How to Build Killer Chevy Small-Block Engines. Mike Mavrigian
crankshafts for the SBC are available in a dizzying array of strokes, including 3.000, 3.250, 3.335, 3.480, 3.500, 3.562, 3.625, 3.750. 3.800, 3.875, 4.000, 4.125, and 4.250 inches.
Standard deck height for a small-block Chevy block is 9.025 inches. If a specific stroke, rod length, and piston CD combination exceeds stock deck height, a tall-deck aftermarket block is required to accommodate the extended distance from the main bore centerline to the piston dome. While stock deck height is 9.025 inches, aftermarket tall-deck blocks are available, usually with a deck height of 9.325 inches, permitting a longer stroke and longer rods.
Deciding crankshaft stroke involves several factors, including the physical dimensional variables of rod length, piston compression distance, block deck height, and the desired operating characteristics. Speaking in general terms, a longer crankshaft stroke provides increased torque, while a shorter stroke provides the ability for the engine to generate higher engine speed (RPM). For example, a drag racing application may call for a longer stroke, while a road race application may call for a shorter stroke.
| Aftermarket Bore/Stroke Combinations | ||
| Engine CI | Bore | Stroke |
| 302 | 4.000 | 3.000 |
| 327 | 4.000 | 3.250 |
| 346 | 3.900 | 3.620 |
| 350 | 4.000 | 3.480 |
| 355 | 4.030 | 3.480 |
| 364 | 4.000 | 3.620 |
| 377 | 4.155 | 3.480 |
| 383 | 4.030 | 3.750 |
| 406 | 4.155 | 3.750 |
| 410 | 4.130 | 3.800 |
| 414 | 4.125 | 3.875 |
| 427 | 4.125 | 4.000 |
| 434 | 4.155 | 4.000 |
| 441 | 4.125 | 4.125 |
| 447 | 4.155 | 4.125 |
| 454 | 4.125 | 4.250 |
We also need to consider crankshaft weight. A lightweight crank, due to a decrease in mass, allows the engine to rev quicker, which is an advantage in drag or sprint car applications. However, in racing where endurance plays a major role, a heavier crank that is not highly modified for weight reduction can provide increased stability with less harmonics, providing increased durability for long runs at high engine speeds, especially where engine RPM doesn’t vary a great deal as the engine tends to run at a fairly consistent RPM. As you can see, choosing the crank stroke and weight involves a variety of factors.
In a small-block Chevy build, you have the option of running 350 main journals or the larger 400 main journals. Today’s aftermarket blocks are available with either main bore size. A crankshaft with 350 mains will feature a main journal diameter of 2.450 inches, while a crank with 400 mains will have a main journal diameter of 2.650 inches. Builder preferences differ depending on their experience and opinions. The small 350 main results in a lower bearing speed, which is preferred for better oil delivery to the bearings. The larger 400 main crank, while slightly beefier, results in additional loss of block material to accommodate the larger journals. My preference is the 350 main.
Always keep in mind that increases in the crankshaft stroke decrease the clearances between the connecting rod’s big end and the block and between the rod’s big end and the camshaft. In terms of cam clearance, rod big ends designed for stroker clearance are vital. They often require rods that feature shorter rod cap bolts, which lowers the profile of the big end’s shoulder. This is the reason that aftermarket block makers offer raised cam blocks that position the cam tunnel about 0.300 inch or more, providing additional cam to the rod’s big end clearance.
Just as the dynamic effect of aerodynamics plays a role in how the vehicle cuts through the air at speed, the profile shape of crankshaft counterweights can affect windage drag inside the crankcase. Although the crankshaft counterweights don’t rotate through the sump’s oil bath, oil that drains down from the top back to the oil pan wet sump or even a dry sump’s pan can drain onto or across the counterweights. The counterweights don’t need oil and should be kept as dry as possible to avoid unwanted parasitic oil drag.
Many builders prefer to knife edge the counterweights, which typically involves creating a rounded/radiused nose on the counterweights’ leading edge, tapering down at the trailing edge, very much like the cross section of an airplane wing. This allows parasitic oil that tends to cling to the counterweights to skim over the counterweight and evacuate quicker, theoretically reducing oil cling and drag, providing what you might call a slipstream effect. While not necessary for a street-driven performance engine, profiling the counterweights can often provide an advantage in a high-revving racing crankshaft.
Another method of reducing drag caused by parasitic oil cling is to have the counterweights treated with a slippery specialty coating that prevents oil from sticking to the counterweights. Specialty coating firms, such as Swain Tech Coatings, PolyDyn Performance Coatings, and others, offer these services for racers who are looking for every possible advantage.
Many aftermarket performance crankshaft makers gun drill a hole through the center of the mains to reduce weight. This photo was taken during the manufacturing process at the Callies Performance Products factory.
OEM factory crankshafts were notorious for journal oil holes that featured no chamfering and had sharp edges. Performance builders commonly addressed these ports by softening the hole edges and grinding a chamfer to promote better oil flow. Today’s aftermarket cranks are already prepped and generally have no need for further modification as seen in this main journal example.
While older OEM factory cranks featured a square-cut fillet at the journal-to-counterweight intersection, aftermarket performance cranks feature a radius transition. This provides superior strength, vastly reducing or eliminating the potential for a stress fatigue.
Rod journal oil holes are also lightly chamfered for better oil delivery to the bearings. Addressing and improving oil flow is just one of the features that aftermarket crank makers provide, eliminating modifications normally needed when dealing with yesterday’s factory cranks.
The trailing edge of this counterweight features a knife edge cut. This slows an increased