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Heavy Duty Details
By Bob Kile
Old Days Are Gone; Now, Reconditioned Con Rods Must Meet Critical Specs
Too much twist and bend will limit oil clearances and can cause engine damage
I can remember in the old days how rods were reconditioned. We had a dry hone with limited mandrels. Rods were ground, assembled, torqued and dry-honed to final specs. Because there was no liquid, it took forever to recondition the big end. Pin bushings were removed then burnished and dry-honed to fit the pin bore and replaced using a vise as the press to install them.
We checked our bend and twist on an antique tester that had a red zone for rods out of alignment and a green zone for good rods that could be reused. The tester used a pointer with a spring-loaded bar on the bottom for straightness and a spring-loaded bar for the twist. Although we never knew what the exact bend or twist was, if the rod was in the green range, it wasn’t a problem to put it on the piston and in the engine. I guess you could call this a "go/no-go gauge."
We have come a long way since the old days. In today’s engines, it is extremely important that rods not be bent or twisted beyond factory specs. Using the old equipment never told us what the bend and twist was. Rods might have .006" bend or more and still be in the green range. By today’s standards most of them would be out of spec, but back then, many rods were never even checked for bend or twist.
Because of higher torque and horsepower requirements, there is more demand that rods be within the specification ranges for size, bend and twist. Oil clearances in the old days were greater, while today’s engines run a little closer with less oil clearance. Also, rpms have been raised as compared to the lower rpm engines of yesterday. They needed to run, not have power to run a 10-second quarter-mile.
In diesel engines the demand for more horsepower, torque and larger loads requires the engine to be perfect in rod reconditioning. Also, in most cases, rods should be machined back to the rod center-to-center specifications. For example, depending on the length of the rod, many diesel rods in the large engines, are limited to a .004˝ bend range and a .008˝ twist range.
Rods with a center-to-center from 10˝ to 12˝ or more are usually what are required in the above ranges. But if we look at an automotive rod that has a center-to-center of 4˝ to 6˝ we would surely try to keep our rods in a range of .002˝ bend and .004˝ twist. A rod with too much bend and twist will limit oil clearances from one side to the other or possibly lock up the engine at the pin end or thrust on the crankshaft.
At RoadBuilders, we build more diesel rods in the truck, agriculture, industrial, generators and construction equipment, but we occasionally work on small truck engines using press fit rods. In Photo 1, you can see the various sizes of the rods we normally recondition. When we recondition a rod, we use a cap grinder that has a floating head that can be offset at 10° or 20°. Photo 2, shows the rod being ground at 10°.
The rod and cap both are ground approximately .002˝. When the rod and cap are assembled and torqued, the rod will have about .003˝ to hone out of the big end. By grinding the cap and rod at 10°, the parting line will have a small amount of material, approximately .0005˝, to ensure that the rod is completely round. Usually when the rods have been reconditioned more than once, the rods will need to be ground at 20°.
In Photo 3, the rods are honed to specifications. It is extremely important not to grind too much off of the rod cap and rod. Too much material to hone can cause the rod to bell mouth or taper from side-to-side. Also, you can lose money if you take too much time to hone the rods because of too much material.
Now that the big end of the rod is to specification, let’s talk about pin bushing removal and installation. With a straight pin end rod, it requires only a tool to remove the bushing and install. It is always important to check the bore size of the rod pin end – you don’t want to install a bushing that is loose.
In many of the diesel rods, the pin end is tapered. As you can see in Photo 1, four out of these five rods are tapered. We make our own tools to remove and install tapered bushings.
Tapered bushings have to be installed correctly, because if they’re cocked, they won’t fully contact the bushing bore surface. This can cause the bushing to spin in the rod and possibly destroy an engine. In cases where the bushing does spin in the rod, we can machine the pin bore larger and make a pin bushing for the rod. This is normally done only when there are not rebuilt rods available and the cost of the rod is considerably above our rebuild price. However, in cases where new rods are $400 to $500 each, it is feasible to machine the pin bore and make a bushing.
Now that the pin bushings are in, we need to size the bushing. In Photo 4, the rod is in the machine ready to have the pin bored to size. All rods that have bushings are set in this machine on a center-to-center fixture. In all cases, the manufacturer will list in its specifications a center-to-center spec. This center-to-center is from the parting line to the center of the bushing.
We have machined the rod and now the center-to-center is short. The machine has a gauge that can set a rod from 4" to 22" center-to-centers. The D-9 rod pictured has a center-to-center distance of 15". Most bushings have enough stock in the bushings that will allow center-to-center to be maintained. In Photo 6, B Series Cummins rods are shown in the completed stage.
The rods are completed but not quite ready to give to the customer. In Photo 5, a 3204 Cat rod is being checked for bend and twist. Remember how important it is for a rod to be straight. If we find the machine did not cut the pin bore straight, we will install another bushing and recut the pin bore.
The machine that checks for bend and twist has three pointers that are electronically controlled by touch. You have a bend, a twist and a center-to-center.
The rod has to be set on center-to-center to ensure the correct reading because twist or bend could give you false readings. In some cases some rods are serrated, meaning that the parting line is stepped. The manufacturer will sometimes offer a .010" oversize bearing for rods.
In Photo 7 a D348 Cat rod has been set on a special fixture. The rod will be bored approximately .008˝ oversize. The big end will be finish honed as shown in Photo 3. The pin bushings will be machined as shown in Photo 4 and checked for bend and size as shown in Photo 6.
Crankshafts have a specific size between the "cheeks" to ensure that there is not too much side play. Too much side play could cause the pin end of the rod to grab the side of the piston and destroy the engine.
Rods that have worn into the thrust sides of the rods should be discarded if worn beyond specs. A small amount can be removed to straighten up the face on the sides. Every time you grind a crankshaft you increase the amount between the side faces.
Customers appreciate rods that can be installed in engines that cause no problems. I realize that all machine shops do not have the same capabilities and equipment, but you can still do the job right with certain machines.
If you replace the pin bushings, at least ensure that the proper bend and twist are correct. There are new machines that check the center-to-center and are affordable for the small machine shop. Doing the job right and knowing that our customers are important is an essential part of our business.
Bob Kile is manager of Roadbuilders Industrial Machining, a heavy duty machine shop located in Kansas City, MO. You may e-mail Bob at firstname.lastname@example.org.