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BOBWEIGHT CALCULATOR

This calculator works out the bobweight for a crankshaft balancing job from the masses of the rotating and reciprocating components, and it carries ProBal's engine reference table: bobweight percentages for 184 engines across 113 engine families, the same table supplied with ProBal DBS balancing software. Weigh the parts, enter them with the percentages for your engine, and read the rotating mass, the reciprocating mass, and the total bobweight in the unit you work in. Free to use, nothing to sign up for.

Worked example. The three percentages are the V-8 entry of the General category in the engine reference table below (100 rotating, 50 reciprocating, 2 cylinders per bob mass). The component masses are example values in grams.

Engine percentages
Component masses
g
g
Rod Small End
175 g computed
g
g
g
g
g
g
g

Results

Rotating Mass
489 g
Reciprocating Mass
339 g
Total Bobweight
1,656 g

ENGINE REFERENCE TABLE

ProBal's compiled reference of bobweight percentages by engine family. Each row gives the reciprocating and rotating percentages, the number of cylinders one bobweight stands in for, and whether the engine is internally or externally balanced. Some entries carry the whole of their guidance in the name, for engines that need no bobweights or are balanced with the flywheel and damper fitted, and those are listed too.

Usage: Free to use at the bench. Link to this page if you reference it. Please do not republish the table.

ProBal engine bobweight reference: 184 engines in 113 categories
Engine Reciprocating % Rotating % Cylinders per bob mass Balance Internal: the crankshaft is balanced on its own counterweights. External: the flywheel, flexplate, or damper carries part of the correction and is balanced with the crankshaft. Select
General
Inline 4 Cylinder No bobweights required Internal balance
Inline 6 Cylinder No bobweights required Internal balance
V-8 50 100 2 Internal balance
60° V-12 No Bobweight Required
GST V6 Dino According to Fentech 56 100 2 Internal balance
56° V-12 Dino 50 100 2 Internal balance
AJS
AJS 1 Cylinder 61 100 1 External balance
AMC
AMC V-8 290 & 304 & 4.7 & 5.0 90° Ext 67 up some 290 mid 66 51 100 2 External balance
Audi
Audi 5 cylinder Correct flywheel and damper at same time 50 100 1 External balance
Benelli Motorcycle all no bobweights required
BMW Motorcycle
BMW opposed street 60 100 1 Internal balance
BMW opposed race 50 100 1 Internal balance
Briggs Race
1 Cyl Race Mini bobweight 67 100 1 Internal balance
Briggs Stock
1 Cyl Stock Mini bobweight 50 100 1 Internal balance
British Royal
British Royal Infield Interceptor 2 Cyl Mini bobweight 70 100 1 Internal balance
Brush
Brush 1 Cylinder Mini bobweight 50 100 1 Internal balance
BSA
BSA Vertical Twin Recip 72 to 80 percent 72 100 1 Internal balance
BSA L-2 Cyl 650CC 80 100 1 Internal balance
BSA 1 Cyl 61 100 1 Internal balance
BSA L-2 Street 60-70 recip 70 100 1 Internal balance
BSA L-2 Cyl race 50 100 1 Internal balance
BSA L-3 Cyl Fentech says no rotating 50 100 1 Internal balance
Buick 90° V-6, street
Buick 3.0 & 3.2 & 3.7 & 3.8 & 4.1 36.6 100 1 External balance
Buick V-6 3.8 with balance shaft 50 100 2 Internal balance
Buick V6 External engines 3.2 & 3.8 & 4.1 & 225
Buick V-6 Race, "solid motor mounts, engine swap into another chassis"
Buick Race V-6 3.2 & 3.8 & 4.1 50 100 1 External balance
Buick V-8
Buick 332 & 455 50 100 1 External balance
Buick V8 External engines 322 & 340 & 400 & 455
Cadillac V-8
Cadillac 4.1 & 252 & 90 deg Fentech says external 50 100 2 Internal balance
Cadillac 5.7 42-46 L Head & 350 50 100 2 Internal balance
Cadillac 6L & 368 60° V-12 50 100 2 Internal balance
Capri V6
Capri 2.8 60 deg 50 100 1 External balance
Chevy 2.8 External on flywheel only (first design)
Chevy 60° 2.8 & 3.1 & 3.4 173 & 191 & 207 50 100 1 Internal balance
Chevy 305 & 350 V8 (second design 1 pc Rear seal) external Flywheel only
Chevy Small Block 305 & 350 full rear seal ext flywheel only 50 100 2 External balance
Chevy Small Block 400 50 100 2 External balance
Chevy Small Block 383 stroker Use 400 dampener and FW 50 100 2 External balance
Chevy 454 & 502, externally balanced on both ends
Chevy 60°
Super 6 50 100 1 Internal balance
Chevy 60° V-6
Chevy 60° V-6 2.8 First generation flywheel only 50 100 1 External balance
Chevy 60° V-6 2.6 50 100 1 Internal balance
3.3 & 3.8 & 229 C.I. 46 100 1 Internal balance
3.4L 44 100 1 Internal balance
351 C.I. 60° not sure on external 50 100 1 Internal balance
5 & 7.8 305 & 478 50 100 2 External balance
Chevy 90° V-6
Chevy 2.3 & 4.3 40 100 1 Internal balance
Chevy 4.3 Split pin 40 100 1 Internal balance
Chevy 4.3 Split pin Race Full Rear seal ext on flywheel only 50 100 1 External balance
4.1 50 100 1 Internal balance
3 throw odd fire 50 100 2 Internal balance
Chevy 262 odd fire 50 100 1 External balance
Chevy 4.3 w/ balance shaft uses balance shaft 50 100 1 External balance
Chevy 4.3 w/o balance shaft no balance shaft 40 100 1 External balance
Chevy Diesel
Chevy 6.2 & 6.5 diesel Heavy metal needed on both ends to balance 50 100 2 Internal balance
Chevy 6.6 Duramax Diesel 50 100 2 External balance
Chevy L-6
Chevy L - 6 140 & 2.3 No bobweights, includes Corvair 50 100 2 Internal balance
Chevy V6 Truck engines 376 & 432 External both ends
Chevy V-8 Big Block
Chevy 396-502 Big Block 50 100 2 External balance
Chevy V-8 Small Block
Chevy Small Block 265 & 350 2 pc seal 50 100 2 Internal balance
Chrysler 340 & 400 & 440 V8 counterweight on cast iron crankshaft
Chrysler 360 V8 Automatic & counterweight is welded to torque converter
Chrysler 360 V8 Manual & counterweight is on flywheel
Chrysler 340 cast crank & 360 & 440 Screwed - cw on torque converter 50 100 2 External balance
Chrysler V-10 8L Fentech says no bobweights needed 50 100 2 Internal balance
Chrysler V-6
Chrysler 3.3 & 3.8 V-6 50 100 1 Internal balance
Chrysler 3.9 V-6 50 100 1 Internal balance
Chrysler V-8
Chrysler 273-318 50 100 2 Internal balance
Chrysler 340 Steel crank 50 100 2 Internal balance
Detroit Diesel
DDC V-53 318 & 426 & 552 5.2 & 7 & 9.0 Special procedure Check with AERA 200 200 1 Internal balance
Dodge 90° V-6
3.0L 50 100 1 Internal balance
3.3L 50 100 1 Internal balance
3.9 50 100 1 Internal balance
Dodge V-8
V-8 50 100 2 External balance
Evinrude
V-6 156 & 2L Weld weights on side of CW's 50 100 1 Internal balance
Ferrari 60° V-6 & V-8
275 C.I. 50 100 1 Internal balance
Fiat V-12 60 deg no bobweights required
Fiat V-12 Dino 56 deg Not sure about cyls per BW 50 100 2 Internal balance
Fiat V2 126 CI Diesel 2 mains
Fiat 126 3600 rpm 55 100 1 Internal balance
Fiat 126 8000 rpm 58 100 1 Internal balance
Fiat 126 above 8000 rpm 60 100 1 Internal balance
Ford
V-8 50 100 2 External balance
Ford 3.8 V6, two different dampener combinations
3.8 Split Pin Special procedure 39.4 100 1 External balance
Ford Big Block
Ford Big Block 460 & 79 and up, CW on flywheel and separate CW on Dampener end
Ford German V-4 60° 1.7L
V-4 Ford 60 Not sure about internal or ext 50 100 1 Internal balance
Ford L-3 Cyl
Ford L-3 Cyl Fentech says 44% recip 0 100 1 Internal balance
Ford Truck, external balance both ends
475 & 534 External 50 100 2 External balance
Ford V-6
60° 2.8 & 3.0 50 100 1 Internal balance
Ford 1.7 German 4 Cyl 60° + 50 100 1 Internal balance
Capri 60° 171 & 2.8 50 100 1 Internal balance
Capri 90° 231 & 3.8 39 100 1 External balance
Ford 2.5L 50 100 1 Internal balance
Ford 2.8L 50 100 1 External balance
2.9L Ranger 50 100 1 Internal balance
3.0L Taurus & Sable 50 100 1 Internal balance
Ford 3.8L 90° split pin 39.4 100 1 External balance
3.8L 90° split pin RACING ONLY 50 100 1 External balance
3.8L Truck & Continental Plus 3 oz. in. V-dampner on end of crank 39.4 100 1 External balance
Ford 3.9L 46 100 1 Internal balance
Ford 4.0L 50 100 1 Internal balance
Ford V-6 SHO
3.2 & 3.4 use one rotating rod weight Fentech says 50/100 0 50 1 Internal balance
Ford V8 429 internal balance Ford V8 externals 221 OHV,260,289,302,330HD,351,361,391,400,428
Ford Big Block All External on 410/428 50 100 2 External balance
Ford V-8 Small Block
221-351 Small Block 50 100 2 External balance
Ford 4.6 50 100 2 Internal balance
General Motors V-6
2.8L 60° manual Manual Tran Ext bal on flywheel end only 50 100 1 External balance
2.8L 60° automatic Automatic Transmission 50 100 1 Internal balance
Geo
3 Cyl Rotating assembly only 0 100 1 Internal balance
GM
V-6 60° 50 100 2 Internal balance
V-8 50 100 2 Internal balance
Late model full seal External on flywheel only 50 100 2 Internal balance
GMC Truck V6 external balance on 305 & 351 & 401 & 478
305 & 351 & 401 & 478 V6 External 50 100 2 External balance
GMC V-6
4.3 90° street external Street Ext on flywheel only 40 100 1 External balance
4.3 90° street Street 40 100 1 Internal balance
4.3 90° race external Race Ex on flywheel only 50 100 1 External balance
4.3 90° race Race 50 100 1 Internal balance
351 cubic inch Splayed Journal 50 100 1 Internal balance
Harley 750cc Class C Racing Motor
Harley 750cc Class C Racing Motor 56 100 1 Internal balance
Harley and Indian V-2 Street
Harley V-2 Street Recip 52-56 52 100 1 Internal balance
Indian V-2 Street Recip 52-56 52 100 1 Internal balance
Harley General Information
Harley Shovel Nichols 55 100 1 Internal balance
Harley Evo Nichols 57 100 1 Internal balance
Harley Twin Cam Nichols 60 100 1 Internal balance
Harley V-2 Race
Harley V-2 Race 66 100 1 Internal balance
Harley V-2 Street HP
Harley V-2 Street High Perf 3000-4000rpm 57 100 1 Internal balance
Honda L-4 no bobweights required
Honda Motorcycle 4 and 6 Cyl no bobweights
Honda V-4
Honda V-4 50 100 1 Internal balance
Honda V-6
Honda 2.5 & 2.8 90° V-6 50 100 1 Internal balance
International V8, External on 304 & 345 & 392
304 & 345 & 392 External 50 100 2 External balance
Isuzu V-6
Isuzu V-6 3.5L 75° 50 100 1 Internal balance
Jacobs 7 cyl radial
Jaguar
Jaguar V-12 60° 0 100 1 Internal balance
Jeep, External
Jeep V-6 225 & 3.7 50 100 2 External balance
John Deere
John Deere H-2 & H & 101 40 100 1 Internal balance
90°V 50 100 1 Internal balance
50° 52 100 1 Internal balance
2 Cyl Hor Inline 180° >1000rpm Above 1000 rpm 52 100 1 Internal balance
2 Cyl Hor Inline 180° <1000rpm Below 1000 rpm 50.5 100 1 Internal balance
Vertical Twin Inline 20 100 1 Internal balance
Kawasaki All No bobweights required
Kohler
Kohler Single Cylinder 50 100 1 Internal balance
Lamborgini
Lamborgini V-12 No bobweight required 50 100 2 Internal balance
Lancia
Lancia V-4 48 100 1 Internal balance
Lincoln
Lincoln V-12 H Series(1941) 50 100 1 Internal balance
Lincoln V-8 Flat Head(1930) No bobweights required 50 100 2 Internal balance
Long
Long 3 Cylinder Tractor 120° Rotating assembly only 0 100 1 Internal balance
Matchless
Matchless 1 Cyl 61 100 1 Internal balance
Mazarati
Mazarati V-6 2.8 50 100 1 Internal balance
Mazda
V-6 60° all 50 100 1 Internal balance
V-6 2.5L MX6 DOHC 44-50% rot 50 100 1 Internal balance
V-6 3L SOHC 50 100 1 Internal balance
Mazda rotary Rot assy only plus 45g oil 0 100 2 Internal balance
Mercedes, On some 6 cyl no balancing on CS & Correct on flywheel and dampener
Mercedes 5 & 6 Cylinder Correct both ends same time externally 50 100 1 External balance
Mercury Outboard
Mercury V-6 Outboard Stock serious heavy metal 30 100 1 Internal balance
Mercury V-6 Outboard Race serious heavy metal 50 100 1 Internal balance
Mercury V-6 outboard
Mercury 2.5 outboard Requires Heavy Metal - Factory static balance 30 100 1 Internal balance
Mercury 2.5 outboard race Race requires Heavy Metal - Factory static balance 50 100 1 External balance
Misc
3 Throw Odd fire V-6 50 100 2 Internal balance
Mitsubishi V-6
Mitsubishi V-6 All 50 100 1 Internal balance
Moto Guzzi
Moto Guzzi 2 Cylinder 750cc 51 100 1 Internal balance
Navistar
Navistar 5.0 & 6.4 50 100 2 External balance
Nissan V-6
3 Cyl 50 100 1 Internal balance
Nissan BG30 & 2.8L & 3.0L 60° V-6 50 100 1 Internal balance
Norton
Norton V-2 street Recip 60-70 percent 60 100 1 Internal balance
Norton V-2 Race 50 100 1 Internal balance
Norton Atlas L-2 75 100 1 Internal balance
Olds external balance on 260 & 307 & 350 & 403 & 425 & 455
Oldsmobile 303 & 324 & 455 50 100 2 Internal balance
Oldsmobile 307 & 371 & 455 & 394 50 100 2 External balance
Oldsmobile V-6
Olds V-6 4.3 Diesel Even Fire 35.2 100 1 External balance
Oldsmobile V-8
Onan
Onan 2 Cyl Opposed racing 50 100 1 Internal balance
Perkins
Perkins 3 cylinder gas & diesel 0 50 1 Internal balance
Peugeot and Renault V-6
Peugeot and Renault V-6 all 50 100 1 Internal balance
Pontiac, ext engines 326 & 389 & 400 & 450 & 455
Pontiac 389, most are heavily compensated on FW to cure bad CS unbalance problem
Pontiac V-8 all external on flywheel only 50 100 2 External balance
Porsche all no bobweights
Saab
Saab V-4 50 100 1 Internal balance
Suburu
I-3 1.7 50 100 1 Internal balance
Suzuki L-4 no bobweights required
Tecumseh
Tecumseh 1 Cyl Garden Tractor 50 100 1 Internal balance
Tecumseh 1 Cylinder 8HP Go-Cart 60 100 1 Internal balance
Tecumseh 1 Cylinder 7.5K-10K 70 100 1 Internal balance
Tecumseh 2 Cylinder 10K 81 100 1 Internal balance
Toyota V-6
Toyota All 50 100 1 Internal balance
Triumph
Triumph 1 cylinder 61 100 1 Internal balance
Triumph L-2 Street Recip 60 to 70 percent 60 100 1 Internal balance
Triumph L-2 Race Recip 60 to 70 percent 50 100 1 Internal balance
Triumph 3 cylinder 50 100 1 Internal balance
Ursus 500-800rpm
Ursus 3 cyl dsl 3 mains 100 100 1 Internal balance
Volkswagon
VW V6 VR6 15° 30 100 1 Internal balance
VW conversion for ultralight 37 100 1 Internal balance
Volvo
Volvo V4 & V6 All 50 100 1 Internal balance
Volvo L-5 50 100 1 External balance
Weight of all
7 cyl radial 50.9 100 7 Internal balance
Wisconsin
Wisconsin VH4D 90° 36 100 1 Internal balance
Wisconsin VG4D 90° 20 100 1 Internal balance
Yamaha motorcycle
Yamaha '82 XZ 550 uses gear driven balancer 50 100 2 Internal balance
Yamaha 4 cyl no bobweights required 50 100 2 Internal balance
Yamaha 6 cyl MC no bobweights required 50 100 2 Internal balance
Yamaha 1 Cyl MC 61 100 1 Internal balance
Yamaha MC V-2 75°Street Recip 55 to 60 percent 55 100 1 Internal balance
Yamaha MC V-2 75°Race Recip 60 to 65 percent 60 100 1 Internal balance
Yamaha MC L-3 50 100 1 Internal balance
Yamaha outboard
Yamaha V-6 Outboard 30 100 1 Internal balance
Yamaha V-6 Outboard Race 50 100 1 Internal balance

HOW BOBWEIGHT IS CALCULATED

A bobweight stands in for the parts that hang on one rod journal while the crankshaft is spun on the balancing machine. The components attached to the crankshaft divide by how they move. Rotating parts spin with the journal: the connecting rod big end and the rod bearing. Reciprocating parts move up and down the bore: the piston, the rings, the wrist pin, the pin lock, the rod small end, and any support rail.

rotating mass = rotating % x (rod big end + rod bearing)
reciprocating mass = reciprocating % x (rod small end + piston + rings + pin + pin lock + support rail)
bobweight = (rotating mass + reciprocating mass) x cylinders per bob mass + oil

What the percentages mean. They are allocation percentages: how much of each group of parts is carried into the bobweight. A V8 entry reading 100 rotating and 50 reciprocating means all of the rotating weight and half of the reciprocating weight go on the journal. Rotating is 100 on nearly every engine in the table; reciprocating varies with the layout, because it comes from how much of the reciprocating mass the crankshaft's counterweights are meant to counter.

A common confusion. These percentages are not the split of the connecting rod's own mass between its big end and its small end, which is typically somewhere near 65 percent and 35 percent. That is a real measurement, and it is a different one: the rod is weighed at each end to get the big end and small end masses, and the bobweight percentages are then applied to those and the other parts. The two get run together constantly in circulating explanations.

Cylinders per bob mass. On a V8 it is 2 because two rods share each rod throw, so one bobweight simulates both. Layouts that do not share a throw carry 1. The table gives the value for each engine.

Overbalance. Overbalancing means adding percentage on the reciprocating side, balancing to something above 50 percent of the reciprocating weight rather than exactly 50. On this page that is done through the Reciprocating Percent field: a builder wanting 52 percent types 52 into that field and the total updates. There is no separate overbalance control.

Oil. The oil allowance is the builder's own figure for the oil clinging to the rotating assembly at speed. It is added once, after the cylinder multiplier, and a blank field means none is allowed for.

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