The crankshaft is the single heaviest component in an engine and it is the foundation of the reciprocating assembly–the rods and pistons. As such, you need to select the right crankshaft for your horsepower target and the application. For most max-performance engine builds, and specifically a Chevy big-block with more than 750 hp, a forged crankshaft is necessity. A stock cast-iron crankshaft cannot tolerate the stress created from the increased horsepower levels and you risk an outright failure.  

This Scat Super Lite crank is just about at the top of our budget for the race engine build featured in this book. This 4.375 stroker went into a 600-ci engine.

A budget-conscious forged crank is the next step up the ladder, if your goal is to have an engine live at a sustained 6,000-plus rpm and make more than about 750 hp. I am only going to deal with cranks up to 4.5-inch stroke because build costs start to escalate somewhat after that.

This end-on view of a Scat Super Lite crank better shows the star flange, hollow journals, and the knife- edged counterweights.

---

 

Find more tech tips like this in the book: CHEVY BIG-BLOCKS: HOW TO BUILD MAX PERFORMANCE ON A BUDGET

Learn everything you need to know about the Small-Block Ford by getting your copy here!

SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in forums, or with any clubs your participate in. You can copy and paste this link to share: https://www.cartechbooks.com/blogs/techtips/chevybigblockcranks

---

 

As for crank fitment, Mk IV and Gen V/VI blocks all accept a 4.25-inch-stroke crank with no block mods. A 4.375-inch stroker often goes in with minor clearancing of the bottom of the bores and the pan rails. A 4.5-inch stroke almost always needs block clearancing, but most blocks are thick enough in the relevant areas to do this. Also when going with these longer strokes, and especially the 4.5-inch stroke, be sure to check the piston-to-counterweight clearance at each piston’s bottom dead center (BDC) position. Interference here is quite common, and the fix is to grind some material off the counterweight. Compensating for this may mean adding heavy metal into the crank counterweights.

This K1 crank went into a street 540 build. The result was 702 ft-lbs and 704 hp. This was achieved with the aid of a short-duration high-lift valvetrain.

At the end of the day, a degree of simplicity is always a good thing. Here, I suggest that for most applications using a factory block, you go with a 4.25- or, better yet, a 4.375-inch-stroke crank. If you intend to use a Dart block, then there is little reason to go with anything less than a 4.375-inch stroke. As stroke length increases, it becomes more difficult to achieve an internally balanced system. This means having an extra counterweight or a correspondingly out-of-balance flywheel.

This is the tail end of a Callies 4.5-inch-stroke crank. Note the flange counterweight (arrow). This crank ended up as part of a 1,200-hp race build.

The same goes for the front end of the crank. Of the two (that is, front and rear), having to go with an external balance damper is the worst because the out-of-balance weight is hanging out much farther than the rear and is on a smaller diameter (the crank snout). Some two-piece steel cranks can be internally balanced at the rear because they have a large external counterweight on the crank flange itself. If you have a crank like this, have your balance tech take weight out of the damper first (use a removable weight damper for this) before taking weight out of the corresponding spot on the crank counterweights.

Let’s next address crank balance factors. A neutral-balance crank is one where the balance factor is 50 percent. That is when the bob weight used to simulate the mass of the pistons and rods is equal to 100 percent of the rotating mass hanging on one journal, plus 50 percent of the reciprocating mass.

An example goes like this: Take the weight of both rod journal ends of the rods and their bearings plus about 5 grams of oil allowance. Typically, a big-block Chevy rod is around the 800-gram mark, with about 560 of that centered on the rod journal end. Add twice this to the weight of two sets of bearings, plus 5 grams, and you are looking at about 1,285 grams. To this, you must add the weight of one piston assembly, which is typically about 750 grams and the pin end of one rod at about 240 grams. All this totals 2,275 grams.

The more stroke that the engine has, the more important it is to have an appropriate pan. Here, I rely heavily on Moroso.

If the bob weight is made heavier than this, the crank is said to be overbalanced; if less, underbalanced. A neutral balance produces a very smooth running engine, so the question here is: Why would you want to over- or underbalance the rotating assembly? The reasons for going either way involve a rather long theoretical explanation, and at the end of the day are hard to prove in practice on anything short of a Top Fuel engine. For a performance build, the general trend has been to overbalance the crank slightly because it theoretically relieves the crank of some small amount of main bearing load. As for underbalancing, it seems there hasn’t been a real trend in this direction.

One of my cranks is being balanced. Note the bob-weight balance fixtures on each of the rod journals. This crank was destined for a particularly successful 482-ci build.

Some of the important elements of a big-block stroker crank are put into practice. Most important of these is the internal balance. This 4.5-inch stroker needed heavy metal slugs as indicated by the lowest arrow. The remaining arrows indicate windage reduction moves.

However, until a conversation with Dart’s boss and Pro Stock maestro Dick Maskin a few years ago, I had never given balance factors serious or additional attention. Dick told me that in a rush to get to the track he had to make do with an underbalanced rotating assembly, and the engine showed better dyno figures than expected. A little more investigation revealed that this was not by some unaccountable chance, but by a repeatable element toward making power from his 500-inch engines. I have dyno’d engines with Dick and I can say that he is pretty meticulous with his test procedures. This puts credibility to his claim, and since that conversation I have vaguely figured out why a minor amount of underbalance might be advantageous for a naturally aspirated, big-inch high-performance V-8. But until I am clearly sure of the theory behind this, I will continue researching it.

Line 1 is for an undamped crank. Line 2 represents an indifferent damper selection. Line 3 is the result of a typical effective damper. Undamped torsionals tend to destroy cam dynamics.

What has so far been said about crank balance leads to one issue that you need to be clear on. The most important aspect of balance, as far as the rotating assembly goes, is to accurately balance the weight of the pistons and rods. Here a tolerance of 2 grams is practical, but I like to get mine better than 1 gram. As far as over- or underbalancing is concerned, if the bob weight is within plus-or-minus about 50 grams, the engine will run smooth enough that you are unlikely to be able to tell the difference between that and a perfectly neutral balance. I recommend that you have the crank company balance the rotating assembly for you at or close to neutral balance. This means if it is necessary to fly-cut the piston valve pockets a little deeper, the overall balance is virtually unaffected. However, since my conversation with Dick Maskin, my cranks for a serious performance application are underbalanced by about 1.5 percent.

As the firing pressures hit the crank, it flexes and produces a vibratory motion superimposed on the crank rotation.

Many forged cranks for big-block Chevy are currently available, but you need to rely on a reputable brand, such as Scat, Callies, Eagle, or Manley. Inexpensive overseas-made cranks, which are not high-quality units, are available, but these often need to be straightened and reworked before installation. In the final analysis, these cranks are not worth the trouble. In the assembly of a max-performance big-block Chevy, selecting the best crankshaft for your budget is a crucial element. So carefully and wisely consider all the relevant factors before making a purchase decision.

The crank snout is also prone to a bending vibration. An external balance damper aggravates this. 

Written by David Vizard and Posted with Permission of CarTech Books