Bigger is Better…Profit!
For the sake of this article, the term “heavy-duty” will apply to large diesel truck engines like Cat, Mack, Cummins and Detroit Diesel (up to larger engines with bores to 8 inches in diameter.) These large bore engines are designed to be rebuilt and are valuable enough to warrant repairing even severe damage.
The first rule in repairing these castings is that you should never do anything that could damage the structural integrity of the metal or the overall casting in any way. Inappropriate welding methods are the number one reason cast iron has a bad reputation for being a cheap, weak, and difficult if not impossible, metal to repair. From my perspective cast iron is the easiest and most predictable metal to work with. Yes it would be nice if you could just weld it as easily as mild steel but I wouldn’t have a job if it was.
1. Damage caused by an accident or an incident.
2. Damage caused by normal operating conditions.
(This type of damage requires the most attention. There are many cases where returning the part to original strength is not enough as the part failed because it was not strong enough to begin with.)
Category 1 repairs are usually very straight forward and can be engineered easily using standard repair procedures. Of course the repair must be strong enough or be able to seal at the required pressure. These repairs include heat cracks, impact damage and freeze cracks.
Category 2 repairs require investigation into the cause. In many cases the cause is unknown and the conclusion is that the casting was not strong enough to support normal operation. Examples are cracked and stripped threaded holes or damage not caused by accidents or incidents (category 1). If at all possible the cause needs to be understood. If you don’t determine the cause and then take steps to resolve the problem, chances are high the repair will fail because repairing it to its original strength is not all that is required.
This issue is frequently not addressed or even considered by the person who decides on the repair process which can be a very big mistake. There are lots of cases where the design, use, or application of the part will result in a failure while it is being used in normal ways.
Common causes include:
1. Core shift
2. Poor or insufficient design
3. Too thin
4. Poor cooling
5. Uneven cooling
6. Bolt holes too close to edges
7. Threaded holes too shallow
8. Casting flaws
9. Uneven mating surfaces
10. Used beyond design strength
On occasion there is no solution to the damage and repairs can only be temporary. Frequently, however, there are solutions if you spend the time to figure out what the cause was, you can design a compensating repair and prevent a future failure.
Remember if the cause of the damage is not corrected, the part will most likely break again. If an engine block cracked due to freezing, any repair you do will not survive if it freezes again.
If, however, an engine block cracks under normal use because the area is less than 1/8” thick, you need to deal with the thin area. If you don’t it will crack again. Sometimes you can cut the thin iron back far enough to find thicker iron, make a patch to fill in the hole and stitch around it. (Often reinforcements can be installed to strengthen weak areas.)
Over the years we’ve seen cases where many castings of the same manufacturer’s model will crack in the same spot with no apparent cause. These are usually category 2 repairs.
Over the past 20 years I’ve worked with many engine manufacturers to develop permanent repairs for both categories 1 and 2 that they use in remanufacturing their engines and components.
Repairing heavy duty engine parts can be more profitable than repairing automotive parts. They are frequently easier to work on because they are thicker and have better accessibility (simply because they are bigger). So, in the casting repair business — “Bigger is Better.” Yes, you have to be able to handle big parts, but a forklift easily converts into a portable crane.
There are two repair types to consider; assembled engines or disassembled engines. Assembled engines can either be repaired in-frame or mounted on a skid. In-frame repairs can include doing the repair on-site which involves setting up for mobile service. This has excellent profit potential. You may charge higher hourly rates and include travel time and expenses. Putting a machine back in service with minimal disassembly saves the customer big dollars in parts, labor and down time. This provides you an opportunity to share in the savings these repairs can offer. I much prefer this kind of work because I like the challenge and the rewards.
There is a higher volume of disassembled engines to work on because many cracks are found during overhauls and remanufacturing. These are usually easier to work on, plus working in your shop with all of your equipment readily available is the easiest. Having the engine disassembled does allow access to all external and internal surfaces.
In order to offer a complete casting repair service it is necessary to have many processes for the repair of cast iron and aluminum. I will elaborate on several of these with specific emphasis on why one method is chosen over another. There are many issues to consider including the cause of the damage, distortion, machining, reinforcements, previous failed repairs, cost, warranty and turn around time.
All in-frame and on-site repairs are constrained to using cold mechanical repairs including metal stitching, thread repair inserts, and mechanical reinforcements. These repairs are usually done on engine blocks and drive train castings like drop boxes on earth moving equipment. Smaller parts such as cylinder heads and manifolds can be removed and sent to the shop for repairs.
Disassembled engines are repaired in-house and include the same methods used for in-frame repairs as well as various hot processes including fusion welding, furnace brazing, and powder welding. Welding cast iron properly requires preheat temperatures over 900 deg. F. but metal spraying (not to be confused with powder welding) can be applied as low as 200 deg. F.
So how do you decide which method to use for each different type of repair process? First, determine and prioritize your objectives.
Not doing more harm than good is always priority #1.
2. Often it is to seal a leak;
3. Perhaps it is to fill a hole;
4. Maybe it is to build up a bore;
5. It could be to reattach a missing piece;
6. Or, to restore strength;
7. Might be to repair a damaged threaded hole;
8. Or, you might just want to fix a crack to prevent it from getting longer.
Once you have set your objectives, you need to decide the best way to achieve them. Of course having access to many different methods is most likely an option you don’t have so you will need to work within the capabilities of the methods you do have.
To put it in simple terms, all of the hot methods are the most difficult to master and the most costly to do, while cold methods are the easiest and least expensive to become proficient in. You most likely already have lots of experience in cold methods like installing valve seat inserts, valve guides, thread inserts, surfacing, boring, honing and many other common practices.
Of coarse there are repairs that are best done by using a hot method but the resulting distortion adjacent the weld affected area usually requires remachining to straighten main lines, cam lines and deck surfaces and the reboring of cylinder bores and counterbores.
Hot welding refers to processes that begin with preheating the part above 900° F. Because cast iron doesn’t stretch or bend and has a very high carbon content it cannot be cold welded without causing additional damage (below 900 deg. F). The two problems to pay attention to are confined expansion and contraction (which is the source of all heat related cracks and hardening which results from heating the iron above 1200° F.) and rapid cooling. Hardening cast iron reduces strength and makes it brittle.
The most important part to remember is that cast iron is stress relieved as it cools from 1800° F down through 1200° F. Cast iron melts at approximately 2400° F. All processes that melt the iron like fusion welding, powder welding, and electric welding (mig, tig and stick), require preheat of 1500° F.
Fusion welding is the process of using an acetylene torch to puddle the cast iron and add cast iron filler to weld cracks and buildup surfaces. Of course this is a very hot process and requires a very slow cool down.
Powder welding is basically the same process, except the filler metal, usually a nickel powder, is fed through the welding tip. The down side is that it has a lower tensile strength than the cast iron.
All electric welding processes have the potential to heat and melt the iron too fast. Using a gas torch allows the localized heating of the surrounding area to prepare it to be fused or brazed making it a two step process. Electric welding bypasses the second step which is why these welds fail. It simply happens too fast.
Brazing does not involve melting the iron so the preheat required can range from 900° up to 1100° depending on the complexity of the casting. Bronze melts at 1775° with a tensile strength of 70,000 PSI and is very close to cast iron in hardness. Brazing is the choice for structural repairs and buildup when the operating temperature is below 500°.
Bronze has a higher coefficient of expansion than cast iron. Using it in high temperature applications can result in the bronze separating from the iron. Bronze should not be used in combustion chambers, exhaust ports, and exhaust systems. These repairs are best accomplished with powder welding where high operating temperatures are present. These are the only repairs that we use powder welding for.
The structure of cast iron retains its shape extremely well even when involved in a high impact accident like a broken connecting rod. Because cast iron can’t bend or stretch, it simply cracks or breaks to release the impact energy. This differs from steel and aluminum which can absorb a lot of energy because they have the ability to stretch or bend when the force exceeds their yield point. This is why cast iron engine blocks almost always survive the event and only suffer localized damage. The rest of the block remains in alignment even with a blow out hole caused by a failed connecting rod.
Freeze damage never affects the cylinder bores or other critical alignments of surfaces other than where the cracks are. Heat that causes cracks is usually confined to a very close area around or adjacent to the crack. Cracked bolt holes are caused by the natural spreading force that always occurs when a bolt is torqued into a threaded bolt hole.
Category 1 repairs are usually straight forward and make up the majority of all repairs. Freeze cracks and blowout holes can look real bad but are actually quite easy to repair by metal stitching. Heat cracks add the issue of hardening the cast iron in some cases that can make it harder to drill and tap. High temperature preheat welding will anneal the hardened areas and return them to their original condition by reversing the hardening effect and restoring ductility and strength.
Repairing heavy duty engine and drive train castings can be a very profitable addition to your shop if you aren’t already doing so.
Cast iron is very predictable and great to work with if you don’t try to bend these rules.
Gary J. Reed is the CEO of LOCK-N-STITCH Inc. You may contact him at 800-736-8261, ext. 244 or e-mail email@example.com. Websites: www.locknstitch.com or www.fulltorque.com.
For a PDF of this article (complete with photos), go to: