Active Member
Wondering how many people are balancing their crankshaft/piston combinations here. I've had to pull the head on my modded motor & am thinking it would benefit from a balance. I've never done this, but am interested in how much it would help. Right now it shakes pretty badly at 3800-4000 rpm, then it smooths out at higher r's.

The basic build is 60363, Arc flywheel & rod, Bullfrog 11:1 domed hemi piston, Black Mamba cam, chromoly pushrods, VM22-133, 26# springs. The new head will be ported a bit when it gets here & I don't have weights on the rod-piston combination. Still a stock crank which has turned 8400 & still seems tight on the bottom. Cylinder is clean & everything is working well enough, except for the head.

I didn't Fit the bracket well enough on the header at the head & the combination of forces due to the slight misalignment & vibration tore a chunk out of the head this weekend. Re-did the header bracket to pick up the lower head screw, so hopefully this will be better. Here's what the old head looks like now with the bolt hole "missing" under the valve cover.


What's going on with people doing a balanced crank on these motors? The Bullfrog piston must be heavier than a stock one, but I'm trying to decide if I should tear down the whole motor & start from scratch again or just put the new head back on once it's ported & call it good.

What if I need extra weight in the crank, tungsten plugs & welding? Shave the bottom of the piston inside a bit?

Thanks in advance for helping me open the newest can of worms in my journey into speed.
Complicated subject but most of the people who claim to balance them are snake oil salesmen. So here are some facts about balancing.
1) you cannot balance a single cylinder engine without a balance shaft or other mechanism to counterbalance the reciprocating weight
2) a single cylinder engine has a rotational component and a reciprocating component.
3) a single cylinder engine has a primary imbalance as well as a secondary imbalance, primary is once per revolution and secondary is twice per revolution.
4) the greater the reciprocating weight the greater the out of balance forces are and they increase with the square of the RPM
5) the lighter the reciprocating weight the less vibration energy so you want to have a light piston. wristpin and thin rings, the pin retainer there is not much to do about as well as the small end of the rod.
6) you can completely balance out the rotating weight with the counterweight but for the reciprocating weight you add a percentage of it to the counterweight. This percentage is called a balance factor.
7) The balance factor moves the out of balance energy in the horizontal and vertical planes relative to the cylinder axis.
8) A balance factor of 100 percent (of the reciprocating weight) basically has the motor in balance at TDC and BDC and is very out of balance at 90 degrees before and after TDC. A zero percent balance factor will be very out of balance at TDC and BDC but in balance at 90 before and after.
9) A 50 percent balance factor ( this is for the purpose of this discussion as it is going to get complicated) more or less spreads the out of balance energy in a circular pattern around the crank rotation and the peak energy of 100% and 0% is basically cut in half.
Now when someone says I can balance your crank the first thing you should ask is what balance factor are you using. I saw a video on you tube of someone you all know using the crank and rod with piston rings ect. balancing that on knife edge rollers which would amount to a 100% balance factor. It would make the horizontal vibrations terrible. People like Falicon cranks that balance single cylinder motorcycle cranks have years of empirical data from trial and error to know what balance factor to use for what bike. The reason they can't just pick one factor for the same motor is the reason you will hear people say I want it balanced for a certain RPM. So if you look at fact 4 which says out of balance force is proportional to the square of the rpm how could that be? The answer is that even though the out of balance forces are increasing, the frequency of these forces are increasing with RPM the primary frequency with the RPM and Secondary frequency at twice the RPM. What happens is the person doing the balancing needs to know the resonant frequencies of the whole structure not just the motor, to learn that is trial and error. just moving the motor like on a slant mount moves the horizontal and vertical vibration components around. So when the engine hits a particular rpm, it just may happen the frequency of the vibration is the same as the resonant frequency of the handlebars and they start shaking like crazy. You could change the balance factor to move the vibrating force in a different plane and it can sometimes stop the resonant vibration even though it is at the resonant frequency. Secondary vibration which happens twice a revolution

So for the purpose of our little engines the reciprocating weight in a stock motor should not cause excessive shaking. Generally there have been many causes I have seen here over the years and almost all were not the engines fault. I have seen centrifugal clutches cause it, aftermarket flywheels with the wrong taper vs the crank go on wobbly, bad engine mounting either loose or broken or cracked welds or parts loose on the bike. Now in your case the bullfrog piston could cause more out of balance force if it is a lot heavier than the stock piston but you would have to weigh it. The best way to reduce vibrating energy is to use the lightest piston, pin and rings but that is easier said than done.

If you want to learn more on this here is one of the best articles to learn about this subject I have seen in many years of balancing my old BSA twin motors which have both pistons going up and down together so its like a single on steroids. Some of my balance jobs were so bad I couldn't see well. I was using .100 over MC forged pistons which had a skirt .200 thick of forged aluminum.



Active Member
Ole, as usual, a wealth of information. It's going to take some time for me to digest this information. Will read the attached PDF & try to think through it.


Active Member
I have to run outboard with the clutch due to the jackshaft setup I'm using. 1st was a MaxTorque (13t), 2nd was an Inferno Flame(14t) with 2 black & 2 white springs. The head broke at some point on 4 or 5 hard, long WOT pulls with the 14t gearing, but didn't notice it until I got the bike back to the shop & started looking things over after the runs. It was the first time I could let it pull as long as it wanted, due to a long paved straight. Prior to that, I'd only been able to run in short bursts due to the road in front of the house.


Guess I could remove the jackshaft & go to a straight chain (inboard clutch location), but it limits gearing choices which I've been playing with on the build. Nice to be able to split hairs with the gearing & so many options for choosing the correct ratio & a jackshaft. Easier to swap jackshaft & clutch sprockets (with the Flame clutch), than change the rear 420 sprocket on a live axle.

I cut the bracket off of the header, relocated it & have a much better fit now with the new bracket. It's the lower bolt hole in the above image. Cut the shroud out & am keeping it for heat & airflow off of the fan, but the new location is better & should be a bit stronger. Was also thinking about using both bolts in the head, but figured a better fit to the lower bolt should work.


Well-Known Member
have you tried running that clutch on another bike? and see if the vibration is still there? it might be just the way the engine and bike is setup? good example would a doodlebug(china) they vibrate bad until you replace the front forks with suspension. this might be little off from what you have going on. in the past I tried to run a 30 series on a built motor with about the same billet flywheel as yours and another motor too with a lite aluminum flywheel and both create a unwanted vibration. when installing that 30 series on a heavy aluminum flywheel or a cast steel flywheel it went away. 30 units are terrible when it comes to spinning high rpm's with billet aluminum flywheels. with the two out pieces are only stamp steel and the inners are cast material. nothing really spins true. more like a egg shape spinning around. when installing a machine out billet aluminum 340 salsbury both engines ran super smooth. most people that I have seen run just a clutch on a built motor normally don't have a problem? definitely change your mounting bracket somewhere else, your asking for trouble in that location. hopefully with trial and error you can figure something out.


Active Member
I've had this happen with this motor & different setups. It's just the way it's balanced with the modded parts. About 4500 rpms it's smooth, just the lighter flywheel, piston, rod, etc. which sets up a different frequency vibration which is rpm related.

Li'l Popeye

Active Member
Interesting thread to be following as I am also dealing with some inbalance.

Here's where I am at:

Perhaps not the most advanced weights used, but as long as it functions.
The flywheel and TC driver pulley didn't change the weight needed and didn't contribute to inbalance in my case.

As I knew the weight needed to get the crank assembly in balance (230gr), I calculated the balance factor and it came out as 45%, that is with the parts I have installed in my GX160.

Because in my case the shaking started after I rebuild 1 engine it must be caused by something that has changed.
I swapped: stock crank with aftermarket, stock connecting rod with ARC 3.303 rod, stock flywheel with ARC ultra lightweight flywheel, stock head gasket with thin metal head gasket. Ignition timing is 4 degree advanced. All other things are stock.
Note: The ARC billet rod 3.303 (with bearings) is (as far as my measurements go) 5gr heavier in total. And these 5 gr heavier is on the big end, small end is the same weight.

This is what I have noticed when comparing the stock crank (left) with an aftermarket crank (right). The stock crank appears to be at an angle, while the aftermarket is level. This could be because of raw casting. Or on purpose, I don't know. Because the stock crank is at an angle it was hard to get the correct weight to make it balance.
The aftermarket crank is heavier as stock.

I have swapped my flywheel back to the heavy stock one.

Just to see if it would eliminate the issue. It did not. It did dampen it a tiny bit. Not enough for me.

But I swapped back to the aluminium flywheel, because the throttle response with this one is much better. And it is safer. The TC driver pulley also adds rotating mass.

It's not caused by tires, because when riding on grass, sand the shaking still exists.

What I am about to do is what ole4 mentioned; lower the mass of reciprocating parts. I think an aftermarket piston is lighter as stock. It might not help all, but if it helps a bit it's worth it.

Li'l Popeye

Active Member
I need to edit my previous message a bit.
The TC driver is in balance on idle. Today I found it isn't when the weights are moving when throttle applied.
But there is an unbalance as I also ran the engine on another minibike, with clutch. It's just not as bad (in a certain rpm range) as with the TC.


Active Member
Curious about the actual rpm range where it starts & stops on your motor. Does it smooth out once you get past the shake & then run up well to top end? I know the iron flywheel will dampen some of the vibration (just more mass to soak up the shake = less apparent vibration), but the problem is still there. A light flywheel will seem to shake more, there's just less mass to soak up the movement. On the linked article Ole posted there's also a discussion of harmonics in the frame, handle bars & forks, engine mount attitude, etc. It's a long list of stuff which affects overall vibration & feel during the ride. I guess that's why experience with certain bikes over time helps change things for the better.

My motor just shakes in the lower rpm range, then it goes away at higher rpms. Same thing on yours as well?
Your balance factor seems low? Generally these engines are built with between 52 and 62 % balance factor but I have no idea why. If you looked at the graphs in the article there is one that shows how at 50% the energy is the lowest but all around the crank. Adding more weight to the crank requires either welding on material of the way it is usually done is to drill a hole or holes opposite crankpin and add Mallory metal (tungsten I think) The Hemi flattop piston is pretty light I am guessing that the Wesco being a forging and depending on which one has a thick top would be heavier, as well as a larger bore piston. But yes the best option if possible is to reduce the piston and pin weight which will raise you balance factor towards 50% if you can. You may be able to taper grind the piston pin. What piston and bore are your running? Some pics from my motorcycle days back in the 70's This was a homemade crude balance stand and some factory weights and a aluminum mold to make my own from led. At the time after my balance job it shook so bad my vision was blurry over 3k rpm. When I went to sell the bike about 10 years ago I completely took it apart used the lightest factory pistons and brought the crank and factory weights to a engine shop here and he dynamically balanced the crank using the stock weights. When I sold the bike it was smoother than it had ever been and faster with a stock bore than the 100 over MC boat anchor pistons.


I agree to what ole4 mentioned; reduce the weight of reciprocating mass, would reduce the vibrations (get you back towards stock balans factor) . As I think there's weight added with the piston you installed. I'm not sure if it's heavier, but it would make sence to me as it is a domed piston. To be certain, you'll have to weigh all the installed / replaced parts and compare them with the stock parts. If possible.
That's what I would and will do.
I'm wondering if adding weight to the counter weights is the only solution to get back to stock balans factor. Would cutting weight at the other side of the crank (where the rod connects) help? I know there's not much material to cut off, but perhaps the hole in the crank could be bored out a little bit (I think this is the reason why they drilled a hole in there, as it's not centered). And maybe some material on the outside lobes. I wouldn't go to crazy on that, but to me it seems as a raw casting, which could still loose some weight. In my case I'll look into that.
The following link is what I'm dealing with at the moment. Torque converter weights causing vibrations:
There's 320 gr of weight running 3 mm of center. With my calculating abilities that's equal to 21 gr at a stroke of 45 mm (GX160 stroke).
I'll need to find a fix for this first, before I can eliminate other vibration causes. And when I have eliminated this, I can tell what an impact 320 gr at 3 mm (or 21 gr at 45 mm) has. If my calculation is not correct, let me know.
By the way: what kind of rocker arms are those? I like that design over the stock GX160 rocker arms.


Active Member
They're just the stock (factory) hemi rocker arms, but they seem pretty well designed & engineered for the job. With the current 26# valve springs & the stock size valves, they've seen 8400 rpms without any trouble (so far). Not sure what going up to heavier springs & bigger valves might cause with them, but so far so good.


Active Member
Ole & Popeye, I took apart the new 60363 motor today, made a crude balance setup & then checked mass on the parts. I got things as level as I could & checked things pretty carefully. If my calcs are correct, the balance factor of the stock motor comes in at 39%. Can this possibly be correct? The piston with rings, pin & clips is 201.8 grams. Small end of the rod is 32.6 grams. With the rod hung on the crank, I needed only 59.2 grams for a balance of the crank at rest.
I added the mass of the small end of the rod to the mass of the weights needed to achieve a neutral balance & got 91.8 grams. This number was then divided by the mass of the piston assembly with the small end of the connecting rod. 32.6 + 59.2 = 91.8 / (201.8 + 32.6) = .3916 or 39.16%

I need a little help here, as these numbers seem very wrong to me. If we're shooting for a 50% balance factor, this is not it with the factory parts. I'm still needing to do the ARC rod & Bullfrog piston combination, but you can see my concern. Would like some input here, or if someone else could check the balance on a factory motor I'd really appreciate it. It's no wonder my build shakes so badly!

The good news is the new crankshaft is within .0005" of being round, so I don't need to be spending a few more days with the wet or dry paper & WD40 to fix it & make it work. I've started a new post on my site to go into the new motor build if anyone's interested.
What's the rod's big end weight? It's part of rotating mass.
Small end weight is reciprocating mass. Together with complete piston assembly.
The 39% is not correct.
If I'm correct:
You need to take the rod off. Weigh rod on both sides and complete. Including bolts and bearings. Small end and big end together should be equal to the total mass of rod.
You will also need the weight of complete piston assembly. Rings, pin, etc.
With the rod off the crank, try to measure how much weight is needed to level the crank at each position.
That weight minus the rod big end weight, gives you a number.
This number/(rod small end+piston assembly weight) gives you balansfactor.
I have not balanced any of my mini/kart motors as none have had any vibration issues. However I do still have my beam balance scale and do have some takeout rods, but I would be hard pressed to tell if they were from old greyhound engines or predators. I could weight them if it would be helpful to have the data. They way I measured them is with the rod held horizontal one end on the scale the other held by a string to a stand arm above it. I measure each end then make sure it adds up to the total weight Popeye has it correct above. reciprocating weight is the grams of the small end added to the piston with pin clips and rings. Take that number and multiply by the balance factor so if your balance factor was 60 percent multiply by .6 In your case I think it would be more important to try to figure out the difference between new motor weights vs your modded motor. I use ARC rods and ARC flywheels (regular 100 dollar one not any of the lightweight ones or adjustable ones) and have had no vibration issues. I would weight the bullfrog piston vs the piston you took out of the shaking motor as well as the wristpin if it is not a stock part. See if the new piston/pin assembly is much different from stock.


Active Member
I've just looked at another site with a different explanation of the method & I have left out the "big end" of the rod completely, as Popeye mentioned. I'd still be interested in someone else's numbers as a reality check on my numbers, just to see.

I'll go back and calculate the new(er) formula with the big end & better numbers, to see where I end up this time. As usual, I think I'm chasing the Whiffle Bird.