I am fully worthy of the abuse I am about to receive.


I was sold a crankshaft, pistons, rods (etc) as an abandoned project. I was told it had been balanced.- I bought if from a very good friend of a very good friend, I was hesitant but I trust people too easily.

I have now got very good reason that the machine work on the crankshaft was never done.

If I was to take it to a shop, is it possible to have the balance checked without going through the entire rebalancing process ?

 

Yes it can be checked but at that point you should balance it anyways. Why would you want a crank that you know is out of balance.

 

Its pretty obvious if it has been done since it was last used. There will be holes drilled that will have no oil staining, varnish, etc.

Checking balance is about 75% of the work of just balancing it. Once its all mounted in the machine and checked, all they do is remove or add weight and recheck it.

 

The "Checking balance is about 75% of the work of just balancing it" is what I seemed to get the impression of, I was just double checking.

Thanks guys.

 

I balanced my own lightened crank, more to see how close I could get it and to reduce the time for the machine shop to take off metal. I used 4x low friction ball bearings bolted to some spare angle and spun the crank gently by hand to find the heaviest side.

The machine shop said it was within a few grams and only needed a couple of small tweaks more to balance each end rather than overall.

Whilst doing my lightened crank I spun up a few spare cranks I had on the shelf. I was surprised to find that one was slightly bent at no4 main bearing and the other two I could find a heavy side. So the 10 mins to make it was worth the time.

It's also quite fun to spin them to a few thousand rpm on the bench with a power drill. They really hum along at those speeds... imagine 9k plus!

 

You're an animal.

 

whycome nobody grinds the weights into an aero shape? seems like a no brainer.

 

Pretty common actually, its called 'bull nosing' and 'knife edging.' Front gets a rounded bull nose shape, trailing edge gets sharpened, so cross section looks like a wing.

 

I'm not sure if reducing the aero drag on a crank actually helps much once you account for the rods and pistons. The mix of air and oil in the crank case would be mashed up into a mess swirling all over the place and pulsing from front to back pumped by the pistons. So there would be no clean airflow for it to spin through. I think reducing frontal area should help a bit plus it drops weight... but machining and balancing teardrop counterweights would be hard to justify.

Its probably easier to just get rid of the air altogether by pulling a vacuum with a dry sump setup.