bebbetufs

Hello.
I have acces to a huge lathe and I'm considering lightening my crank for my DE engine. Chris has mentioned that reducing the diameter of the counter weights could be preferable to knife edginge, I agree as it will help keep the rotating mass away from the oil and help reducing frothing which is a very likely contributer to the #2 bearing failures.

I should be able to reduce the overall diameter of the counterweights on the lathe. Are there any concerns to be aware of other than protecting the journals? If I reduce one counter weight at a time and weigh the crank in between each time I should be able to remove the same amount of weight from each journal. What about the large counter weight at the rear of the crank. Do I reduce that with the same amount of weight as the others or do I need to remove a weight proportional to its larger size?

How much weight can be removed from a crank without reducing streetability too much?

I realize that the crank will have to be professionally rebalanced afterwards.

Paulyy

It's possibly in theory. But i've never done it before so i cannot say yes go do it. You probably can weigh the crank and take off equal amount on each counterweight. I think if you set the tool correctly and 0 the Y axis as soon as you touch the counterweight and take off and equal amount on each side would work. But i'd want to get advice from a proper engine builder before going ahead. But yeah it most likely need to be rebalanced

bebbetufs

I'm guessing it would need to be cooled during the process to avoid changing the hardening and other properties of the metal.

Paulyy

most lathes should have coolant. either to cool the tool bits and or the material. no material likes being cut hot

bebbetufs

True, but the ones I'm used to operating are smaller non cooled ones so I will have to check wether the one I'm thinking of using actually has coolant.

How much weight do people normally remove for a street car?

Paulyy

We've got 3 lathes. the small one isn't cooled. which is 500mm. our 800mm one has coolant, but that is a production/boring lathe. the 2000mm lathe has coolant also.

I'm not sure if the small lathe is big enough to do a crank. i wouldn't do it on the small one.

rlm328

You do not change the over all diameter per se but bevel the thickness of the counter weight, thus knife edged. I would send it to some one who has done it before.

carlege

Is it really worth putting it back together and then having a huge mess if you dont know what you are doing?

lee101315

I'd vote no, it will be an enormous waste of time if the machine shop cant balance the crank because too much was removed in the wrong places. I dont think the rotating assembly is something to "learn" on. You should either do it under the supervision of someone who knows what theyre doing, or just send it out to be done.

Chris White

A couple of thoughts -
don't work about cooling - the area of the crank you are working on is not heat treated and unless you get it really hot it won't have any issues.

How much to remove is a problem - You should weigh the crank before you start and then work on one counter weight and then weigh the crank to figure out how much was removed - then remove the same amount from the counter weight that is opposite the one you just did. Repeat for all the other counter weights and you will be close enough that a crank balancer can get the crank back to spec.

don't get to carried away with trying to lighten the crank for street use, a lighter crank will also decrease the smoothness of the engine.

bebbetufs

Thanks Chris.
I realize that reducing the diameter has more effect on the balance than removing material further closer to the centre. Have you actually reduced cranks this way? If so how much would you recommend to remove from a street car. How bad will it throw the engine off balance if the pistons and rods are not lightened at the same time?

bebbetufs

Agreed. I would use a huge one they have at my mate's work. I just haven't used that particular lathe before.

If I was knife-edging it I would not even contemplate doing it myself. However, I'm considering simply reducing the diameter, keeping the counter weights close to flat. This makes it much easier and should keep the crank away from the oil better, provided it won't cause excessive imbalance in the rotating assy.

Voith

Crank is self balancing so pistons and rods can not throw it off balance if you lighten the crank.

bebbetufs

I'm not sure I understand. According to everything I have read abut cranks the counterweight is there to counteract the exact weight of the piston, rings, rod, pin, bearings and even the rod journal. There is a formula for calculating this.

Normally you would not want to reduce the weight of one, say the crank, unless you also reduce the other as this will create an imbalance in the complete rotating assembly. Depending on how bad thiese vibrations are, and where in the rev band they are harmonically amplified, they can cause the crank to fail.

Is this not correct?

Voith

Counterweights on 4 cylinder crankshafts are on opposite sides. This means they balance each other out and are not there to counter piston and rod.

Also crank counterweight weights at least 5 times more than piston&rod combo.

If you put lighter pistons in or knife crankshaft, this will have no effect on first order vibration of engine (as long as parts are balanced), second order depends only on (reciprocating) piston and rod weight, and balancing shafts should be lightened acordingly.


http://en.wikipedia.org/wiki/Engine_balance

Weights are there to store inertia so that the engine is smoother.