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How to Stroke Any Engine - Stroker Science

The long and short of building a big-inch street engine

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Bob Weight

Balancing a stroker crankshaft assembly is usually an easy affair, but it can get complicated if you have purchased a separate crank, rods, and pistons. The problem is that crankshaft counterweights are designed to accommodate a given amount of weight for the piston and connecting rod. The big variable is the length of the connecting rod. Longer connecting rods weigh more, which can affect the balance. Crank builders and balance shops use something called bob weight to manage these combinations. Bob weight is the weight attached to each connecting-rod throw on the balance machine that simulates the amount of weight for a pair of reciprocating piston assemblies. Often, you will see a particular crankshaft listed for a given stroke with a given amount of bob weight, expressed in grams, such as 1,800 grams.

To boil it down to the essentials, bob weight is 100 percent of the rotating assembly weight plus 50 percent of the reciprocating weight. Rotating weight consists of the big end of the rod, the bearing, and perhaps a few grams of oil. The reciprocating weight is equal to the small end of the rod, piston, wristpin, rings, wristpin locks (if used), and also a bit of oil. Let's say our rotating weight for a small-block Chevy is 474 grams and the reciprocating weight measures 872 grams. To find the bob weight, we add 474 plus 436 (half of 872) to equal 910 grams. We multiply this by two because we have to balance the crank for a pair of rods and pistons to come up with 1,820 grams. If the crankshaft has a bob weight of 1,850 grams, it's easy to drill a small hole in the counterweight to reduce the counterweight to equal 1,820 grams.

If the crank counterweight is only good for 1,800 grams, we either have to lighten the mass of our piston assemblies or add Mallory (heavy) metal to the crank throw to increase its weight up to 1,820 grams. Enthusiasts get into trouble when they assemble mismatched parts that require expensive machine work to balance properly. This is where balancing can more than offset any savings from these parts.

Bore and Stroke Chart
To find displacement, merely follow the bore from the left to match up with the stroke amounts listed across the top of the chart, match the up and you have your displacement. For example a 4.50-inch stroke with a 4.50-inch bore equals 572 ci.

STROKE
BORE 3.50 3.75 3.875 4.00 4.125 4.375 4.500 5.00
4.000 352 377 389 402 415 440 452 502
4.030 357 383 395 408 421 446 459 510
4.060 362 388 401 414 427 453 466 518
4.090 368 394 407 420 433 460 473 525
4.125 374 401 414 427 441 468 481 534
4.155 379 406 420 434 447 474 488 542
4.165 381 408 422 436 449 477 490 545
4.250 397 425 440 454 468 496 510 567
4.280 403 431 446 460 475 503 518 575
4.310 408 438 452 467 481 510 525 583
4.350 416 446 461 475 490 520 535 594
4.470 439 471 486 502 518 549 565 628
4.500 445 477 493 509 525 556 572 636
4.530 451 483 499 516 532 564 580 644
SOURCES
Bulldog Performance
www.bulldogperformance.net
GM Performance Parts
www.gmperformanceparts.com
Callies Performance Products
Fostoria
Oh
419-435-2711
www.callies.com
Indy Cylinder Head
8621 Southeastern Ave.
Indianapolis
IN  46239
3-17/-862-3724
indyheads.com
Competition Products
N/A
Lunati
4770 Lamar Ave.
Memphis
TN  38181
901-365-0950
www.lunaticamshafts.com
Crower Cams & Equipment Co
San Diego
CA
Mopar Performance
Mopar.com
Dart Machinery
353 Oliver St.
Troy
MI  48084
248-362-1188
www.dartheads.com
Scat Enterprises
3-10/-370-5501
scatenterprises.com
Eagle Specialty Products
Southaven
MS
6-62/-796-7373
eaglerod.com
World Products
Ronkonkoma
NY
6-31/-981-1918
www.worldcastings.com
Ford Racing Performance Parts
44050 N. Groesbeck Hwy.
Clinton Township
MI  48036
5-86/-468-1356
www.fordracingparts.com
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