| Watts Shop Performance |
| Cranks |
| Cast-nodular-iron cranks are commonly used in production engines. OK for mild street use, they marginal in performance applications. Unfortunately, economical steel cranks aren't offered for every engine, particularly niche big-blocks like Buick, Olds or Pontiac. Those iron cranks survive as well as they do thanks to large-od main and rod journals that aid strength by both increasing the overlap area between the rod and main journals, as well as spreading bearing loads over a greater surface area. But big bearings don't do well at high engine speeds. Under 7,000 rpm, this can be crutched with lightweight rods and pistons. OEM forged cranks are usually made from 10xx medium-carbon mild steel alloyed with some manganese. A few of the better GM engines used a 5140 forging, which has some chromium added for greater fatigue resistance. But any production crank is subject to the vagaries of mass-production methods. The cost of reworking an OEM forging into a decent crank raises its price close to that of a premium 4340-steel cranks. In fact, small-block Chevy 4340 cranks are now in the $600 and up range. The 4340 nickle-chromium-molybdenum alloy steel cranks- whether forged or machined from billet stock- have great ductility, fatigue resistance and tensile strength when properly machined and heat treated. Right now, billet cranks get the nod over forgings because the billet can be custom- tailored for individual applications, with optimized counter-weight location and shape, plus superior control over heat treating process. Effectively making a high end billet crank 25- 50 percent stronger than an equivalent forging. Several methods have been developed to improve the hardness of a crank's bearing journals, including Tuftriding and gas nitriding. Old tech heat treating methods used to apply these coatings sometimes bent the crank, then the straightening process formed stress risers. Modern sophisticated ion-plasma nitriding minimizes these problems. |
| Reducing Crankshaft Bearing Rod and Main Bearings |
| Reduced friction is free horsepower. By reducing crankshaft rod and main journal diameters as much as possible through the use of crankshaft machining and custom rods, bearing surface area can be reduced for a combined drop in operating friction and oil demand. For example, cutting edge small block Chevy builders reduce the stock 350 style 2.100 rod journals to 1.888 to run Acura rod bearings in their custom made matching connecting rods. Though it would seem bearings would see greater average loading, Federal Mogul says that the reduction in bearing speed more than makes up the difference in bearing wear and many racers report improved life. Smaller bearings can also require less oil volume to maintain a protective film. Modern 500 cube Pro Stock motors safely use 2.000 rod journal diameters and those nitro guzzling Top Fuelers make do with 2.200 inch crankpins. A side benefit to smaller rod bearings is that the smaller big end is lighter, more compact and will more readily clear the crankcase in stroker applications. The crankpin becomeslighter, too. How can you take advantage of this with your engine? We can research the crankpin diameters and bearing widths, then determine if there's a common, smaller size bearing and rod combo that will work for you. For example, a small block Mopar has a 2.125 crankpin that might be able to be cut down to 2.000 inch and use small journal Chevy rods. Offset grind the crank for more stroke while your there. |
| Offset Grinding the Crankshaft |
| Examples the using the Chevy mouse motor. Mice came with two rod journal sizes: 2.000 inch for the early 283's and 327's and 2.100 inch for anything after 1968 or so. The beauty is that aftermarket rods remain readily available for the small journal size, so you can grind your 2.100 crank down to 2.000 to get smaller bearing sizes for reduced bearing speed and more power. While you're there, offset grind the crank to stroke it or destroke it for more or less cubes. If you start with a standard 2.100 crank, you can usually alter the crank throw by as much as 0.080 of an inch if you grind it to 0.100 undersized. The extra 0.020 is often needed for clean up. |