bebbetufs

Or are they balanced separately and assembled to tolerances?
The reason I'm asking is because I'm considering having the rods balanced on their own as they need to be refreshed. I will have to ship them to get this done and I really don't want to mess with shipping the crank at this point. Will this throw off the balance of the entire rotating assy?

Arominus

I would think the entire rotating assembly would need to be balanced at once in order for it to be worth it. Crank/rods/pistons/rod bolts all together.

FRporscheman

My machinist (who mainly does 944s) said he balances the entire rotating assembly, but I don't know how the factory did it. Do you have the workshop manuals? There's a surprising amount of detail and information in there.

bebbetufs

I have the manuals but I can't remember reading about balancing.
I know that to balance the crank you have to have bob weights simulating the pistons, rods, pins, rings and lock rings attached to each journal. My question is what if you don't want to rebalance the cranck, only weight match the pistons (yes, I know, but money and shipping are limiting factors). If the factory does it like it is done when blueprinting an engine then all parts are balanced together and altering one will throw everything out of whack. However, if they are balanced individually to set tolerances I should be able to safely weight match the rods and pistons and see an improvement even if I don't touch the crank at this stage.

87 944 C

the flywheel balances them. it's called external balancing. if wanna run a lightwight flywheel, it's better to have your internals balanced. the internals are balanced, but not perfect

ferrari, 911 gt's, turbo's and lambo are the only one's i know that factory are balanced internals

bebbetufs

The way I read this the flywheel is balanced to the weight of all the complete rotating assembly meaning I can't mess with the rods and pistons. Is this correct?

87 944 C

usually you can mess with rods and cranks, i've never heard of changing weight on a piston itself though

GregBBRD

4 cylinder in-line cranks (924, 944, 968) are balanced independant of the rod and piston weight. The balancers do not have to do a "bob weight" calculation which involves the rod and piston weights, like on a V-8 engine.

You can change piston or rod weight, without any regard to the balance of the crankshaft.

FRporscheman

I thought the counterweights on the crank have to be shaved when using lighter rods and/or pistons?

I remember reading somewhere on rennlist that the 951 (and I suppose the 944) engines are not perfectly balanced from the factory, and it becomes more apparent when using a lighter flywheel. But this doesn't necessarily mean they didn't balance the rotating assembly at all - maybe they did it to large tolerances.

bebbetufs

Thanks guys.
The crank has definitely been factory balanced. They removed quite a bit of material on mine to do so. I have never seen that the pistons and rods need to be lightened to match the amount of material removed when cranks are knife edged. The way some cranks are ligthened it would be impossible to match the weight reduction on the pistons and rods anyway.

@Greg. Did you have a look at my measurements? Would you say all the rods need refreshing or only the worst one?

GregBBRD

Yes. The "factory tolerances" for rod and piston variance was very high. The cranks are balanced, but not as close as someone building a "race" engine would make them.

bebbetufs

Ok, I just weighed the rods. I did not manage isolating the small end enough to get super accurate readings, but when weighing the rods themselves they are withing 0.2g of each other. Pretty impressive I must say.

The pistons themselves where not as close, but within 1.5g of each other. Where is the best place to remove material from a piston?

I have not weighed the pins yet.

fortysixandtwo

I'll second what Greg pointed out. The crank is balanced by its self. The connecting rod big end is considered to be part of the rotating assembly, so you'll want to match those to each other. Then the pistons, rings, wrist pins, and connecting rod small ends are part of the reciprocating assembly.

Before you go removing material, measure all of the parts. You may end up really close by mixing parts of various mass. The goal would be to not have to remove any material, or remove it from the fewest number of parts as possible. Also keep in mind, while you try to chase that last 0.5g of imbalance, these parts are going to be thrashing around in oil. So the variation in the amount of oil stuck the parts surfaces, may be greater than that 0.5g.

bebbetufs

Thanks guys. Is it ok to mix used parts? I was under the impression that once a part has bedded in it should stay in the bore it belongs to.

f1rocks

Your lifters are typically the one thing that should be monogamous.