Yes - you should be fine, you can feel an unbalanced motor. If the vibration goes away when you put the weight on - leave it.

 

so if the flywheel does not have enough weight on it to blance the crank it will shake pretty bad, but if you put more then enouh weight on the flywheel it will be fine?

 

Yes. To a certain extent.

 

so i guess it is still possible that my flywheel is not 100% neutral blanced with the weight off and as soon as i throw a weight on there everything becomes overblanced and its fine?

 

Anything is possible - your machine shop should have supplied you with a balance card which lists the numbers and weights for rod, piston, rings, pin, oil allowance, etc they extrapilate bob-weight and balance to a +/- and list that number.... The only way you are going to KNOW is to check that card and talk to them about the problem, or pull the motor and re-spin the rotating assy.

 

I don't think so. Unbalance is unbalance whether it's too much weight or not enough weight added. If you were to put two of the weights on the flywheel at the same spot you'd probably have the same amount of unbalance but in the opposite direction. (don't try this at home with a running engine!)


Some folks might confuse "overbalance: with "unbalance". Overbalance usually means the bob weights used during balancing are a few % heavier than the theoretical weight, which is all of the reciprocating weight and 1/2 the rotating weight. The problem comes in estimating what part of the rod is reciprocating. Just weighing the little end and big end isn't necessarily exact, but it's real close.

"Unbalance" means some amount of weight some distance from the center of rotation causing the vibration. In the case of the flywheel add-on weight, it's the weight of the piece times the distance from the center of the crank to the center of gravity of the add-on weight. If you were trying for 1/2 in-oz of maximum unbalance, that's a 1/2 oz (14.2 gm) weight located 1 inch (2.54 cm) from the center of the crank, which is still within the main bearing diameter.

Your add on weight might be 4 oz (a guess) at 6 inches from the center or 24 in-oz of unbalance, 48 times what you might want for out of balance, or 10 times what you might get with a 2% "overbalance". You'll definitely feel that.

Clear as mud?

 

consider the traditional sbc 400.
how much 'imbalance' was on the, front, the back.

I think the correct answer would be some thing like...
the front needs 30 oz-inches at 18 deg atdc,

or some such gibberish.

 

well got to the bottom of it
The motor is externlly balanced with stock flywheel
Thanks
Marcin

 

You're lucky.

I had the EXACT same issue a few years ago with my 396 and it wound-up breaking the oil-pump drive that runs off the back of the cam @ 6800 rpm with hours on the new 6-speed setup.

Neutrally-balanced flywheel on an externally-balanced motor =


Mike

 

I have a queston.
I bought a street twin from Jason Short. It was previously used behind his 396 and was balanced internally I believe. I plan on using it with my stock short block. I figured that I would have to have it rebalanced. It presently does not have a bolt on weight on it. If I was to get one from Mcloud and installed it would I be able to then install the clutch as is? I tried calling mcloud but they are really hard to get ahold of. Any thoughts?
Thanks,
Kory