Homemade crank balancing

velardejose

New Member
Sep 9, 2008
120
0
0
Lima, Peru
Hi
I disassembled my engine to balance it
Vibration and a rattling sound I noticed a week ago
Before disassembling I discovered a broken mounting stud, rear mount
At the machine shop we took the broken stud out, and went to balancing
Piston+pin+rings+clips = 70 gr
Light rod end = 35 gr
Total reciprocating weight = 105 gr
I fixed a 55 gr weight to the rod journal
And checked the crank balance using two metal rods (horizontal bubble level checked) and suspending the light rod end with a string
Too heavy
I started drilling the crank added side pieces, and found it had holes in it
Was not enough
I had to drill 1/4 holes through the center crank plates
And balance was perfect
Thing is, when taking the parts to the machine shop, to the local representant to use his extractor, and to the gasket shop to make a center gasket the front gear dissapeared
Grrr
I will see my mechanic today, hope it will be assembled, maybe the gear is in my bag...
Jose
PD I am replacing all studs with automotive grade ones
 
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velardejose

New Member
Sep 9, 2008
120
0
0
Lima, Peru
Re: Homemade crank balancing (sort of)

Hi
Crank balancing (sort of)


Piston mod, there were 2 mm skirt in the intake path with the piston in the top dead center


And cyl+head after a light pass using a 320 wet sandpaper
You can notice it is not flat exactly...


Hope it helps
Peace
 

velardejose

New Member
Sep 9, 2008
120
0
0
Lima, Peru
Hi
I visited my local representant looking for a front gear replacement (right crankshaft end)
He lent me a used gear to use it as a template to make another one for my engine
I got the duplicate gear today (after 5 days and 30 usd)
My engine will be assembled tomorrow
Can't wait to try it and see if the mod reduced the vibration
Peace
 

Goat Herder

Gutter Rider
Apr 28, 2008
6,248
12
38
N.M.
Glad to see more people are noticing the piston clearance at TDC. This mod will make the little china run with lots more power. When you had the jug off hope you played with the porting. [matching exaust &intake]. Do look closely at the gasket to the bottom of the jug.[trimmed mine considerably it blocked half the ports at bottom of jug].Not all of these chinas have piston clearance issues at TDC some modols put the intake hole lower, clearing the piston skirt.()
 
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velardejose

New Member
Sep 9, 2008
120
0
0
Lima, Peru
Finally
I tried my mb today
Loud gear noise, just as new, because of the 'new' crankshaft gear
Easy for a couple blocks, and then I tried high revs
Smooth as silk!
Danger is there is no 'vibration rev limiter' anymore and thing goes fast
I highly recommend it
Jose
 

velardejose

New Member
Sep 9, 2008
120
0
0
Lima, Peru
Report
2 mods at the same time: crank balanced and piston skirt filed to clear the intake path
All studs upgraded to automotive grade ones
Result
I can cruise at comfortable speeds, no variation
I can accelerate to higher revs (higher speed too), and I am too scary (old?) to keep it for more than a few secs
Engine is normal at lower revs and at higher revs vibration seems to be in the pedal area, not the whole mb
Idle seems to be more stable
Mileage seems to be the same
Will make a diagram with the holes I did in the crankshaft
Peace
 

europorsche914

New Member
Jun 18, 2008
168
1
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Lancaster, PA / Newark, DE
2 crankshafts later and finally I have been successful in my efforts. I drilled two holes 3/8" in diameter through the existing holes already in the side plates. I only feel vibration at severely high rpms which I achieve from my boost ports so if anyone is attempting this it will be balanced for the rpms normally used in these engines (non boost ported) since you will not achieve the same insane rpms as i do... Good Luck! -Justin
 

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2 crankshafts later and finally I have been successful in my efforts. I drilled two holes 3/8" in diameter through the existing holes already in the side plates. I only feel vibration at severely high rpms which I achieve from my boost ports so if anyone is attempting this it will be balanced for the rpms normally used in these engines (non boost ported) since you will not achieve the same insane rpms as i do... Good Luck! -Justin
Id like to see the work on your cylinder
got any pics?
take care
JAK
 

robin

Member
Nov 15, 2008
618
3
18
74
Penticton British Columbia Canada
Great--just ordered an 80cc--will do mods before using--my mechanic friend says i can use 50 to 1 oil mix any comments--i would like specs too please
Do you bore holes all the way thru--also how do you port and polish --thank you!!
P.S. on a new engine do i need new gaskets?
.cargo
 
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jasonh

New Member
Jun 23, 2008
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Longmont, CO
usually it's recommended to run a little heavy on the oil during break-in. 16:1 or 20:1 for the first gallon or so. After that, I use 32:1 of a good synthetic. 50:1 of a good synthetic may be ok, but you never know, because of the rough tolerances on these motors.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
236
63
up north now
Great--just ordered an 80cc--will do mods before using--my mechanic friend says i can use 50 to 1 oil mix any comments--i would like specs too please
Do you bore holes all the way thru--also how do you port and polish --thank you!!
P.S. on a new engine do i need new gaskets?
.cargo

With all due respect to your mechanic buddy, I would say 50:1 is NOT O.K. for these engines, unless you are willing to risk the big bang theory.

24:1 and if you are adventurous, go to 32:1 after break in.

24:1 with synthetic will never let you down, but may cost a few pennies more....not enough to count really.
 

tylerlough

New Member
May 28, 2008
109
0
0
Mishawaka, IN
This would have been good info to have a few weeks ago when my engine was almost totally apart. But it's all back together now. I discovered a problem when I went on a 5 mile test ride, it's damn cold out.
Thanks for posting the pictures.
 

Egor

New Member
Jan 30, 2008
714
0
0
Hurricane Utah
I have a few engines that have the two piece cranks, they still vibrate. I am going to take one apart and see if they are close. I wonder if the cavities would be the same as the small holes in the one piece crank. I will let you know. I only have one engine that is close to being smooth running, it has a two piece crank, that is my Green Hornet, it has a Dax engine. Have fun, Dave
 

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