|Watts Shop Performance
| 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,
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.