Balancing & Blueprinting Odds & Ends

“Balanced and blueprinted,” are often said a one. They sometimes go together, but are not the same. Blueprinting means the builder has set clearances and other measurements inside the engine to within a range of specifications provided by the manufacturer, or come up with his own. Balancing means that the builder has subtracted or added weight/mass to the flywheels to offset forces exerted by combustion and two groups of components. The first, composed mostly of top end parts that move back and forth, makes up reciprocating mass. The second, located around the outer diameter of the flywheels, makes up rotational mass, which moves in a circular direction. Parts that comprise reciprocating mass include the pistons, rings, wristpins and keepers, and the upper portions of the connecting rods. Parts that make up rotational mass include the crankpin, bearings and cages, crankpin nuts, lock plates and screws (with bolt- together flywheel assemblies only), and the lower portions of the rods. As you’ve probably heard before, balancing a V-twin flywheel assembly is always a compromise, because the reciprocating parts are constantly at odds and never doing the same thing at the same time. In a V-twin, balancing can only reduce vibration in a specific RPM range, not eliminate it.


To offset the forces exerted on the crankpin area of the flywheel, flywheel manufacturers make the opposite side of each flywheel thicker and heavier in an area known as the counterweight, but that’s just the beginning. After installing an adjustable bobweight in the crankpin bore of a flywheel and a support shaft in the mainshaft bore, the engine builder places the assembly on a balance stand. Gravity causes the flywheel to turn and shift the heaviest area to the bottom, which indicates that the flywheel is out of balance and weight must be removed from or added to the counterweight area. Holes, usually 3/8” to 1/2” in diameter, are drilled in the counterweight area to remove weight. If weight has to be added, which is rare in street engines unless the flywheels are fubar’d and have more holes than a box of Krispy Kreme glazed, a tungsten alloy called mallory, which is heavier than steel, is pressed into drilled holes.

The biggest challenges in balancing are to make sure the balance stand is solidly mounted and perfectly level, and weigh the ends of the connecting rods accurately. It’s also important to con rm that the support shaft and balance stand surfaces are free of grit, nicks, and rust that could interfere with movement of the flywheel and support shaft. Weighing the rods and other components requires a precision gram scale with at least a 700 Gm. capacity, a level, a length of fishing line, and a support of some sort for the sh line. Weighing most of the components is just a matter of placing them on the scale and recording the figures for future reference, but not so the connecting rods, since the two ends have to be weighed separately. To weigh the crankpin end, place it on the scale, support the wristpin end with a length of sh line hung from the support, level the crankpin and wristpin hole centers, and record the weight. Reverse the procedure to weigh the wristpin end, then con rm that the weights of the two ends match the total weight of the rod. If the numbers don’t add up, con rm that the rod is level and try again.


With the different parts weighed and the weights recorded (separately for each cylinder), you’ll be able to calculate the weight required for the bobweight after settling on a balance factor- which represents a percentage of the total reciprocating weight. Truett & Osborn has used a 60% factor for decades, while S&S switched from 60% to 52% a few years ago, as did Harley- Davidson. The balance factor in fluences not only the force, but also the direction of vibration and the RPM range in which the engine will be smoothest. According to S&S, lower balance factors make vibration more vertical in nature than lateral, while higher balance factors lower the rpm at which vibration becomes a problem. I’ve ridden engines balanced at 52% and 60%, but other variables like crankpin diameter, connecting rod length, bore and stroke, compression ratio, and so forth make it hard to say which one’s better. If I had to choose one, I’d cheat by going with 60% for a solid-mount highway engine, 52% for a rubber-mount street engine or solid-mount street-strip engine. The best plan for most of us is to choose a good engine builder and go with whatever he recommends- he’ll know what works best in his engines.


The balance factor is also important because it’s a key element in the formula used to calculate the weight of the bobweight, which can be adjusted by adding or removing shims. 

                      The formula is:

Everything we’ve just talked about applies to static balancing. Dynamic balancing has become popular over the last few years for a couple of reasons. Believers feel that it is more accurate than static balancing, because spinning the trued flywheel assembly up to just a few hundred RPM will allow computer sensors to detect imbalances, especially those that occur in a lateral direction instead of in the direction of flywheel rotation and would be undetectable with static balancing. Dynamic balancing also determines exactly where and how much weight must be removed from the ywheel assembly to correct the imbalance. (The holes in statically balanced flywheels are drilled along the inside surface of the

flywheels, those in dynamically balanced flywheels are drilled along the outside.) A clear advantage of dynamic balancing is that it is faster- an important consideration for manufacturers and large volume engine shops. BTW, I stumbled across a balancing service that claims flywheels from aftermarket manufacturers are shipped unbalanced. Bull**it! S&S and T&O do sell unbalanced flywheels, but only if the customer requests them that way or fails to specify the engine application or reciprocating mass.

The most important thing to understand about balancing is that excessive vibration is not always caused by a balance problem. More common causes are a problem with the tires, wheels, fork, swingarm or frame; poor sprocket alignment; a bent transmission mainshaft; loose transmission or motor- mount hardware (especially that sneaky bastard up top between the heads), a loose exhaust support; off-kilter ignition timing for the rear cylinder in an ignition with separate timing adjustments for the two cylinders; lean carburetor jetting; a worn or improperly adjusted chain or worn sprocket; damaged, loose, or out of balance alternator rotor; or something really obscure like mismatched combustion chamber volumes.

As said before, blueprinting refers to insuring that clearances and other measurements inside an engine are as they should be. Some clearances require a fair amount of skill and special tools to measure and set. Others, camshaft endplay, for example, require only a feeler gauge and a pack of endplay shims. While H-D service manuals list clearances and other dimensions, aftermarket specifications are usually tighter. If not taken to the extreme, tighter clearances makes for a quieter engine and, as with precision balancing, one that is smoother and longer-lasting. It’s tempting to set clearances up tight, based on the theory that tightening them to the max will make the engine last longer, but the down side is that tight clearances prolong break-in and make the engine more susceptible to heat damage during that time. Tight clearances may damage parts like the crankpin, rod bearings, and piston skirts, even if a conservative break-in program is followed. (More on that later.) For those reasons, setting most clearances near the middle of the range is a better plan. In case you’re interested, S&S provides clearances in its product instructions at www.sscycle. com, under the Tech Information tab. The flywheel, piston, balancing kit (located under “Special Shop Tools”), and Long Block instructions are a good place to start.

Here are a few odds and ends to finish things up.

• Truing refers to insuring that the flywheels are concentric on the crankpin once assembled with the rods and bearings. Preferences vary, but S&S recommends measuring run-out on the pinion and sprocket shafts with a dial indicator while rotating the flywheels in a truing stand. Some builders place the indicators against the rims of the flywheels instead, but S&S feels shaft concentricity is what matters. For the record, most sources recommend using a stout c-clamp or hardwood wedge to adjust the flywheels on the shafts. Off the record, a good smack or two with a lead hammer may be required to square things up. In that case, be sure to remove the flywheel assembly from the truing stand before beating on it.

• Rowe’s truing stand has long been the industry standard. It’s available directly from Rowe (www., as well as from Jim’s and other distributors. S&S handles a good balancing kit, but buying the tools for flywheel work seldom adds up financially except for shops and maniacal DIY’ers.

• I’d rather pay someone else for flywheel work, but installing the wheel assembly in the cases shouldn’t be a big deal if you have the tools and can follow instructions. Jim’s makes an outstanding tool for installing the flywheels in the left case, but a press works too.

• Rod sideplay is important, but no more so than making sure the connecting rods are straight, and the wristpin bores square with the crankpin. Tools that are no more than elongated wristpins are available for that, and pretty cheap. Considering the cost of the tool, checking the rods and wristpin bores is always a good plan, even if you pay someone else to do the flywheel work and button up the cases. Bent rods and tilted wristpin bores are all but guaranteed to make a wristpin keeper come loose and trash the cylinder. If you have keeper problems, buy the tool and check the rods/wristpin bores before going to buttons, which aren’t compatible with all pistons because the wristpin bosses may be too narrow to support them. It’s possible to use double keepers with some pistons, and won’t hurt if they leave enough clearance for the pistons/pins to swing freely enough on the rods to accommodate heat expansion. The few additional grams of reciprocating mass won’t affect the balance factor enough to matter.

• Heavy flywheels make for a smoother engine, are easier to kickstart, and store momentum better than light ones. Light flywheels rev quicker.

• When installing the oil pump in an Evo, Shovel, etc., turn the drive gear inside the crankcase to rotate the pump gears as you torque the case down, and be sure nothing binds. If it does, loosen the body, remove one of the outer gears, rotate it half a turn, reinstall it, and try again. Some oil pump gears are dimpled to index them. Placing the dimples next to each other should keep the gears from binding.

• When installing the snap ring on the oil pump drive shaft inside the case, be careful not to spring it. Con rm that the opening between the two ends is not wide enough to let the drive key slip out.

• Everyone has his own preferences, but I’ve had better luck with paper oil pump gaskets than plastic. Install them dry. When you do use sealant, apply it sparingly so it doesn’t squish into an area where it doesn’t belong and block an oil passage. “Thin lm,” should be the operative phrase if the gasket surfaces are good.

• Most of us know about the chance of mismatching the cam and pinion gears when changing cams. Too little lash (clearance) will cause a whine, too much will cause a tick that sounds like a bad lifter. It may sound cool, but the whine is bad news because the gears are binding against each other and slowly wearing out their heat treatment. Next will be worn metal and new parts. The tick is annoying but harmless.

• To me, it makes more sense to replace the pinion gear with a larger or smaller one than the cam gear, if you have the tools. Being off just a hair can throw the cam timing off enough to have a major affect on performance. If the cam’s too advanced, it will cause hard starting and ignition knock. If too retarded, it will kill torque and horsepower. In comparison, changing pinion gears is a shoe-in.

• Light pistons cause less vibration than heavy ones. Some builders try to equalize piston weights, others don’t. Matching the weight of the wristpin ends of the rods is worthwhile, the caveat being that it’s possible to compromise heat treatment and structural integrity if you get carried away with the grinder.

• When measuring ring gaps, push each ring an inch down into the bore and be sure it is square in the bore before measuring the gap. It’s possible to increase ring gaps with a fine file, but easy to cut the ring ends at an angle. Better to use a ring grinder, and be sure to position the gaps on the piston correctly, according to the manufacturer’s instructions.

• Before installing the heads, seal the valves with a lm of grease, install the spark plugs, ip the heads over, and measure the combustion volume to be sure it’s the same in each head. To do it right, you’ll need a burette, a 6x6 square of 1/8” or thicker plexiglass with two holes drilled in it (a 1/16” hole for a vent, another one large enough to accept the dropper end of the burette), and enough colored alcohol (use food coloring) to ll the custom chamber. After sealing the plexiglass to the gasket surface with grease, carefully ll the chamber with alcohol, record the volume, and repeat for the other head. A difference of a cc or two should be ok, but much more than that will increase vibration and could make the engine more likely to ping. It’s trickier because you’ll have to tilt the engine to make the spark plug opening the high point of the combustion chamber, but a better way to cc heads is to put the piston at TDC, seal the bore with grease, install the head, and then measure the volume of alcohol required to ll the chambers to the bottom of the spark plug hole.

• The traditional step-by-step break-in procedure that took thousands of miles to complete is somewhat obsolete. First suggested to me by a pal at Zipper’s, a technique called heat cycling is much quicker and seems to work equally well or better . With a powerful fan standing by, start the engine and run it at 1500-2000 RPM while holding a hand close to the cylinders but not touching them. Heat will increase quickly and abruptly, at which point you turn the fan on and kill the engine. After the cylinders are cool enough to touch for a second or two, restart the engine and repeat the procedure, then do it again five or six more times. Each time, you’ll find that the engine takes longer to heat up. When additional cycles fail to extend the cool-off time, it’s safe to consider the engine broken in. Put fifty miles on it, then change the oil and filter and you’re done (except for following whatever ritual you have for retightening the top end hardware- not recommended for the head bolts in S&S P-motors, by the way. 

Tom Johnson

                                                                                                    Tags: balancing, flywheel -

Back to Tech Tips for Building Custom Motorcycles

Leave a comment

Please note, comments must be approved before they are published

Explore Articles

Search for a Topic

Explore Sections