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MG MGB Technical - Balancing flywheel

If I have my flywheel lightened, can it be balanced separately from the crankshaft, or do they have to be balanced as a unit

T J Malloch

Although the 1st best option is to balance the crank and then bolt on the flywheel and balance it as an assemgly and then the same with the clutch cover----- If you don't have your engine apart it is quite common to have the flywheel balanced on it's own without the crank,and then bolt the clutch cover on and balance it as well, Also when lightening your flywheel you get better results by removing material from the larger diameter sections of the flywheel - Willy

Terry, do you know if the crankshaft is balanced? If not you are totally wasting your money just doing the flywheel.
The best, and in my opinion the only way to do it, is to do the crank, then put on the pulley and do it again, then the flywheel and do it again, then the clutch and Bob's your uncle!!

Not forgetting the pistons and rods of course!

Colin Parkinson

Colin That's fine if Terry has his engine apart- but if he only has the flywheel off there is absolutley no reason why he can't get it balanced seperatley- And it would be needed to be balanced one way or the other if it's going to be machined down Willy

Willy, so what is the point of only having 1 bit of it balanced??

Nothing at all. The complete assembly is still out of balance!!

Colin Parkinson

The crank is balanced from the factory. Then the flywheel (or other rotating piece) is put on and balanced, etc. If the crank is right, then the flywheel etc. will be also, and independently. The point of balancing as an assembly is to make sure that small discrepancies within tolerance at each stage do not happen to add up all on one side, putting the total out of tolerance.
If you are changing some part of the total, ideally you might rebalance all from the start. This is not realistic, so you balance the replacement bits on their own. Carried to extremes, this would mean that if you had to replace the clutch driven plate, you would have to rebalance everything, but you would in fact not change anything but the part you substituted. {I should like to see your reaction if you brought a car in for a clutch plate and I presented you with a bill for a complete engine dismantle, rebalance, and rebuild !}
In this case, since he is cutting the flywheel, which might drastically unbalance it and it alone, it should or must be balanced separately. If all pieces or subassemblies are balanced to tolerance, the total should certainly be acceptable.

Fletcher R Millmore

As Fletcher says, the crank is balanced in manufacture, but only within a tolerance so may still be slightly out.
It is likely to be significantly out if the crank has been reground and not rebalanced.

The flywheel is the most important item to balance as the moment of inertia is greater at the larger diameter of the flywheel. One gram out on the fly will have roughly 3 times the effect of 1 gram out on the crank. If removing material then the balance will be thrown out as the density of the steel varies slightly in places and even though you take an even cut, the weight distribution changes.

The chap who does my balancing is a perfectionist and the crank is balanced to zero, then the flywheel added & balanced to zero and then the front pulley, again to zero. Finally the clutch cover if I am supplying a clutch with the engine.

Chris at Octarine Services

As Chris says, any imbalance out on the periphery of the flywheel can be a problem. OTOH, they aren't likely to be very far out as the factory balance was at least different.

I wish I could say the same about replacement clutch covers - we end up with bits of metal welded to the outside of them sometimess to get them back into balance as the manufacturers seem not to be too worried about it.
Bill Spohn

This thread was discussed between 05/03/2010 and 07/03/2010

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