Mathematical formula for balancing 69cc Chinese Bicycle motor

Discussion in '2-Stroke Engines' started by Fabian, Oct 22, 2009.

  1. Fabian

    Fabian Well-Known Member

    G'day all

    I've stripped down my Happy Time style 69cc Chinese bicycle motor.
    It has a 40mm stroke and 47mm bore.

    The crank has been split to remove the big end bearing and to allow for accurate phasing of the crankshaft halves.
    I've found high quality replacement big end and small end connecting rod bearings, also high quality replacement left and right side crankshaft bearings - these also happen to be the same specification as the clutch shaft bearings.

    Ok, i am not knowledgeable about crankshaft balancing or the mathematical formula to create a bob weight for dialing in crank balance.

    Can someone with expert knowledge in this area advise how (or give a tutorial) to complete this task.

    Last edited: Oct 22, 2009

  2. Flapdoodle

    Flapdoodle Member

    Not sure I qualify as a "knowledgeable expert", but I have balanced small engines and reciprocating components for manufacturing equipment.

    The easiest way for the home mechanic to do it is by placing the crank with connecting rod, piston and rings (don't forget the wrist pin retainer clips) between two smooth straight edges. The blades from two machinists squares are ideal.

    Make a jig that hold both blades at a distance equal to the crank bearings. Plywood is fine. Use a good bubble level to get both blades a level as possible. Reverse the level so that the bubble is the same if measured either direction. (do this is in case the level is not accurate)

    If you have a flat surface such as a ground steel plate, use it to make sure the two blades are parallel. If you don't have one you can do it by eye with fairly good results.

    Put the crank with the rod and piston between the blades. It should not rotate no matter what angular position the crank is at. Tap the blades to see if the crank rotates. Remove a small amount of metal from the inside of the piston, or the crank counter balance as necessary.

    This sounds awfully difficult, but it really quite easy.
  3. Fabian

    Fabian Well-Known Member

    so what you're saying is that the connecting rod and piston should be able to hang from the crank pin as you rotate the crankshaft on the two straight edges.

    If the piston/rod combo pulls the crank pin downwards, you remove a quantity of metal from the piston till it reaches equilibrium and stasis with the crankshaft counterweight.
    If the crankshaft counter weight pulls the crank pin upwards, you remove a quantity of metal from the counterweight till the combination reaches equilibrium and stasis with the piston/rod combo.

    Seems like an unbelievably simple method to achieve perfect primary balance.

  4. Al.Fisherman

    Al.Fisherman Active Member

    The area that material needs to be removed should always be on the bottom (baring a bad bearing). Think of this as a plumb line.
  5. clay

    clay Member

    no. if it is balanced that way, it will be balanced until the motor fires up then will progressively become more out of balance until it shakes apart. a well balanced motor will shake at idle and will become smoother as rpm's increase.
    i explained it a little here
  6. Fabian

    Fabian Well-Known Member

  7. Fabian

    Fabian Well-Known Member

    so, what method does the home handyman use to balance variants of Happy Time style Chinese Bicycle motors?
  8. clay

    clay Member

  9. Fabian

    Fabian Well-Known Member


    After splitting the crank on my 69cc Chinese Motorised Bicycle engine to replace the big end bearing (that always fails on these Chinese engines) with a high quality item, i've been able to measure the dimensions for a replacement part.

    It's an oddball size needle roller bearing: 16mm inside diameter, 21mm outside diameter and 10mm in width, also known as a timkin style bearing K16x21x10
    I have been trying to find a Japanse motorbike that uses this bearing so i can order a genuine quality Japanese part.
    All i've been able to find that uses this bearing is the Suzuki FZ50 and one website listed the following, though they did not give a Suzuki part number which complicates the process enormously, as the motorbike shops just want a Suzuki part number; they are not interested in the dimensions of the bearing.

    JM118 JAULA DE MUNON DE BIELA - 16 x 21 x 10 HERO PUCH 65 TUBU / SUZUKI FZ50 2,13

    I just need an Illustrated Parts List for the Suzuki FZ50 to work out the part number
  10. Fabian

    Fabian Well-Known Member


    This is looking better - a complete connecting rod kit that includes big end and small end bearings for the Suzuki FZ50.
    I have no idea on the centre to centre length of the Chinese Bicycle Engine connecting rod as compared to the FZ50 but if it's the same dimensions, the quality of bearings and connecting rod would be significantly better - check out the pic
  11. Fabian

    Fabian Well-Known Member

    More info

    The Suzuki connecting rod part number of FZ50 for the models 1979 and 1981 are both 12161-04001

    The big end bearing part number for 1979 and 1981 are both: 09263-1501c or 09263-15016
    This is because the print isn't very clear and i'm not sure if it's a c or a 6 on the end.

    The Small end connecting rod part number for both 1979 and 1981 are: 09263-12003
  12. srdavo

    srdavo Active Member

    I've not seen, or heard of many complaints of 'big bearing' failures.

    I've seen lots of wrist pin bushings worn out. I've never had a roller bearing wrist pin, but have heard of a few failures.

    The only bearing that has given any problems, on any of my kits, was a left side main crank bearing. This bearing was noisy, right out of the box, but I ran it several thousand miles. When I finally pulled the seal, some extra metal fell out.


    great pics of your balancing & engine. I am not into extreme speed or power, but a smoother, vibration-free engine would sure be a plus.
    Shame they don't engineer this in the factories!!
  13. Fabian

    Fabian Well-Known Member

    Scroll down to the "16" section - you'll find a K16x21x10

    At least this is one company where the bearing can be sourced.

    SrDavo, your comment is interesting
    The retailer who is selling Chinese Bicycle Engines has had a large number of failures from the big end connecting rod bearings (including my first engine) and no failures from the small end bearing.
    The other failures are from engine seizure due to incorrect oil ratio and plain mechanical failure.

    I've spent the last two days with the importer/retailer and we have put together a large PDF doccument with photos attached of the big end failures.
    It's been sent to the manufacturer in China, to fix their product.

    Really interesting that you have only seen small end failures - surprising as the diameter of the small end bearing has significantly lower cage speed compared to the big end bearing.
    If a bearing is going to fail, it's more likely to be the big end, come to think of it, my Kawasaki KX500 blew the big end bearing after a vicious over rev trying to hold the bike on the back wheel, going through the gears and maxing it out in top gear for too long on a long straight dirt road.
    The small end didn't disintegrate, but it was replaced when the engine was rebuilt.

    Last edited: Oct 24, 2009
  14. srdavo

    srdavo Active Member

    that's good news.... more improvements on the way. Do you mind telling us who the manufacturer is?

    My daily rider... a 55cc grubee, (lots of miles) is on the 2nd wristpin bushing & is starting to 'tick tick tick'. It's about time to tear it down & have a look.
  15. Fabian

    Fabian Well-Known Member

    Gees, this is taking me back quite a few years now - a very long time indeed

    In the early years of motorcycling my second bike was a Yamaha RZ250 with a set of modified carburettors and a set of race pipes.
    It wasn't all that long before it blew the big end connecting rod bearing, after over rev chasing an early Suzuki GSXR 750.
    Must have been about 115 miles an hour when it blew - certainly an intersting ride getting it to a stop.
    After the thing was rebuilt with RZ350 cylinders, i left Melbourne and moved to Far North Queensland, a ride of some 3600 kilometers (2200 miles).
    I loved that RZ250 with the 350cc engine capacity.
    For a small engine, it really had some get-up and go.

    Must have been up there for 2 years and i found a nice quiet straight road out the back of Cairns on the Atherton Tablelands.
    I made another fatal engine mistake.
    I maxed out the speed (loved it) speedo reading 220 kilometers (140 miles an hour) and it blew the big end connecting rod bearing
    Made for another interesting stop from speed.
    After the engine was rebuilt, someone told me that i should have used Yamaha TZ big end bearings as they were far more durable - if i only knew that before it was rebuilt, the first time.

    From my experience, 2-strokes seem to have their achilles heel set in the big end bearing - they just don't tollerate over rev because of the limitation due to a needle roller bearing big end.

    Last edited: Oct 24, 2009
  16. Fabian

    Fabian Well-Known Member

    I have no idea which manufacturer the Chinese importer/retailer is using, i never even thought of asking.

    The guy who prepared the PDF is Chinese and wrote the thing in Chinese.
    He should know who the manufacturer is.

    One thing is interesting to note - the early engine kits used a brass bushed small end and 8mm head studs, yet, when we checked a few of the motors from the current shipment, they all use 6mm head studs and a needle roller bearing small end.

    Last edited: Oct 24, 2009
  17. Fabian

    Fabian Well-Known Member

    It also looks like the ATALA RIZZATO Califfone uses a K16x21x10 big end connecting rod bearing - haven't been able to find an Illustrated Parts List to confirm this though.

    It seems that all these 50cc 2-stroke mopeds use very similar crankshaft, connecting rod and bearing sizes compared to our Chinese Bicycle Engines.

    Maybe someone can find out the dimensions of a Honda 50cc moped's connecting rod and small and big end bearing sizes.
    Honda has some of the best engineered and most durable motorcycles.
    If i could find a genuine Honda K16x21x10, i would buy it on the spot.

  18. 210061741

    210061741 Guest

    the suziki bearing is 15X21X10
  19. keystone

    keystone New Member

    Have you tried Timken I tried to get the link posted,but won't work. They have a listing for 2 cycle small engine, outboard motor applications.
  20. Fabian

    Fabian Well-Known Member

    Hi Rich

    That Suzuki part is not useable for our Chinese Bicycle Engines because the crankshaft pin is 16mm in diameter and the inner diameter of a K15x21x10 needle roller bearing is 15mm.

    The only way it could be made to work is by having the working face that needle roller bearing rides on, offset ground to 15mm diameter.
    This would have an added bonous of increasing the crankshaft stroke to 42mm from the standard 80cc engines 40mm.

    Total capacity would go up from 69cc to 73cc with a boost in the compression ratio.
    Fact is that we can get a K16x21x10 and it works with the standard engine parts.

    Last edited: Oct 24, 2009