Connecting Rods
Connecting rod failure is rarely less than catastrophic. Fortunately, factory rods are fairly stout pieces but carry a lot of excess balance-pad mass. With work, they can be lightened considerably. Also, you can install a set of ARP bolts. But subsequently, your local machine shop needs to cut the caps and resize the big-end bore.
Piston and Rings: Final Specs
Many factors allow a good piston selection to become a great one. Here are the points you need to keep in mind:
• Select a piston and pin with the minimum weight to make an internal crank balance more easily.
• Maximize compression for the fuel octane to be used; first by minimizing cylinder head chamber volume and then by using a piston with a suitable crown.
• Use the thinnest rings your budget allows.
• Be sure the piston crown conforms to the porting specs.
• Deck the block to achieve a net quench clearance of 0.032 to 0.037 inch.
• Do not shroud the spark plug. (For further information see Chapter 11, Ignition Systems.)
• If the cam you intend to use has more than 280 degrees of off-the-seat duration, do a dummy assembly of just the number-1 cylinder with the cam installed to check the valve pocket clearance. (See Chapter 10, Valvetrain Optimization, for more on this subject.)
• If the budget allows for thermal barrier and anti-friction coating, have them applied.
Fig. 2.17. A stock rod can be converted to a fully floating pin by simply honing out the press-fit pin bore to give a 0.0006 to 0.001 clearance on the pin. Here, the pin has had a Tech-Line anti-friction coating applied; it is an option (not a necessity) if the right lubes are used.
Fig. 2.18. Swapping out stock rod bolts for ARP bolts is easy enough, but the big-end bore needs to be resized. The cost for that has to be added to the cost of a rod bolt upgrade.
Fig. 2.19. Before installing any stock-style bolt rods, be sure to put a sleeve over the bolt threads to protect the crank journal from damage during the installation.
Fig. 2.20. This Scat I-beam low-shoulder stroker clearance rod (PN 2-ICR6385) is made of 4340 steel. It’s strong, light, and affordable.
Fig. 2.21. I have used a number of these Callies stroker rods to good effect. The design seeks to minimize the material cut from the block necessary for clearance.
Fig. 2.22. These Scat rods (PN 2-454-6385-2200) look somewhat bulky, but they are, in fact, as light as most other quality rods. I have used them in nitrous engines up to 1,500 hp without failure.
Fig. 2.23. These K1 rods (PN CF6385APRB8) are also a cost-effective buy that is worth checking out. I have limited experience with these rods, but those I have used have held up in high-output builds without problem.
Fig. 2.24. Crower rods are certainly far from the cheapest available, but the quality is almost unbeatable. This billet rod (PN B93911PF-8) has just come out of a 900-hp bracket race engine after three busy seasons of racing. All of the rods look just as they did when they came out of the box.
Fig. 2.25. Manley has a great range of quality rods and is a good source if you are looking for a special lightweight rod for a specific application in which some race-rule-mandated aspect is limiting power, such as the use of a dual-plane intake.
When all of this is done, you will have about half the money into them that a set of good aftermarket rods cost. I use the word “good” here because I have seen failures with one or two brands of “off-shore”–sourced rods. Analysis has shown that the material spec was way off what it was supposed to be. If you stick with the rods I show here, you should be in about as good a shape as can be expected.
The rod I use most (because it has proven time after time to deal with the prolonged dyno sessions my mule engines go through) is Scat’s ICR6385 rod. In addition to being strong, it is typically lighter than a stock rod even though it is 1/4 inch longer between centers. Also, it is very affordable.
My gas dragster that has a touch of nitrous and produces a little more than 1,100 hp uses these rods. As of 2014, these rods are on their sixth season. In engines of up to 850 hp and 7,500 for a quarter-stroker, they have so far appeared bullet proof.
If you want to spend a little more and get rods with even more strength that are 100-percent machined, you can do so without breaking the bank. Options to consider include those from Callies, Crower, K1, Manley, and Scat. When it comes time to spend money on connecting rods, remember that longer is always better. Usually, when building a short-deck block, a 6.385-inch-long rod is the best choice if you are looking for maximum inches. If you are building a tall-deck block (10.2 to 11.1 inches), rods are available up to 6.8 inches long.
There is more to crank selection than simply deciding what stroke and rear seal style is needed. All the bigger stock-displacement factory cranks are externally balanced, so it was either inconvenient to accommodate enough counterweight mass within the confines of the crankcase or there was simply not room. To balance the crank required additional counterbalance mass incorporated into the crank dampener and flywheel/flexplate. Although this Band-Aid fix is passably okay for the rear of the crank, it is undesirable for the snout because it leads to unnecessary bending moments about the number-1 main journal. Unless cost considerations require it, absolutely do not go with an externally balanced crank and dampener system; it needs to be internally balanced.
Fig. 2.26. The Scat 9000 1/4 stroker crank is by far the most popular aftermarket Chevy big-block crank. They are available in both internally and externally balanced form. To tell which one you have or may want, check the machining at the points indicated here. Internally balanced cranks
