The connecting rod plays a vital role in the engine. But a connecting rod is under tremendous stress, with the weight of the piston sitting on top, changing direction thousands of times per minute. This continuous stopping and changing of direction combined with the weight of the piston and speed of the engine hammer on the bearings and torture the rod bolts which hold everything together.
Proper geometrical alignment and bearing surfaces that are smooth and perfectly round is the best way to ensure long engine life and happy customers. Any unwanted lobing, chatter or misalignment, particularly in engines that operate at high rpms, will affect the engine’s efficiency.
One of the most important aspects of rebuilding an engine is to recondition the connecting rods. There are many different types of connecting rods from cast steel to powdered metal “cracked” rods to all the various types of performance rods. The processes used to recondition this engine component may vary a little from one rebuilder to the next, but the end goal is always the same – straight rods with round bores. Sounds simple, but like anything worth doing, there’s always more to it than you think.
Typically, reconditioning rods involves cleaning them thoroughly, then checking them with magnetic particle inspection for any cracks. Then the rods are checked for straightness because any bend or twist in the rod may result in oil clearance problems and likely lead to a failure.
A visual inspection of the rods will also include looking for any signs of overheating, which may be indicated by a “bluish” appearance. If the rod has been overheated, its structural integrity may have been compromised, according to the rebuilders we interviewed for this article.
“If a guy has overrevved his engine, we go through and magnaflux all the parts,” says Kenny Burns, Harry’s Machine Works, Dodge City, KS. “Connecting rod material is typically pretty good, but sometimes the machine quality leaves something to be desired. Therefore, we check everything, even brand new rods.”
On the performance side, some rebuilders will check the hardness before declaring them fit for the junk pile. “You can test the heat treatment with a Rockwell tester,” says Roger Friedman, Dyer’s Top Rods. “We know what our rods should be. They’re usually in the 42-43 range on the Rockwell C scale. If a rod got hot enough to change that, it’s junk to us.”
Friedman cautions that color isn’t always a true indicator, however. “We’ll sometimes see some rods that were affected when an oil pump belt came off, for example. If you catch them soon enough, they will still turn color, but if the rods test okay, we will shot peen them, re-cut and resize them and reuse them.”
After the rod has been cleaned and inspected thoroughly, it should be put back together with the rod bolts torqued. With a stretch gauge, check each rod bolt for proper stretch. If the stretch is out of spec, then replace the bolt. While some engine builders say that it is safer and less expensive to just replace the rod bolts instead of measuring the stretch, others say that practice really depends on the application, because some high performance rod bolts can be quite expensive.
After the caps are torqued on with acceptable rod bolts, measure the big end bores. This will help you to determine how much to take off the caps and to what size you’ll need to hone them. In general, you want to take off as little material as possible to make the bore round again. After you hone the big end, measure the rod to see what size bushing you need to put in the pin end.
“The ultimate goal when reconditioning rods, is to come up with a set of rods that are straight and of the correct length,” says Jay “Dr. Diesel” Foley, of Foley Engines, Worcester, MA. “In common four- and six-cylinder gasoline and diesel engines, the rods must be machined back to original specs with no more than .0025″ of bend and no more than .00425″ of twist. A rod with too much bend will limit oil clearance from one side to the other and possibly lock up the engine at the pin end or at the thrust on the crankshaft. In addition, they must have round and concentric bores, and the fasteners must also be able to withstand the stresses of a modern engine.”
Once the connecting rods are bored and honed, then you can put the proper size bushing in the small end, which itself is a very important step. “We are very concerned about the piston-pin bushing relationship,” says Foley. “We always press out the old piston-pin bushing and install new ones. But that is only half the battle. To ensure that the bushing won’t rotate, we expand it to conform to the small end bore. To expand this bushing we press a hardened steel ball through the ID of the new bushing. This will lock in the new bushing and prevent it from spinning in the bore. If you heat the rod to install the new bushing, you should allow it to cool before you expand the bushing with this broaching technique. Then grind the cap to the correct center-to-center dimension and hone the big end and install new rod bolts.”
According to Harry’s Machine Works’ Burns, keeping the rod straight is very important. However, one of the difficulties his shop faces is finding aftermarket wrist pins that are the correct size. “Some aftermarket suppliers are making wrist pins that are supposed to be the same size as OE but they are not,” he explains. “We have had a hard time trying to get the right sizes. Sometimes the pins are .001? to .003? off the OE specs. Within that application we have seen differences of up to .003? and we’re trying to keep the tolerance within .0002″ to .0003″.”
Burns says that his shop uses a similar process to Foley’s for reconditioning connecting rods. Both Foley Engines and Harry’s Machine primarily rebuild diesel engines. Rebuilding diesel rods isn’t much different than gas engine rods, except they’re much bigger and the center-to-center distance is has to be exact. Since diesel engines operate on a compression cycle, rod lengths have to be correct. “We can shorten them or lengthen them,” Burns says about diesel rods, “it just depends on how much has been taken off the block. So the center-to-center distances are critical.”
Fractured rods are a fairly new phenomena. Ford was one of the first on the automotive side to use a fractured rod in the 4.0L engine, which was one of its first new generation engines in 1990. The fracture method has proven to be less expensive for manufacturers and it produces better quality because it is forged in one piece and then ‘cracked’ at the rod cap.
“Before fractured rods were invented, conventional rods were two components,” says Dave Hagen of the Engine Rebuilders Association (AERA). “You would have one piece, which was the cap and another that was the beam. The two pieces were close enough to bolt together; then you would have to do several machining operations to get the center-to-center distances correct. The fractured rod, on the other hand, is a powdered metal rod that allows the manufacturer to pop it out like an egg with very little machining to make the size exactly right. It comes out essentially the final size and then is broken at a scored line that is part of the design. Once it’s broken or ‘cracked,’ it’s done. It can be manufactured for far less cost than a traditional rod and it’s a more durable component.”
According to Hagen, the inside diameter of a fractured rod bore is scored and then some pressure is applied until it snaps. The resulting split is like a piece of china that has been broken. It has a very distinctive surface that custom fits together. The fracture has more surface area because you have peaks and valleys, and the alignment is more accurate since the cap only fits together one way.
For rebuilders, there’s not much you can do with fractured rods. You can’t cut the caps because of the unique break on each one. And for the most part, you cannot hone the bore because there are very few oversize OD bearings available for them. Hagen says that some suppliers carry oversize OD bearings for the big end of the more popular models, like the modular 4.0L and 4.6L Fords, but it’s uncertain if there are any bushings available for the pin end. So there’s a little bit of rebuildability with fractured rods but not much.
Now, some heavy-duty manufacturers are going to fractured rods. “There are some heavy-duty manufacturers making them, like John Deere is coming out with some now, but with fractured rods we can’t do much with them,” acknowledges Harry’s Machine Works’ Burns. “We measure them to check the size, and that’s about all we can do until there are oversize OD bearings available.”
For years, connecting rods have been honed on specialized rod honing machines, which are available from many leading manufacturers. These machines have been the standard, produce excellent results and are still widely used throughout the industry. However, Sunnen and Rottler have both recently come out with new systems that take a totally different approach to rod reconditioning.
Sunnen’s system is called the KGM-1000 Krossgrinding System®. The KGM system utilizes an easy-to-use computer control, diamond tooling and a feed system that gives the operator high accuracy and speed in the production of precision reconditioned connecting rods. The company says the system is extremely accurate for honing connecting rods, and is capable of holding very tight tolerances, achieving accuracies of .00001″ in straightness and .00015″ in roundness.
Rottler has also designed a completely new system that works with the F-65 and F-67A multi-purpose machines. According to Rottler’s Anthony Usher, the company wanted a system that could bore both the big end and the small end in one setup.
“When we decided to get into the rod reconditioning business,” Usher says, “one of the big problems we saw was that rods bend and twist. When you have two setups you can sometimes create other problems. We decided to design a system where a rebuilder could lay the connecting rod horizontally and set it up so both the big end and the small end could be open. With both ends open you can machine both ends in one machine and in one set up and achieve perfect parallelism between the centerlines of both ends.”
Rex Crumpton Jr., of Memorial Machine in Oklahoma City, OK, says his shop has both a Berco rod honing machine and a new Rottler system. According to Crumpton, both systems work excellent and he can achieve good results either way. Memorial uses the Berco machine for doing smaller stuff and the Rottler for reconditioning larger rods.
“Honing is excellent, there’s nothing wrong with doing rods that way,” says Crumpton. “But boring can be a bit more precise. You don’t have to worry as much about stones loading up, which could produce taper. As long as you have a good operator who is paying attention, both methods work fine.”