Crack repair has become one of today's hottest topics because good rebuildable cylinder heads for many late model engines are getting harder and harder to find. Some are nearly impossible to acquire at any price. So many engine rebuilders are now repairing heads they would have thrown out only a few years ago as unsalvageable.
There have always been options for repairing cracks in cast iron. Pinning and furnace welding are both viable options that have long been used to repair cracks and other kinds of damage. But pinning has its limitations, and furnace welding is not something a novice can learn overnight.
Repair also involves a certain amount of risk. That is why most of the crack repair activity in the past was limited to high cost heavy-duty diesel heads, antique heads or other jobs that justified the time, effort and risk. Most passenger heads were too plentiful and too cheap to bother fixing, unless a crack was relatively minor and could be easily pinned. Even then, it was often cheaper to simply replace the head. But no more.
In todays high competitive market, crack-free cores are a scarce commodity. Thin-wall castings and higher operating temperatures cause many heads to crack, and often in areas that are difficult to repair. Cracks between the seats, that extend deeply into ports or run all the way across the top of the head are commonly encountered in many engines: Ford 2.9L V6, Ford 2.3L and 2.5L, General Motors 2.5L, GM 250 six-cylinder heads with integral exhaust manifolds, and 1987 and later Chevy small block V8 "Vortex" heads.
"Aluminum heads are relatively easy to fix, but cast iron heads are not," said Randy Neal, of Cast Welding Technologies, Norcross, GA. "For a long time, pinning was the only repair alternative available to most rebuilders. Pinning can stop leaks but adds zero structural integrity. Welding, on the other hand, restores integrity and allows you to fill cracks, holes or even change the shape of the metal. But hot welding (furnace welding) is a black art that is hard to learn. It takes a lot of heat and cannot be used on blocks because the casting will distort. It also takes more time to heat up and slow cool the head.
"The other alternative is powder welding (flame spray welding). The basic technique has been around for 40 years but it is been misapplied in the past. The nice thing about it is it gives you just as strong a repair as hot welding without as much heat. You still have to preheat and postheat the head, but not as hot nor as long. Only the weld area gets hot so you do not get the distortion you do with hot welding."
Neal says his company's approach to flame spray welding is the outcome of research and development work done for the OEMs on cast iron repairs, and is a refinement of previous methods. It makes use of special powders and a redesigned torch that makes it easier to get good results.
Flame-spray welding requires a special oxyacetylene torch with a trigger-operated hopper that feeds nickel powder into the flame. When the powder is exposed to the flame, it melts and fuses to the sides of the crack to fill the repair area. The process requires preheating the head to about 700 degrees, and post-heating the head after welding to relieve stress.
"We are able to achieve better adhesion and get a faster fill rate with our torch by carefully controlling the particle to fuel ratio in the flame. We also swirl and slow down the particles as they hit the work surface so they stick and do not ricochet off. This allows us to get in, fill a weld area and get out much more quickly.
"We can fill a three inch hole in less than a minute with our process, and without shocking the head. Hot welding the same hole would probably take at least three minutes, and then you would have to slow cool the head for seven or eight hours to keep it from cracking. We prevent cracking by post heating and stress relieving the head after it has been welded.
"With this process, five welders can do the same amount of production work as fifteen people doing hot welding. Welding can also eliminate the need to machine a head to accept a valve seat insert if the old seat is damaged. A seat creates a barrier that can inhibit the flow of heat away from the valve. You do not get that with this process. The high performance people really love it."
Neal says his welding powders, which are all nickel based alloys, come from Germany and are available in five different grades to match different applications. The base powder for general repairs provides a hardness of 18 to 22 Rc. Another can provide hardness from the high twenties to the low thirties for spray welding valve seats. There are also harder powders for welding camshafts and crankshafts.
Cast Welding Technologies offers training courses in flame spray welding, which Neal says is the key to getting good results with this process. The basic three-day course is $495.
WAS SKEPTICAL AT FIRST
"When I first heard about flame spray welding, I was very skeptical. But we have been using the process for two years now with a better than 90 percent success rate," said Eddie Browder of Eds Automobile Machine in Abilene, TX. Browder recently attended the Cast Welding Technologies school and said he was "very impressed" with what he learned there.
"The secret is 95% preparation, 4% welding and 1% Murphys Law. Every head is different, so you have to vary the procedure if the casting is thick or thin. It takes patience and previous welding experience to get good results."
Browder said he uses the flame spray welding process everyday on everything from diesel heads to high performance heads. He says he has had few problems with porosity or cracking. If a head did show porosity after welding, he blamed it on failing to prep the head thoroughly, not the welding process.
"We can now recycle cast iron heads that would have been taboo before. Being able to weld heads allows us to chase a niche market because we can fix heads other people cannot. The neatest thing about this process is that we can weld a head and be completely finished with the job in less than an hour and a half. Furnace welding can take seven or eight hours because it needs a long cool down period afterwards. This is also a much cooler process. We work the heads at 700 degrees instead of 1200 to 1400 degrees," said Browder.
Mike Schaefer at Southern Illinois Crankshaft, Red Bud, IL is another who has discovered the benefits of flame spray welding. Schaefer said he found the process to be a "little shaky" at first, but is now getting good results on diesel engine blocks.
"We are fixing blocks we would have thrown away a year ago. I wish I had some of those blocks back because now I could weld them."
Schaefer also had one of his people attend Cast Welding Technologies school. "We probably gained a years experience with that class," he said. "We still have to learn as we go, but now that we have a better understanding of the process it is much easier."
John Spangler of Franklin Power Products Welding Division, Peru, IN, says he is just now in the beginning stages of flame spray welding. "We have always used fusion welding to repair cylinder heads here, but I can see some real possibilities with this new process."
FLAME SPRAY WELDING: A HOT ISSUE
Those who have not had good results with flame spray welding say they have had problems getting the powder to stick or fill properly, or that heads often recrack because of the stress created by welding.
Marlo McGraw of Waterhouse Motors, Tacoma WA, said he tried flame spray welding three years ago with disappointing results. "If we cut a welded head to put in a seat, the weld would fracture. We also found the welded areas were hard to machine. The overspray was also difficult to get off."
McGraw says he tried different heating and cooling procedures but could not achieve consistent results with the equipment he was using. He says other rebuilders he is spoken with say they have had similar difficulties getting the process to work well for them.
"We found the torch was a high maintenance item. If you did not keep it doctors office clean, it would plug up and backfire. We also had problems with the plastic powder cup distorting and even melting from the heat."
"If a head needs repairs, we furnace weld it. We do not do any pinning. Furnace welding works well for us, but we cannot use it on blocks because blocks cannot take the heat like a head can," said McGraw.
Jim Anderson of Maintenance Welding in Laguna Hills, CA, says his company pioneered flame spray welding for the aftermarket. "If someone is not getting good results with the process, it is because they are not following the proper procedures. We have developed detailed tech sheets for specific applications to help our customers use the equipment properly."
"We have been using the process for four years and think it is one of the biggest blessings we have ever come across," said Jim Wright of Motorheads in Elyria, OH. Motorheads does mostly passenger car and performance engines and heads.
"We tried furnace welding cast iron heads, but just could not do it profitably. So when we saw this at an AERA show, we thought we would try it. We are glad we did because now we use it on everything."
Wright says with any type of welding, there is a learning curve. "It is no different than trying to TIG weld aluminum if you have never TIG welded before. It is not something a novice can pick right up. It takes practice. A beginner will either try to weld too hot or too cold. If the metal is too cold, the weld won't stick and you will get porosity. If it is too hot, you will get cracks.
"This process has a very narrow temperature envelope. When you put the torch on the metal, you have to be patient and allow the heat to creep into the weld area until it turns red. Within a couple of seconds the area will form a skin and start to sweat. So as soon as you see the sweat, you have to hit the powder to fill the crack. If you wait too long, the area will get too hot and you will overstress the metal."
Wright says he recommends flame spray welding with untinted safety glasses so you can see the metal as soon as it starts to sweat. "Once you get how to do it, there is nothing to it. You can weld in the combustion chamber, down the ports, virtually anywhere. You can create and fill port walls one eighth of an inch thick with no problems. The welded areas are the same hardness as the base metal so machining is not a problem either.
Wright admits that over the years he has had a few comebacks. But said it was not the fault of the welding process. He blamed it on failing to do a good enough job cutting out all of the crack prior to welding it.
Ralph Picariello of Casting Salvage Technologies, Falmouth, VA agrees that successful crack repair requires careful preparation of the crack beforehand, but does not agree that flame spray welding is the best way to repair such damage. Cast Salvage Technologies repairs heads and blocks, and also offers training courses and videos on all aspects of head repair including pinning and furnace welding cast iron as well as welding aluminum.
"Spray welding is okay for repairing cracks in the tops of Ford 2.9L heads and some other things. But if a crack can't be repaired by pinning, which 80 percent can, I prefer either furnace welding or braze welding. Braze welding works great on applications like damaged bolt holes and even cracks in head ports because it is a simpler process.
"If you look at the operating environment, the outside of most castings only run at about 200 degrees. Even the inside of an exhaust ports will only see about 400 to 500 degrees at the metal's surface because of cooling in the head. So I have successfully brazed cracks in exhaust ports with no problems at all.
Picariello says he likes braze welding because the process can be done at 800 to 900 degrees, which means the casting only gets to about 400 to 500 degrees and does not distort or melt.
"I have brazed the tops of Ford 2.9 heads very easily. You can see where the repairs were made because of the color difference, but other than that it is a good repair."
"If you are furnace welding a head, heat management is extremely important. You cannot just heat up a head with a weed burner. The right way is to preheat the head in an enclosed furnace or oven so the heat comes up through the head from the bottom. You have to keep the head insulated with a blanket and prevent any drafts while welding it otherwise you will get cracking and porosity in the weld. The head then be slow-cooled afterwards."
Picariello says the need for combustion chamber crack repairs may be lessened by the use of new high tech sealers. Many engine rebuilders currently use some type of sealer after pinning or welding a head as added insurance to prevent seepage or plug porosity leaks in the casting. There are many such products on the market, but none compare to a new sealer that Picariello says he is currently testing. "It is like putting an inner tube inside a head."
Picariello says the sealer uses a chemistry similar to that
used to coat the inside of underground pipes. The chemicals react and form a coating on the side of the cooling jacket that prevents leaks through small cracks in the combustion chamber and elsewhere. "You still have to drill and pin the ends of the crack to keep it from spreading, then peen it shut. But the sealer is what actually plugs the leak. If you are cutting the head to put in a seat, you do the internal coating afterwards."
Furnace welding cast iron is often called the "black art" of crack repair because it requires high temperatures and a lot of skill. Learning how to "recast" damaged heads is not something an inexperienced welder can pick up quickly. Those who have mastered the process say it takes at least six months to a year of constant practice to master it.
"If furnace welding was easy, everybody would be doing it," said Rick Geertsema, vice president and general manager of A & C Casting Rebuilders in Kelseyville, CA. "It is a hot job that does not exactly appeal to a lot of people."
Geertsema served his apprenticeship in the black art by learning how to furnace weld diesel cylinder heads for Northwest Motor Welding. Four years ago, he started his own head repair business. Today, most of the work he does is on passenger car heads, including a lot of specialty restoration work on older heads off classic cars and antiques.
"We repair about 80% of the cracks we see with pins. We mostly use tapered pins with a sealer. After the crack has been pinned, we circulate our own special ceramic sealer through the head at 180 degrees and pressure test it at 140 lbs. to force the sealer into any crevices or porosity in the casting. If a crack cannot be pinned, though, then we will furnace weld it."
Geertsema said the price of the head usually determines the type of crack repair technique he uses. "If it is a $200 head, we will usually pin it. If it is a $500 head, then it is worth the effort to weld it."
Geertsema said he may use either furnace or powder welding to repair a crack depending on the application. Either way, the first step is to fully identify the cracks, then grind them out with a hand-grinder.
If a crack is being repaired by furnace welding, Geertsema first preheats the head to 1300 degrees F (cherry red) in an oven. Other engine rebuilders we interviewed who also furnace weld heads say they use preheat temperatures that range from 1000 to 1500 degrees F. All agree that preheating is absolutely essential to minimize thermal shock and to relax the metal so it won't distort when the torch is applied to the casting.
When the head preheat temperature has stabilized (it takes about an hour), a "neutral flame" oxyacetylene torch with slightly more acetylene than oxygen is used to melt the cast iron (which melts at 2400 to 2600 degrees F). Geertsema says he uses a Tucker cast iron filler rod (Martin Wells is another good brand) and borax flux. The trick here is to keep the weld clean by adding a little flux so the impurities will rise to the top. The impurities can then be floated out of the repair area with the torch.
"If we are building up a valve seat, we will make a carbon graphite plug to fill the hole, then weld up around it. The puddle will be about half an inch deep and maybe two inches in diameter. It takes a lot of heat to do this, about 5000 degrees."
After the crack has been filled comes a long, slow cool down. This step is also absolutely essential to prevent the head from recracking. If cast iron cools too quickly, one of two things can happen. The surrounding metal can shrink away from the weld causing new cracks to open, and/or the carbon in the iron can turn to carbide making the metal too hard and brittle to machine. The casting must therefore be cooled very slowly to prevent these undesirable metallurgical changes.
Geertsema says he wraps the heads in an insulating blanket and keeps the head in a hot box so the head will cool at a rate of no more than 200 degrees per hour. Others we spoke with also stressed the importance of slow cooling but quoted cool down rates that ranged from 50 to 100 degrees per hour.
At this rate, it can take quite awhile for the head to cool down to ambient temperature: 8 hours to overnight. So one cannot be in a hurry when furnace welding cast iron heads. Geertsema says once the head has cooled, it is cleaned to remove the scale, then rough machined and submersion pressure tested in hot water at 100 psi for leaks.
Steve Spradley of Hills Cracked Block Service in Indianapolis, IN says after he furnace welds a head, a process he says generally takes anywhere from 40 to 120 minutes depending on the extent of the damage that is being repaired, he pressure checks the head three times: once after the head has cooled down and has been cleaned, a second time after the valve guides and seat outside diameters have been rough machined, and a third and final time after the valve seats have been installed. To some this might seem like a lot of unnecessary effort, but he has found that this procedure has virtually eliminated any chance of leakage.
Spradley says another step that is necessary after furnace welding a head is to replace the exhaust valve seats. The high temperatures of furnace welding destroy the induction hardening in the valve seats. So the seats have to replaced to keep the valves from pounding the seats out.
Spradley also said that furnace welding can be affected by a number of things, including ambient temperatures, drafts and humidity. If the preheat, welding and cool down temperatures are not closely monitored, you can end up with leaks and cracks.
PINNING CRACKS: A STITCH IN TIME SAVES NINE
Pinning is the most commonly used technique for repairing cracks in cast iron head, blocks and manifolds because it is fast, reliable and cheap. Pinning has been around for a long time and is familiar to most of our readers. It is a relatively easy technique to learn and use, does not require much in the way of tools except a drill, guide fixture and tap, and uses no heat. That is why almost everybody relies on pinning to one degree or another to repair many types of cracks.
There is not a lot of mystery to drilling holes in a crack, installing overlapping pins to fill the crack, then peening over the pins to seal and blend the surface. Even so, there are two basic methods of pinning cast iron: tapered pins and straight pins. Each type has its advantages and may work better in certain situations than the other.
Tapered pins pull themselves into a crack as they are tightened to provide a tight seal along the entire length of the pin. This occurs because the threads on both the tapered pin and hole have an interference fit. Sealer really is not necessary, but is often used for added insurance. The holes for tapered pins must be carefully hand tapped and the pins hand tightened for a tight seal. Straight pins, on the other hand, are faster to install because an ordinary tap and power drill can be used to install them. Straight pins are designed to twist off when tightened, eliminating the need to cut or grind the heads off (as is necessary with tapered pins). Straight pins also have an interference fit over their entire length, and a tapered shoulder that crushes into the surface. This eliminates a thread line on the surface enabling the surface to be machined. This type of straight pin can also eliminate the need for installing valve seat inserts in cast iron heads. Thus each type of pin has its place.
Ordinary tapered and straight pins cannot handle certain kinds of cracks, such as those along an outside edge or corner that require support to hold the sides of the crack together. The same goes for cracks in an area of a casting that would open up or pull apart when the casting is under load or gets hot. For these types of crack repairs, locking devices and/or pins with a self-gripping ability can provide the required reinforcement and strength. "Castmaster" pins (manufactured by Lock-N-Stitch) have a unique "spiral hook" or "reverse pitch" thread pattern that grips both sides of a crack when the top shoulder of the pin bottoms against the metal. This type of pin can actually hold a crack together rather than just fill it, and may be used to repair cracks that cannot be fixed with ordinary tapered or straight pins.
Another crack that is difficult to repair by pinning is any crack that occurs in very thin (less than 1/8 inch wall thickness) area of a casting. Pins will not hold unless there is enough metal to grip and support the threads.
TROUBLESHOOTING FLAME-SPRAY PROBLEMS
If you are having trouble achieving good results with flame-spray welding, here are some hints that may improve your success: