How much can you lighten a 292 crank and still be durable for the street? Thank you for your reply.

I've taken 12 pounds off of quite a few of them for race engines before. As long as you rebalance the crank, it won't affect street longevity at all.

I did not weigh my altered stroker but I would not be surprised if I took 12 pounds off clearancing for wristpin piston boss clearance. I've built three engines around this same crank without evidence of a problem.

I'd like to see pictures of these lightened cranks. It is funny that on another site we discuss adding weights to cranks that don't have any. There must be a happy "balance".

I wish I had taken pics of some of the ones I did. You can get a bunch of weight off by rounding the counterweights up, but the rest comes from knife-edging and radius cutting the front and rear of the rod check regions.

I cut round as far as I could, then pendulum cut more. This crank actually will clear 5.7 rods. 6.0 rods in the photos.

That is a lot of cutting. Thanks for the pictures.

Yep, i'd say its a lot lighter than it started out!

I understand that the reason for counter weights is to offset effect of the weight of the rods and pistons moving up and down in the block and the resulting stress of turning that into rotating energy, but how close is the counter weight to the rods and piston weight? How do you figure how much weight is best? Also what is the best shape?

Beater, that is a very complex question. Mechanical balance as you suggest is only the beginning. Vibration modes torsional and otherwise, manufacturing considerations, mechanical stresses, bearing loads, and more I am sure.

I justified it in my mind that my pistons are much lighter than stock so I can lighten my counterweights and let it go at that. I also use a fluid damper with the thinking that it is not tuned like an elastomer damper so should be effective for my altered rotating assembly.

I attempted no calculations, just clearance to fit and see what happens...

Beater, we lightened most all of the inline race engines we built, not just 6 cylinders but 4 cylinders as well. An inline crank has much different balancing rules than a "V" engine does. In a "V" engine, you have a formula that has to be calculated based on all the weights of the individual rotating components including rings, pistons, wrist pins, wrist pin locks if it has them, a percentage of the connecting rods small end weight and total weight(including bolts, nuts and washers) and bearings. Then a bobweight is constructed and bolted to each rod throw of the crank based on this formula you calculated and it is spun in a balancing machine with these weights attached to the crank. Also, if the crank is externally balanced, then the appropriate balancer and flywheel is also affixed to the crank as well as its balanced.

But for an inline engine, the weights of all these rotating components is insignificant to the balance of the crank. The crank is spun in the balance machine independent of anything else being attached to it. As long as the total weight of these parts for each cylinder weighs the same, it doesn't affect the balance of the crank. The counterweight weights cancel out the weight of opposing counterweights to balance itself. In the same way, the piston, rod, wrist pin weights also cancel out the weights of opposing components kinda' like a seesaw.

Now whether there is a magic number as to how much counterweight can be removed before it affects things. I really doubt it since the mass of the counterweights has no relation to the weight of the rotating mass of the rods and pistons and other components to achieve balancing. If you spin the crank in the engine with all the related components attached to it to 7000 RPM, or remove all the rods and pistons and spin it to 7000 RPM again, the crank is still balanced with or without these parts bolted to it.

CNC, you are correct but you are only talking about the mechanical balance of the rotating assembly. Or to put it another way, balance effects that you might feel outside the engine. If these were the only concern there would be no need for counterweights at all on an inline six.

Counterweights reduce crankshaft bending stresses. I suspect this is the primary reason GM went from 6 to 12 weights on the 292 cranks. Nothing to do with mechanical balance but major effect on bearing loads and crankshaft durability. It is also likely that 6 large weights versus 12 smaller weights will exhibit different torsional vibration characteristics.

True, we always used the 12 counterweight cranks because they had better vibration qualities. But I think the number of counterweights had more to do with this quality than the mass they actually had. The more mass we removed from these cranks the less we experienced harmonic shaking. It didn't eliminate it, but it did reduce it significantly compared to our non-lightened 292 cranks.

Interesting. Removing counterweight mass but not appreciably changing the shaft stiffness would raise natural vibration frequency which is probably favorable. Must be what you observed.

It always seemed to be beneficial in our applications to reduce counterweight mass. Whether this contradicts engineering theory or not, I don't know. But we never experienced any detrimental side effects from it. Crank longevity didn't seem to affected by it either way. In a street engine, it can't help to also be a benefit I would think. It obviously hasn't had any ill side effects in your stroker combos either, has it, or you wouldn't have kept doing it, right.

This is interesting. I probably could have removed some weight from the 181 crank in my 153 four cylinder but somehow think that weight my be my friend driving the little four on the street. Then there is the question of the '28 chevy four that has no weights at all. Many have welded weights on. A friend has a set I may get.

That was also popular with the Model A cranks to add counterweights to them that had no counterweights originally.

How does it affect the RPM's? Pros and cons vs cost?

Reducing the rotating mass allows the engine to rev faster since its not having to spin so much weight. Its like swapping from a 30 lb. flywheel to a light 5 lb. aluminum one. it is much easier for the engine to rev because it doesn't have to extra weight to spin.

As for the cost, all shops have different shop rates, and some may not be knowledgeable about these types of mods. So you'll just have to shop around.

Not a 292 crank but a 250,
IIRC, Mike Kirby had told me he had lightened a couple 250 cranks by about 18 lbs & it did not help his ET or MPH one bit.

Those cranks he lightened just made his wallet lighter with no benifits.

So he just uses standard 250 cranks that last just as long as his lightened one do if not longer.

lightening the crank has little effect because the mass you can remove is so close to the center line. You can shape the crank lumps to a wedge shape to reduce drag (oil/air mist) and polish the crank, but you would not notice any real life application difference at the speeds you would be spinning it. Lightening a flywheel from the outer rim is very effective because the weight is so far from the center line. Reciprocating masses matter too, lighter pistons and rods help.

It was always our experience being many time NHRA National Record Holders(over 40 division and class records) and many time National Champions(11 NHRA National Championships) in professional racing and in the record books that kept us doing these mods. We didn't spend two days lightening our cranks if it didn't benefit us in faster ET's or more records. Other peoples experiences may vary, but for us and our many customers we also put in the record books, lightening the crank always improved performance numbers.

Which is why I said "you would not notice any real life application difference at the speeds you would be spinning it" this is a question for a street engine, not a race engine. I too have experience in building race engines with lightened cranks But they spin 10 grand. A street driven 292 will most likely not see past 6k :P

Mike Kirbys 250 engines turn 9000 RPM & he saw no difference racing with the lightened 250 cranks. Maybe others have had better numbers with them?

I do plan on knife edging my 250 crank.