66cc HT Crude Balancing

[AussieSteve]

From what I've read here, the moving parts in the 48cc and 66cc HT engines are exactly the same, apart from the piston and rings.
Further reading tells me that the 48cc engines rev out smoothly whereas the 66cc engines vibrate badly above about 4000 RPM. (Mine sure does)

So, it appears that any factory balancing was done on the 48cc and the 66cc piston & rings are too heavy, probably causing the extra vibrations.

Also, the inlet port is partly covered by the piston at BDC, which, it is said, can be cured by shaving the bottom of the piston skirt on the inlet side.

It therefore follows that shaving the piston skirt will also improve balance, (in 66cc versions only), allowing higher max RPM without destroying the motor.

My question(s) - won't shaving one side of the skirt throw the piston off-balance?
And what would be the result of shaving the exhaust side of the piston skirt to match, thereby improving piston balance and hopefully reducing piston slap, as well as improving overall engine balance?

I'm looking forward to any input on this topic.

... Steve

 

[210061741]

Ok lets see.
The thing you have to remember is shaving the piston skirt changes the port timing.
Changing the port timing affects directly how the engine runs.
I added 1 bottom jug gasket to my cylinder.
My ebgine now runs like ****.
I can easily remove the extra gasket.

The point is the jug gasket is only .045 inches thick.
I raised my ports .045 by adding a second gasket.
This changes the port timing.
My engine runs like **** now.

Cutting the piston skirt would have the same effect.
It will change the Port timing.

So unless you have an extra piston i definatly wouldn't cut both sides.

I have heard also that cutting the skirt on the intake side works well.
I haven't done it yet.

The smart thing to do would be to make a Port Map.
Then you can figure out exactly where the timing is now and plug the same numbers in some software to figure out the optimal timing

I am in the process of this.

I have found out by error that a small change in timing can make a big diff.

I personally would like to know what perfect is before i try to make somethin perfect.

Either way not a bad idea to have a spare jug and piston around.

The smallest changes sometimes have a very profound result.

BTW:
If you cut the piston let me know how you make out.
I have heard it is a good thing.
But i have not verified it yet.

 

[Scotchmo]

Ok lets see.
...
The point is the jug gasket is only .045 inches thick.
I raised my ports .045 by adding a second gasket.
This changes the port timing.
My engine runs like **** now.

Raising the inlet port only .045 is not that much of a change. But you also raised the head .045 and thereby reduced your compression ratio significantly. That is probably why the engine is low on power.

Anything that you do to lighten the reciprocation mass should reduce the overall vibration.

 

[210061741]

I milled down the head and the Jug to keep the compression in the cylinder.
And yes i agree .045 isn't much but it made a big diff.