw. Many castings are just too thin and after running through many thermal cycles, the casting relieves itself and cracks."
Head bolt holes and main bearing bolt holes are the most prone to cracking under normal operation, according to Reed. There is a very high amount of radial pressure created when a bolt is torqued into a hole. The threaded hole in these cases becomes grossly prestressed. Reed says that LNS has been able to remedy this problem for rebuilders with a thread insert called Full-Torque. It acts much like LNS’s CastMaster pin with a pulling force. It eliminates unwanted radial pressure on the cast area and encapsulates it in the insert instead.
Accidents: These are things like a thrown rod through the block or if a vehicle was in an accident, e.g., broken motor mounts. "One thing we see fairly regularly is the really big engines get dropped and big pieces break off," says Reed. These accidents are generally structural in nature. This isn’t always the case – a little window in the side of the block can be repaired with stitching pins. And with these large engines, the cost of a new replacement is so high that stitching is the best option. Profits for rebuilders doing these high-end repairs can be quite nice, too.
Elevating The Image
El Monte, CA-based Irontite by Kwik-Way, has been manufacturing threaded tapered pins since the ’50s. "We refer to our process as ‘cold repair,’ says Irontite by Kwik-Way’s Rich Wengatz. "We don’t refer to our process as pinning. The industry knows everything about pinning already. We’re trying to elevate the image of pinning. What we’re really doing is a sectional repair, replacing the cracked metal with new metal."
Wengatz and our other experts agree that following the basic principles of crack repair is usually the difference between a good crack repair and a bad one. And you start with magnetic particle inspection. Once the crack is found, you drill the ends of the holes. And – this is key, says Wengatz – you should magnaflux the piece again. If you’re looking at a 360° hole and the crack is right at the top (at 360°), you’ll want to inspect 359° opposite the break in the metal for any signs of a crack, especially water jacket areas, up to the surface.
"Many cracks occur from the water jacket side up to the surface, which are undetectable until you open the casting up," says Wengatz. "A lot of cracks happen in areas that can’t be seen. I’ve actually drilled into a combustion chamber following a crack that went 3/4 of the way around a valve seat. And the only way to find these hidden cracks is to remagnuflux it."
Wengatz recommends using a strong magnet to detect cracks. Coincidentally, Irontite by Kwik-Way manufactures one of the strongest electromagnets available at 92 lbs. lift. Most magnets are rated at about 30-40 lbs. lift, which can leave some cracks undetected. "We visited one shop that said our pins don’t work, but when we tested their magnet it wouldn’t hold 35 lbs. When we used a high powered magnet they could see all the cracks they missed before. It was the same head, the only thing that had changed was the magnet," says Wengatz.
Another thing that often is overlooked, says Wengatz, is remembering to use a reamer. "Some people ask: why do you ream the hole? Why can’t you just tap it? The reason is that a drilled hole is a square hole," he explains. "If we take a tapered tap and put it in a square hole we might have tight threads on top and shallow threads at the bottom. If you use a reamer you get more even threads when you tap it because you make the hole tapered."
In addition, it is a good idea to use a lubricant on your taps. Dry taps make inconsistent threads which pins can become hung up on and break off prematurely. Wengatz also recommends using a sealer around the threads before inserting a clean plug.
After the plug has been installed and cut off, use an air hammer to peen the plugs. "To properly peen you start from the center of the plug and work outward. You’ll see all the various metals blended together if it’s a good peening job," says Wengatz. However, you have to be careful because peening can add stress to the stitched area.
"Pinning is my first choice on cast iron before going into welding." says Seal-Lock’s Picariello. "Simply because of the fact that once you start welding on cast iron you must preheat it and you’ve got a lot of work to do after that. In fact, with cast iron as soon as you start welding, you’re asking for trouble. Not that you can’t do it, but it’s infinitely easier to pin a cylinder head than it is to weld one. If you make a mistake, you just drill another hole. If you’re welding, it’s another story."
Picariello has his own 80/20 rule in regard to crack repairing cast iron and aluminum. He says: "when you’re repairing cast iron, 80 percent of the time you’re going to use a process that has nothing to do with welding. Twenty percent of the time you’ll have to do some sort of welding in order to effectively repair the crack. With aluminum it’s just the opposite."
New cold repair technologies offer rebuilders other ways to repair blocks and heads that they may not have been able to do previously. For most shops welding is not an option, especially on cast iron. However, the cold repair process is inexpensive and relatively easy to learn, as opposed to the years of training and expensive equipment it takes to weld. Cold repair is also the most profitable type of crack repair. Often times it only takes a few minutes to permanently repair a casting.
There are several training courses offered by pin manufacturers. Lock-N-Stitch’s Gary Reed and Seal Lock’s Ralph Picariello both have classes that teach the finer points of cold repair. And, according to Reed and others, there also is a lot of profit potential in repairing non-automotive related equipment. But first you must know the basics.
Manitowoc Motor Machine’s Scott Wichlacz sent his most qualified technician to one of Reed’s courses and said it took him to a whole new level. Wichlacz says, "Training and education is everything if you’re going to try and make money at it." Wichlacz, also an Engine Rebuilders Association (AERA) board member, says there are funds available for AERA members to take training classes through the Engine Rebuilders Education (EREF) fund. All you have to do is apply for it. So, he says, "why wouldn’t you?"