How Much Can 12 cwt Crank Be Lightened & Knife Edged ?

[milnersXcoupe]

Given the Weight of a 12 cwt crankshaft & inherent dynamic balance
how much can the counterweights be trimmed to reduce overall weight ?

I like the idea of uniform balancing via counterweights just not
the some 20 extra pounds of cast iron to spin up .

[BADASYJ]

Not sure anyone here has gone as far as knife edging a crank for these motors. The extra weight does help with the harmonics.
Honestly though, it would be way more cost effective and lighter for you to just go with a 4 cwt crank then to have your 12 cwt crank knife edged then balanced. Unless you plan on racing and spinning up to 7000 rpms you won't be gaining anything but a lighter wallet. Even then you wouldnt even notice a difference.

[Cheromaniac]

Given the Weight of a 12 cwt crankshaft & inherent dynamic balance
how much can the counterweights be trimmed to reduce overall weight ?

I like the idea of uniform balancing via counterweights just not
the some 20 extra pounds of cast iron to spin up .

On an engine that's rev limited to a modest 5250rpm, why bother?

[IH 392]

Trade your 12CW crank for a 4CW crank.
Properly balanced, I don't believe its been proven that a 4CW crank will spin up any faster than a 12CW crank?

[Russ Pottenger]

I agree with everybody statements up to this point.

Knife edging cast Crankshaft in a low RPM tractor engine is a waste of time and money.

[gradon]

The 20# lighter 4-weight will absolutely spin up(and down) faster than the 12-weight. I was even contemplating a 10# lighter fidanza flywheel at one time. Even 5# lighter makes a noticable difference in the rotating assembly(I'm in the BMW world too with my '95 M3--there are many lighter flywheel and clutch assembly options). The bottom line is if you are off-roading you want the 12-weight and if not, you want the 4-weight.

[gradon]

My brother Nick just told me his knife-edged(or semi-knife 'cause I thought it could be more aggresive), offset-ground(4.060" stroke) 12-weight 505 crankshaft weighs 60#, so that'll give you a clue how much you can lose.

[Russ Pottenger]

I guess another question that needs to be asked is what is the application of your stroker project?

I ask because sometimes the purpose and budget of the build will help to determine the practicality of a upgrade.

For example. If you had a budget of less then 5K to build your stroker, knife edging a crankshaft would be a waste of money because you'd receive better gains or benifits if you applied your budget and other areas

There is definitely pros and cons to both 12 and 4 counterweight Crankshafts.

[IH 392]

A lighter flywheel will give more benefit than a lighter crank, the mass is at a greater distance of the radius.
On our race car we cut a 199 9" clutch flywheel right down to the pressure plate bolts and carved it a lot more than that too, and used a flex plate to start it with, it really spins up fast!

[BADASYJ]

The 20# lighter 4-weight will absolutely spin up(and down) faster than the 12-weight.

I used to think that as well. The crank is pretty much center mass so the extra 20# is negligible on spining up n down. Only a larger stroke could add rotational mass. The advantage that the 12cwt crank brings to the table is that it softens the harmonics at higher rpms and thats about it. Honestly a 4cwt crank would probably be more benificial for 95% of the people running 12cwt cranks. All for the simple reason that it would drop 20# off the nose of the jeep.

[Cheromaniac]

A lighter flywheel will give more benefit than a lighter crank, the mass is at a greater distance of the radius.

Agree.

[Cheromaniac]

knife edging a crankshaft would be a waste of money because you'd receive better gains or benefits if you applied your budget in other areas

Like adding a crank scraper, welding an extra baffle inside the oil pan, adding a "kickout" to the sump to increase its capacity, and swapping in a HV oil pump. Perhaps an oil cooler too if you do a lot of offroading.

[jsawduste]

Darn it.
Wrote a long explanation on the physics of rotational mass. Musta pushed the wrong button 'cause it anit in the thread.

Let's just say the guy with a bad haircut knew a few things way back when.

You cannot beat physics. You might think you can but it isn't gonna happen.

[gradon]

My brother told me the wrong weight. His scale might be off, but he weighed it to be:

[Russ Pottenger]

Did your brother mention how much or how involved it was to rebalance the crank?