Undersizing Crankshafts


Your customers need clarity in terms of the old 10/10" myth. 
Smaller does not mean weaker. 


Unlike subjects such as engine displacement, garage space, swimming pools and computer hard
drive capacity, size doesn't matter when considering crankshaft journal diameters.  Chances are, some (or many) of your customers get a bit squeamish if their existing crank needs to be ground more than -0.010" on the rods and mains. By the same token, they may not want to buy a reman crank if it's been ground further than -0.010". Their reasons typically include concern about the safety of the crank (the smaller journals make the crank too weak) or surface hardness (gee, if it's ground that far, I'll lose the hardness and my crank will break). 

In a nutshell, that's sheer baloney.  For one thing, both cast and forged crankshafts are extremely over engineered from a strength standpoint.  The OEMs design all crankshafts with a wide margin of error, simply to ensure against any potential for crankshaft breakage. Yes, cranks do break from time to time, but not because they were undersized. If removing as little as, say .040" of material from the diameter of a main or rod journal makes the crank so weak that it will break, then the crank had some serious flaws to begin with, and the undersizing was not a contributing factor. 

In the old days (back in the 1930s and early 1940s or so), if a crank was ground to an  undersize, bearing makers compensated for the additional clearance by making bearings with the same thickness of steel backing as their standard bearings, but simply added to the thickness of the liner (Babbitt) to take up the additional clearance and to maintain the specified oil clearance. If the crank was a bit on the unbalanced or distorted side, the eccentric movement of the crank tended to beat out the thicker soft Babbitt lining, which created excessive clearance, which dropped oil film pressure, which took out the bearings and maybe the crank itself. Unfortunately, this old wive's tale has perpetuated over the years, and for absolutely no good reason. Today, an undersized bearing employs a standard thickness liner, with a thicker steel backing, so there's no excessively thick, soft, mushy liner to beat out. Besides, with today's bi-metal and tri-metal bearing construction, lining thickness becomes much less of a significant issue in terms of fatigue resistance issues. 

HARDENING LOSS? 

As far as hardening is concerned, cast cranks are usually not surface-hardened in the first place (something your customers probably don't realize). In practical terms, the only real advantage of a concentrated hardening layer is for bearing failure service. If you wipe out a bearing (due to a non-crank-related cause), it's easier to scrape the bearing material off of the journal during the repair and cleanup. that's it. In addition, if the hardening of the crank is so much of a concern, consider this: if the crank is placed in a situation where the bare journal contacts the rod or main bore, you have a drastic problem to begin with that is in no way a dimensional issue. In short, surface hardening is simply not an issue in terms of grinding a crank to a moderate-to-severe undersize. 


HOW FAR IS TOO FAR? 

Let's put it this way: if an established bearing supplier makes a bearing in a specific undersize, it's safe to grind the crank accordingly. The leaders in the bearing field are not about to offer a bearing for an application where it isn't designed to endure and perform. If XYZ Bearing Company offers a -0.040" rod and main bearing set, they know it will work properly, so there's no reason for alarm. They simply would not offer undersizes if they thought a problem existed in their use. 

Let's refer to the racing industry as an example of undersizing success. Many of today's leading, cutting-edge competition engine builders routinely grind their cranks as much as .300" undersize in order to reduce mass and the resulting frictional energy. A specific example involves some NASCAR Winston Cup engine builders who reduce connecting rod journal size by .210" to achieve a 1.88" rod journal, and use Honda-dimensioned connecting rods. If these guys are whacking off that much meat, and still running 8,000 or so rpm, there shouldn't be any concern about taking a mere 0.020" - 0.040" from the journals for any street or high performance crankshaft application. 

According to John Havel, noted industry bearing expert at Clevite Engine Parts, cast iron crankshafts sometimes exhibit a degree of shrinkage in the centerline of the crank, but if this is significant, it wasn't a good crank to begin with. There aren't many hardened cranks out there anyway. When grinding a crank, you might remove the tuftriding (this applies to a steel crank only) but you don't have to have extraordinary crank hardness to begin with.  In terms of how undersize bearings are made, the lining will not normally vary to a great degree. In most cases, a standard and a -.010" bearing will utilize the same steel back thickness. A -.020" bearing and a -.030" bearing will share a thicker steel backing, and a -.040" bearing will use an incrementally thicker steel backing.

Basically, bearings fall into three families in terms of steel backing thickness: standard and -0.010"; -0.020 and 0.030"; and -0.040".  The crankshaft journal fillet is the most significant element of the crank in terms of the reconditioning process. If a crankshaft breaks, it will occur in the fillet area, so attention to fillet profile is critical. However, this isn't because the crank is being undersized. Dimension has nothing to do with this concern. The fillet is the interruption area -- the transition, if you will -- between the journal and the rest of the crank. Naturally, a stress area exists wherever you have a corner or an edge, so the shape of the fillet is all-important.  

When a crank is ground (regardless of the degree of undersizing), the fillet must provide a smooth transition to minimize the creation of a fault-inducing stress riser. If the fillet is removed during grinding, resulting in a sharp 90-degree corner transition, the chance for failure is increased if subjected to enough flex and torsional twist under load, again, regardless of the journal diameter. If the transition area features a ground fillet (where the radius has been created by the profile of a stone), this needs to be maintained or re-created in a similar fashion. If the crankshaft journal fillets were rolled at the OE level, this involved an extreme compression (compacting) of the fillet area, where the metal was compressed under incredibly high force, to create an undercut and radiused roll area that is smaller in diameter than the adjacent journal bearing surface. If the fillets were originally rolled, the rule of thumb is that you simply cannot grind to an undersize that would bring the journal surface down to a smaller diameter than that of the roll. If you can grind the journal undersize by .040", .060", or even .300" without disturbing the rolled fillet, this is perfectly acceptable. Again, the concern is not the final
diameter of the journal, but the condition of the fillet. Examples of OE-rolled fillet cranks include the Cadillac 4100 series alum block V8 and the Ford 3.8L V6.  

The only problem with grinding a cast iron crank to a drastic undersize involves not the dimension as it relates to strength, but the nature of cast (nodular) iron itself. The surface finish is important, especially on cast cranks. Since this is nodular iron, the graphite in the structure is spherical in form. When you grind, you have a tendency to break into this graphite. If that roughness is not removed by a proper polishing job, you may create microscopic burrs that will abrade the bearing. It's critical to grind to a smooth finish, and to polish to a smooth finish. The best practice would be to polish in both directions anytime you grind a cast crank (although this is not done on a routine basis, it's nonetheless a good idea).  

In practical terms, because of the metallurgy of cast iron, the practical limit for a routine passenger car or light truck cast crank would be an undersize of -.040". Again, this has nothing to do with final diameter, and everything to do with surface finish. Many builders are able to use -0.060" bearings (depending on availability of that size increment for a specific motor) with no problems whatsoever. 

Caution: when a crankshaft is ground, some folks are tempted to touch the thrust face in order to clean it up and provide an acceptable appearance. However, care must be exercised, since this can create a wavy face if the wheel chatters. Also, if you grind with the side of the grinding wheel, a crisscross pattern may result, which is detrimental to maintaining a proper oil film. Since thrust faces are difficult to final polish as well, and since it is more difficult to achieve correct oil film on a flat surface as opposed to a radiused surface, the best advice is this if there's nothing wrong with the thrust face, leave it alone. 

In short, tell your customers that there is absolutely nothing wrong with undersizing a crank further than -0.010". They are not being short-changed by accepting a crank that's ground 10/10, 10/20, 20/10, 20/20, 10/30, 20/30, 10/40, 30/30, 40/40, etc. As long as the grinding is performed accurately and the fillets are shaped correctly, and the correct size bearing is installed, the result will be a perfectly performing crankshaft that can be used with confidence, both from a standpoint of performance and durability. simply put, there is no disadvantage to a crank that has been ground further than 10/10. If anything, the engine should run stronger, since undersizing effectively reduces frictional losses. 

by Mike Mavrigian 
May 2000 - PRECISION MACHINE SHOP