A motorcycle crankshaft rarely needs to be balanced, as they come from the factory with good balance. If repairs or modifications have been made and the crankshaft has been disassembled for any reason then a crankshaft alignment and balance is necessary.
The motorcycle crankshaft needs aligned before any balancing is attempted. Alignment puts the crankshaft pieces centerlines straight and in line with no wobbles and the crankpin (that the rod goes on) parallel to the crankshaft. This is a trial and error then try again process until all centerlines are straight and parallel. Once this is accomplished you may then proceed to balancing the crankshaft.
Many people confuse crankshaft alignment with crankshaft balancing. They are two distinct operations requiring tools and skills for that operation.
Alignment of the Crankshaft
To align a crankshaft, a few tools are needed. Some you can make, but you really need a dial indicator to indicate the actual shaft runout in a miniscule amount such as 0.001” or 0.0254mm.
Assemble the crankshaft by installing the crankpin between the two crankshaft pieces. This operation requires a press and some precise care when joining the pieces. Be sure the rod and rod bearing are installed on the crankpin before final assembly.
The crankpin is held in place by interference fit with the crankpin being just slightly larger than the receiving holes in the crankshaft halves measured in thousandths of an inch (usually 1 or 2 thousandths).
Use your eyes to check alignment of the counterweights. If they are too close at the far end from crankpin they can be pried apart with an aluminum wedge between the counterweights and lightly tapped with a hammer. If they are too far apart, they can be squeezed closer together using a sturdy vise.
The crankshaft should rotate freely around the rod with no binding, bumping, or noticeable wobble of the rod.
Acquire a pair of V blocks or make some from thin material. If you make them the V blocks should be equal heights. Place the assembled crankshaft in the V blocks and set up your dial indicator to indicate the runout of one end shaft. Rotate the crank in the V blocks while observing the dial indicator for needle movement. Any movement greater than 0.001” will indicate you need to align the crankshaft.
Alignment is accomplished by marking the high spot on the counterweight and then striking it in that place with a soft face hammer. The hammer can be a lead hammer, an aluminum one, or a plastic face dead blow hammer. You don’t want to strike the crankshaft with a steel hammer as that will cause damage and deformation of the crankshaft counterweight.
After striking the crankshaft with the hammer, place it back in the V blocks and check again with dial indicator for any improvement in runout.
Sometimes you can strike the counterweight too hard and cause the runout to increase dramatically. This is when you do the opposite location on the counterweight in striking with the hammer.
This operation is repeated as many times as necessary until the indicator shows no more than 0.001” runout. This is done to both sides of the crankshaft to get the crankshaft center line straight so there is no wobble. The dial indicator test should be repeated for both ends of the crankshaft assembly to be sure it is in alignment.
Static Balancing the Motorcycle Crankshaft
When you achieve closeness of no more than 0.001” runout on each side of the crankshaft and the crankpin is parallel to the crankshaft center line you are ready to begin the crankshaft balancing procedure.
Place the assembled piston on the small end of the rod to ready the assembly for balancing.
Motorcycle engines are balanced in an attempt to minimize vibration when the engine is running. Engine vibration can cause operator fatigue, shorten engine life, cause frame damage, and make the overall motorcycle driving experience less pleasurable.
A well balanced motorcycle engine will add years to the engine’s life and make for a happier riding experience.
There are two methods of balancing a motorcycle crankshaft; static and dynamic. This discussion will cover static balancing in the home shop. Static balancing will be “good enough” for the average use of a motorcycle.
A balancing stand can be purchased to aid in balancing the crankshaft. It should come with instructions explaining the proper use of it.
A simple one can be constructed quite economically in the home shop. Use two 2”X8” lumber boards approximately 24” long that are turned with narrow side up and level, secure round metal rods (1/4” or larger) to the top, one on each 2”X8” board. Duct tape is quite sufficient for this task.
Space the boards parallel and far enough apart to accommodate the crankshaft center section while the end shafts are on the steel rods. Adjust the makeshift balancer tool until it is as level as you can get it.
Levelling may require the use of shims at one end of the levelling device. This will take downward slope of the tool out of the balancing equation allowing the crankshaft assembly to be thoroughly tested for correct balance.
Patience and time are the main factors used at this time. The balancing process does take a lot of time and sometimes goes the wrong way. Patience will get you past the difficulties you encounter and practice will increase your knowledge and expertise.
Place the assembled crankshaft on this makeshift balancer and rotate it to another position. If the crankshaft is balanced it will stay in the position you stop it at without rotating further. If it rotates to another position tape a small washer on the counterweight at the top position.
Rotate the crankshaft to another position and see if it will stay in that position. Continue this process until the motorcycle crankshaft will remain in the position you stop it at.
The end result you are searching for is the crankshaft will not rotate any from the position you stop it at no matter where in the 360 degrees of rotation.
To aid in the initial balancing small washers may be taped to the crankshaft counterweights until the crankshaft is balanced. These washers and their tape then need to be weighed on a gram scale to ascertain how much weight needs added to the counterweights.
If this is not working as you want, then you can try adding weight to your piston to achieve balance. This indicates there is a need to remove weight from the counterweights as there is not a good method for adding weight to the piston and rod assembly.
Mallory metal (an alloy of tungsten) that is approximately two times as heavy as steel is used to aid in the balancing of crankshafts. Holes are drilled where additional weight is needed for balancing and the Mallory metal is pressed into the hole, therefore adding weight.
Mallory metal should be placed in holes drilled in the sides of the counterweights. Drilling the sides will aid in retaining the Mallory metal when the engine is being operated at high rpm’s.
Centrifugal force will be exerted on the Mallory metal if it is placed on the circumference of the counterweights. If it happens to come loose it can totally destroy your motorcycle engine and possible cause the rider serious injury.
If the rotating assembly is too heavy for the piston and rod assembly, you can drill holes into the counterweights but not through them to reduce weight or a grinder may be used to remove metal lowering the weight.
Care should be taken to wrap the rod to keep any grinding particles out of the bearings at each end of the rod.
If you grind away metal it is best to grind the side of the counterweight rather than grinding the circumference.
Weight removed near the outside of the counterweight will have a much more pronounced effect than weight removed near the center. Remove the metal in miniscule amounts and check balance often to avoid removing too much metal from the counterweights.
This is check and then check again after adjustments are made. It takes time and patience to get balancing done right.
Dynamic Balancing the Motorcycle Crankshaft
Dynamic balancing of the motorcycle crankshaft requires the use of a highly technical machine that will spin the crankshaft at high rpm (revolutions per minute) and through the use of electronic sensors will indicate the exact positions and the amount to add or remove weight to achieve “perfect” balance.
This procedure is employed when building high performance high rpm engines that are used in motorcycle competition. The dynamic balancing machine is quite expensive and it does take some time to learn how to use it and be proficient balancing a motorcycle crankshaft.
If you want the best balance possible take the crankshaft to a good reputable motorcycle repair shop that has a dynamic balancer and the experience to use it correctly.
Your riding pleasure will be increased considerably with the vibration reduction and overall performance enhancement. There will be some expense and down time involved with going this route, but the end result should be worth it to you if you are a serious motorcyclist.
Gain the Knowledge and Experience
It is a good idea to train yourself using an inexpensive crankshaft assembly from a small motorcycle before moving to a larger more expensive crankshaft assembly.
Many times shopping classifieds on social media will turn up a free or nearly free motorcycle you can acquire for your training experience.
After acquiring a low cost “trainer” you can disassemble the engine and remove the crankshaft assembly. This crankshaft assembly will aid you in seeing just what tools you need to balance the crankshaft assembly.
Follow the instructions outlined in this article and balance the crankshaft assembly.
Once you are comfortable using the tools and achieving success balancing a motorcycle crankshaft, you will be ready to move on to the next step of doing the same for your larger more expensive motorcycle.
You will appreciate the learning experience obtained from the “trainer” bike and be ready to do the bigger jobs.
If you discover that you have a knack for motorcycle crankshaft balancing, it might translate into a “side hustle” that could yield a small income and lots of satisfaction for you as you balance crankshaft assemblies for friends and family.
This income could offset some of the expense that will be incurred in acquiring the equipment and tools necessary for motorcycle crankshaft balancing.
Crankshaft balancing still is somewhat of an art, but does require the science to do it properly and efficiently.
Tools for Crankshaft Balancing
- Sturdy workbench
- Tools for disassembling engine
- Press and press tools
- Dial indicator and stand
- Two 2”X8”X24” lumber boards
- Two ¼”X24” round steel rods
- Duct tape or masking tape
- ½” drill bit
- Drill press or hand held drill
- Right angle grinder
- Large lead hammer
- Assorted washers (to be used as trial weights)
- Gram scale
- V blocks
- Paper or cardboard for levelling shims
- (5 gallon bucket of patience)
Is Motorcycle Crankshaft Balancing For You?
Motorcycle crankshaft balancing has been briefly described here. If, after reading this article, you feel you have the time, knowledge, tools, and patience to embark on this journey, then jump right in to the world of motorcycle service and improvement.
Basic engine skills and knowledge are required to be able to do the disassembly and reassembly of the engine that will be balanced.
If you are doing motorcycle competition, a well balanced engine will give your bike much better performance and could be just the edge you need to top your competitors.
The well balanced motorcycle engine will give the rider many enjoyable hours of travel time with less fatigue and irritation than a less well balanced engine can.
Motorcycle crankshaft balancing can improve and enhance your motorcycle riding experience.
Get ready for your new experience with a well balanced and tuned motorcycle for your next trip.