Chevy Big Blocks. David Vizard
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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 have a large scallop machined from the big-end journal cheeks.
Fig. 2.27. I have had great success with top-of-the-line Callies forged cranks. However, what you see here is a little different. This budget Compstar crank is an offshore-sourced forging that is finished at the Callies plant in Fostoria, Ohio. As such, it is machined on the same equipment as their high-dollar cranks and undergoes the same stringent quality control.
Fig. 2.28. This is Scat’s entry-level forged 1/2-inch stroker crank (PN 4-454-4500-6535). Although counterweighted for a 6.535-inch-long rod, it can be paired with a 6.385 rod and installed in a stock short-deck block. With 0.060 oversize on the bore, this crank yields 525 ci.
Setting Bearing Clearances
It is very important to have bearing clearances within functionally acceptable limits. In a pro shop, this is typically done with expensive measuring equipment that is normally beyond the budget of a home engine builder. However, the use of Plastigauge can establish whether or not the clearance is acceptable. Your targets here are: mains, 0.0025 to 0.0033; rod journals, 0.0022 to 0.0027.
In my shop, we use this Fowler bore gauge to measure the bore diameters so we can establish bearing clearances. However, this gauge is expensive.
By assembling the bearing housing and bearing, then crushing the Plastigauge strip, the bearing clearance can be established within required accuracy.
A decent micrometer from one of the consumer tool houses, such as Harbor Freight, is affordable for the budget-minded home engine builder. Having this micrometer allows you to establish the crank journal sizes accurately.
When using Plastigauge with rod bearings, measure them in pairs. To prevent the rod skewing on the journal, insert feeler gauges (arrow) between the rods to take up the side clearance during nut/bolt tightening.
Crank end-float needs to be between 0.004 and 0.008; the target is 0.006. Use a fine emery cloth laid out on a machined flat surface to remove material from the bearing thrust face if it’s too tight.
Fig. 2.29. Manley has a good range of cranks available, with and without center counterweights.
Another aspect of crank design you may want to consider is whether or not to go with a design having center counterweights. The subject of center-counterbalanced weights may not have previously entered your thoughts. To understand what it’s about, see Figure 2.30.
Without center counterweights, a “couple” (a rocking effect caused by two forces) acts on the center main bearing; it is brought about because of the displacement of the two rod journals on either side of the main bearing. By fully counterweighting each throw, this couple is considerably reduced. But how important might this be in the grand scheme of things?
In terms of engineering finesse, a center-counterweighted crank is the way to go. It relieves the center main of some bearing loads and reduces the bending action caused by a lack of counterweight at this position. But it’s not an open-and-shut case. Having those extra counterweights usually means a slightly heavier crank, although not by as much as you might think. In a center-counterweighted crank, some of the mass for the center counterweights is taken from the next counterweights out. This means that, in part, some of the mass for the extra pair of weights is realized by the crank designer moving some of the mass from adjacent counterweights to the center counterweights.
Fig. 2.30. Shown here is the difference between a non–center-counterweighted crank (top) and one having center counterweights (bottom).
Building a street or street/strip engine with an internally balanced crank means you have eliminated an out-of-balance mass at either end of the crank. That’s such a big step in the right direction that it makes the issue of using a center counterweight or not far less important. The center main is big enough to take the added loads of the couple around it, so reliability is not likely to be an issue, at the