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 level likely to be seen for engines up to approximately 7,500 rpm and 900 hp.
Fig. 2.31. This Scat non–center-counterweighted 4.5-inch-stroke crank weighed 63 pounds after balancing. This compares to about 75 pounds for a stock 4-inch-stroke forged crank.
If I’m building a cost-is-no-object engine and call the shots on a billet crank that needs to be as light as possible, I would go with the center counterweights for an endurance race engine. However, for anything short of Pro Stock or Pro Mod, the inherently lighter weight of a non-center-counterweighted crank is a good, although minor, option to consider.
The crank for one of my 598 builds (see Fig. 2.31) is a relatively high-dollar custom item, and I elected to go without center counterweights. If your application involves endurance and/or RPM at a relatively high level for extended periods, such as marine use, a center-counterweighted crank is your best option.
Fig. 2.32. This 1/4 stroker K1 crank is in the moderate price range and features center counterweights. It went into a GM-blocked street 540, 10.5:1 CR, build that netted 730 ft-lbs and 690 hp. All with a 550-rpm idle and plenty of vacuum.
