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I am just starting a mild performance M10 engine build. I’m using a 1984 1.8 M10 donor motor. It is being bored to 90 mm with custom forged pistons, stock rods with ARP bolts and a 80mm stroke 2002 crankshaft. The other parts from 1984 engine utilized will be the flywheel, which needs resurfacing, pulley/balancer and new single row chain sprocket. The shop that is doing the machine work says that this build does not need balancing since it is an inline 4 cylinder and BMW does a good job at the factory with balancing. They have experience doing BMW engines (they had 3 M10s in the ship and do the machine work for a well known BMW performance/restoration shop). I really don’t know the history of the crankshaft and would think some type of balancing would be wise. Since it is an inline 4 I don’t think dynamic balancing with bob weights is necessary. Comments. Recommendations. 

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The rods are a “matched set“ (3 paint dots) from the 1984 318i donor as well as the flywheel and pulley. I purchased the 2002 crankshaft separately from a member of this forum. Don’t have any other history. It was magnifluxed and polished and is within stock specs.

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From what I've found:

 

usually the BMW balance is very good. 

 

Since it's an I4, there's only so much you can do, but:

 

the pistons should be weighed,

the rods should be weighed, by end.

 

and the clutch/flywheel assembly should be balanced, too.

 

At that point, it's hardly any more to balance the whole thing-

you'll 'usually' find that nothing else needs to be touched,

but sometimes the crank needs just a bit.

 

I view balancing kind of like insurance- you might not really need it

or use it, but knowing it's there is reassuring.

 

t

has been known to assemble engines without balancing them.

Nor balancing his checkbook.

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