How to Balance the Engine for Less Vibration and More Top RPM

(This version of this page is only for those with two stroke bicycle engines. For all other engines click here.)

The fix is two-fold. The first half of the fix affects vibration all through the rpm range. The second half of the fix lessens vibration only at the top rpm range where it is caused by the standard too-advanced ignition timing.

1. After you have settled on an appropriate cylinder compression (in psi. more compression gives more power) then you can change the port timing just a bit for better high rpm power and balance the engine by removing weight from the piston and wrist pin. (Drilling holes in the flywheels only offsets the weight of the piston and wrist pin.) On bikes that self-limit their mph by vibration you can easily tell if you are making it better (more in balance) by the max velocity. The first part of the fix is to lighten the combo of piston and wrist pin. I had my wrist pin drilled out from a 5.8mm diameter hole to a 7.5mm hole. With these weak engines there is no need to worry that the pin will be too weak after drilling. The same holds true for the piston. Your machinist can use a 9/32" (7.1mm) carbide drill bit to use in his lathe on the wrist pin (available for $8 from Grainger) or you can buy a 33mm long wrist pin for the 48cc piston that already has a 7.5mm hole in it, available from Treatland. Next are 11gm wrist pins with a 7mm hole (1.2mm more than stock) from that will work in the piston for a 55cc or 60cc Grubee engine:
Generic 36mm x 10mm wrist pin & bearing $13
Generic 36mm x 10mm wrist pin $9
They probably also work in a 69cc engine since the piston is only 2mm wider. They used to also sell a 37.5mm long 10mm diameter titanium wrist pin (6.3mm I.D.) which is really good since titanium has around half the weight of stainless steel. You can use a grinding wheel to shorten it if needed. It is 3 grams lighter than the 55cc/60cc wrist pins. But if you have a high compression head it is better to use a steel wrist pin though since the titanium pin recommended here is not treated and wears down faster under severe conditions or when muddy water is splashed onto your filter and sucked into the engine. Use high quality oil and a bronze bushing if you use the titanium pin.

You can also lighten the piston by drilling holes in it that will be no more than 5mm from the side edges of the exhaust and intake port. Drill size is 9/32" and each hole shouldn't be closer than 10mm center-to-center from the next hole. Here's a photo showing where you can drill up to 9 holes on each side. Unfortunately, if the piston wall is 1.9mm thick (as mine is), 18 holes will remove only 3.3 grams.

I have a reed valved engine and with the additional holes on the intake side mine totaled 27 holes and I have rev'd it to 37mph (8400 rpm) and it held up OK. Use an exacto knife to trim the outer edge of each hole to not be a sharp edge. I used a small bit with my dremel to grind a dent in the piston where each hole should be drilled. Then I used a small drill bit to make the pilot holes. (Otherwise the drill bit would wander off center.) Then I used the 7/32" bit to make the final size. Aluminum is very light so even 18 holes won't make a huge difference in weight but when it comes to engine balancing every little bit counts. Once you find the best balance you can change the porting a bit. With these two changes the vibration should be much less and the top rpm much more.

2. The second part of the fix is to buy the Jaguar CDI which is designed to spark later at rpm above 3600 to lessen the combustion/compression forces that the piston and flywheel have to push against. This approach is standard with 2 stroke engines. I think the CDI that comes with these engines is made for a 4 stroke since it does not have that essential change in timing at high rpm. That causes reduced power and reduced rpm, both of which are wanted by the company so that it can pass all countries regulations. But you don't want that because you want power and a bit more rpm without having to endure excess vibrations at the handgrips and seat.

Vibration is reported to be much more with the 69cc (80cc) engine and so this cure is even more vital for it. If not enough then you have to balance the crank assembly. Here are some reports on the motoredbike forum:

"Is the 2 cycle [69cc] motor known for a lot of vibration? I have some rubber pads between the mounts and the frame, but anything over about 15mph is just about unbearable."   post

"My seat seems to be vibrating a little too much to comfortably ride long distances at full throttle. It seems to have gotten worse recently. Any idea how I could figure out where it is coming from? The engine mounts are solid I wrapped the frame in thick leather under the mounting hardware. I'm running the Chinese 66cc 2 stroke on a mountain bike."  post

"I just finished my first motored bike build- a "Black Stallion" 66/80cc from Kings Motor Bikes. the end of yesterdays ride I cranked it wide open to see what it could do. As it built RPMs, it passed a certain range and the entire bike began vibrating like the engine was totally mis-balanced! The gas tank loosened and shifted, and I had trouble keeping my hands on the handlebars! I dropped RPMs, and the vibration went completely away. I tried doing this several times, and each time I crossed that certain RPM barrier, the bike would go into wild vibration!" post

"I have a huffy panama jack bicycle, with a 80cc engine on it. I have such a bad vibration in the bike, its terrible.Ive tried rubber motor mounts, does anyone have any ideas? please help!!!!!!!!!!!!"  post

Crankshaft balancing (for even less vibration):

An imbalance in the crankshaft in relation to the reciprocating weight of the upper end causes vibration and a loss of power. Making sure your engine is balanced correctly is essential, especially if you are modifying the engine to work in a different RPM range than what it was designed for. Using a lighter wrist pin lightens the balance area by 4.5 grams which can make a big difference in a stock 48cc. If it has higher compression and is ported for higher revs then the Jaguar CDI will be needed. Click here to read why I don't recommend using the balance factor method.

I had two different 55cc top ends of different cylinders and pistons to go on the bottom end of a 48cc engine. One was with piston port intake and the other was reed valved. The results I got testing those, along with online calculators for upper piston assembly inertia force and the centrifugal force of the counter balance is what I based my theory of balancing on. The piston port engine was way off in balance, and the other was perfectly balanced. Using it as a base point I developed an Excel file for calculating the rotational forces on the crankshaft which allows you to input different counter-balance hole sizes till the numbers are right.

In calculating the offsetting centrifugal force of the imbalanced flywheel we treat the weight removed by the flywheel holes as an additional weight on the opposite side. For example, if you removed the flywheel and put it on two weight scales you will see that the non-holed side has weight in excess of the holed side equal to the actual weight removed when the holes were made. So we use that "missing" weight to make the centrifugal force calculations.

1st test:
Piston port intake 55cc engine (see engine details below) ported for 10,000 rpm but that achieved only 9100 since I just did the test runs with the standard exhaust pipe instead of an expansion chamber with the correct header length for 10,000 rpm. Anyway here are the details:
upper assembly weight: 105.6gm (piston, rings, bearing, wrist pin, end of conrod)
additional counter balance weight removed: 9.8gm (via 7.2mm diameter hole on each crank wheel)
The engine vibrated between 5600 and 7900 rpm and ran smooth before and after that rpm range.

specs of piston port intake engine:
55cc Grubee cylinder/head on 48cc bottom end
port durations: 185 exhaust, 119 transfers, 125 intake
transfer port walls removed for greater transfer area
stuffed crankcase
155 psi cranking pressure
18mm Mikuni
custom intake manifold
piston port intake
slant plug head with squish band .65mm from piston
Kawasaki KX65 piston and rings (adapted for use with piston port intake)
Jaguar CDI with Kawasaki KX high voltage coil
44 tooth rear sprocket
26" wheels with mountain bike tires
peak head temperature: 425F

2nd test:
My other 55cc engine (reed valve, Honda piston, torque pipe, 18mm Mikuni) with 96.6gm upper assembly with 15.8 grams removed via a 9.15mm diameter hole thru both flywheels (centered between the two balance holes) allowed my engine to go up to 9150 rpm (downhill) without any bothersome vibration.

In figuring the counter balance weight it's important to include everything that would affect it. As example: my flywheel came with two 11.5mm diameter holes through both flywheels. The stainless steel there removed adds up to 50 grams. The conrod pin added 3.3 grams after the weight of the conrod pin holes weight were subtracted from it. Its weight was only calculated using the steel weight calculator listed below, not measured. The part of the conrod that is around the bearing, and the bearing itself, weigh around 30 grams. The centrifugal force has to be figured at the distances of 19mm of the conrod, 36mm of the additional balance hole, and the distance of the two counter balance holes.

Here are two useful online calculators. The second one may be needed if you use two different sized drill bits in the same hole with the largest bit only drilling a portion of the full depth. You can do that if you need a certain amount of weight removed but you don't have the right size drill bit.

centrifugal force calculator  (don't enter linear speed. change m to mm, change kg to grams, change N to lbf)

steel weight calculator  (multiply kgs by 1000 to get grams)

Here is a picture of my crank assembly with an additional balancing hole just above the conrod pin. The 6 blue holes are lightening holes for better acceleration (although I wouldn't recommend any more than 4 if the bike is for street use). The blue is foam filling half the hole. The ends of each hole were later filled with JBWeld. I used foam just to reduce the amount of expensive JBWeld used. The conrod hole and two factory balance holes are already filled with JBWeld for increased crankcase compression (which isn't important unless your engine revs to 9000 or more).

Concerning determining the weight of the lower conrod bearing and the part of the conrod that is around the bearing: I figured that by dipping the two into a measured amount of water and seeing how many cc (ml) they raise the water level and then multiplied their ratio by the total weight. My 48cc rod end with bearing had an equivalent 30 grams.

I have made my own crank balance calculator which you can buy from the following pages but here are the results that you can readily make use of:

For the following data set for a 69cc engine I see that the extra two holes you can drill need to be close to 9.8mm in diameter. (use 3/8inch)

69cc Grubee engine
connecting rod 80mm center to center (weight 65 grams)
piston/rings/bearing 91 grams
wrist pin 14 grams

If the connecting rod is 85mm then use a 10.5mm drill bit (13/32" #1G901 from for $21)

You can drill these holes with any good electric drill although it's a bit tough. Much easier to take it to a machine shop and let them put it on a drill press. Also the holes can be drilled at the TDC location of the crank wheels without even taking it out of the crank cases. Just put duct tape on the crank wheels (after cleaning them with alcohol) to keep metal shavings from going into the crankcase, and then keep the crank in correct position by using vise grips on the primary gear above and below where it meshes with the clutch gear. Each hole should be 15.5mm deep. Measure from halfway through the angled tip and mark the drill bit at the 15.5mm location with black electrical tape. That way you have a visual reference while drilling.

Here is a way to drill holes without taking the cases apart. Take the engine out of the bike and take off the cylinder and piston. Clean the crank wheels with alcohol and secure the position of the cank with vise grips on the primary gear. And use duct tape around the hole sites to prevent metal fillings from going down into the cases.

Click here to read more about my spreadsheet which can be used to calculate the size of counter balance holes needed in any crank.