The Act of Crank Balancing

[StrontiumEthics]

Hey guys, ive been doing a lot of reading about crank balancing these HT engine. It seems as thought most people use between a 53%-55% balancing factor which is fine. I'm trying to get up to about 8500-9000 rpms without vibrating things apart. I have an extra crank that I don't need to practice on from a 66cc engine, and im sure that any sort of balancing would do these engines justice.

I know that the 48cc crank was used and because of the heavier top end the engine vibrates like a mofo at higher rpms. In general for the 66cc do I begin with adding weight or removing weight to the crank?

As for the Crank balancing factor, is it that I add 53%-55% to the lower end of the crank (flywheel)?

I will be working on a crank balancing device that I can use to measure the weight of the crank.

 

[Fabian]

From what i understand the subject of engine balancing has been covered in depth, maybe many times over.
Try using the search function for crank balancing, otherwise Google is your friend.

I've never gone so far as crank balancing because my application has the engine operating in the midrange speeds, between 2,500 and 4,500 rpm.

Unfortunately single cylinder engines cannot be balanced for a broad rpm spread, but only in a narrow rpm range; vibration progressively getting worse outside of the chosen rpm sweet spot.

If it's set for reasonable balance at 9,000 then it will vibrate appallingly at 4000.
The only real solution (aside from a balance shaft) is to add a second cylinder at 90 degrees to the existing cylinder centre line.

 

[Deleted member 12676]

Before going to the extreme trouble of drilling on the crank wheels why don't you just try to lighten up the wrist pin? It worked for me. Obviously these things aren't too far out of whack for such a small change to make a big difference. And drilling a bigger hole in the wrist pin doesn't weaken it too much because a few horsepower engine without excessive compression doesn't require much strength there.

 

[StrontiumEthics]

Drilling a hole in the wrist pin might be a bit risky with the high compression setup that I have.

 

[Purple Haze]

Before going to the extreme trouble of drilling on the crank wheels why don't you just try to lighten up the wrist pin? It worked for me. Obviously these things aren't too far out of whack for such a small change to make a big difference. And drilling a bigger hole in the wrist pin doesn't weaken it too much because a few horsepower engine without excessive compression doesn't require much strength there.

I think Jag is on to something, KB pistons does this routinely on their racing pistons, and they have waaaay more stress put on them than our little motors are capable of. My trick is to match the weight of the piston assembly with the 49cc assembly. Works for me.

 

[StrontiumEthics]

100% I actually tried drilling out that wrist pin yesterday. No joke that stuff is some HARDDD ****. I couldn't get through it with my industrial drill press. Im in the process of figuring this out

Purple Haze what do you mean by " match the weight of the piston assembly with the 49cc assembly" How is that possible the assembly is much heavier than the piston no?

 

[Fabian]

You will need a "full cobalt" drill bit to drill through hardened material.
Can you remove some of the weight from the piston skirt?

Much easier than lightening the wrist pin.

 

[Purple Haze]

The assembly consists of the piston, rings, pin, bearing, and clips. The heaviest by far is the pin. Concentrate most of your effort in lightening there, you can get real close. The piston itself doesn't weigh much, but can be lightened some. Any balancing on these engines is a compromise, as Fabian states so well. As for weakening the the pin, it could be case-hardened for peace of mind, though I don't think it necessary. As long as you can get close to the 49cc weight, it'll really help smooth out the vibrations.

 

[Purple Haze]

You will need a "full cobalt" drill bit to drill through hardened material.
Can you remove some of the weight from the piston skirt?

Much easier than lightening the wrist pin.

Yes, the best place to start is on the skirts. These pistons are waaay too long, but have to be because of the port design. I try to trim the bottom of the skirt where it doesn't affect any port timing. Don't worry about equalizing side to side weight, it has little to no effect on balance. It's the total weight (moving up and down that matters. Don't remove any material where it will compromise the strength of the piston in the pin area.

 

[Deleted member 12676]

Removal of weight from the piston is only for fine tuning because aluminum is so light. Drill out the wrist pin with a 7.5mm drill bit. If there's still some vibration then you can try to drill a couple holes in the upper part of the connecting rod.