I have several 250 that I have been robbing parts off of and selling to support my habit. The other day a friend and I were going through some of my stash of parts and comparing against similar parts to determine what we were going to put in a more performance oriented engine I am buildiing soon.

We were looking at three crankshafts and he noticed that the bob-weights on one were significantly smaller in size than on the other two. The width at the bottom of the weight as well as distance from the crank journals was much less. This got us to thinking about weight.

When we picked up the crank, the one with smaller bob-weights had to weight at least 5lbs if not closer to 10lbs less than the other two.

I remember the smaller crank coming out of a 250 that was installed in a late 60's pontiac. I think it was a transplant.

It's definitely a stock crank, without any machine work done to it. Does anybody know anything in regards to this subject? I will try to get the casting numbers to help.

Just thinking that a crank that is that much lighter would be pretty nice for a more performance oriented motor.

If memory isn't failing me the crank with "407" as the last three digits is the lightest 250 crank....it was a passenger car application.....fats

The casting number is 460407. So it would appear that you are right. Do these have any inherent desirable characteristics for performance aside from light weight? Any draw-backs?

The other two's casting numbers were: 3876802 and 3876802N

Not sure what the "N" means.

I believe the lighter weight,more desireable (SP) cranks are from 250's were built in the 80's.

I have heard the circle track guys love them,I have one for my next build. Can't remember the P.N.

Mike Kirby has had crankshafts lightened up a lot, & said,they do not make his race cars any faster or make more HP & they do not last as long.

Having big heavy counter weights actually help make the engines/cranks last longer.

MBHD

This may be slightly off topic; sorry guys if it is--just let me know and redirect; I've got thick skin!--but on the subject of crank weights:

I'm looking at a '66 292 block, with the original crank. It's my understanding that the '64-66 292 cranks have six counterweights versus the 12 counterweights in later years, and are lighter than the later stock crankshafts.

Any reason not to stay with that crank in a non-race application? We're talking driving RPM (below 5000, generaly) and looking for longevity in the build. (It will replace the 250 in my 1969 Olds!)

In general, more mass in the crank damps vibration, and also lowers the frequency at which vibrations and harmonics occur.
Depending on the track, chassis etc. less weight may not help, especially with dirt.
On a time-to-speed dyno a lighter crank will develop more power every time at every speed (unless the valve gear can't keep up).
The improvement to the chassis as to acceleration will be roughly proportionate to the square of the overall gear ratio: 1st gear × axle × tire OD. If you go from a 2.20 1st gear (Muncie close 4 speed) to a 2.52 the change is over 31% - but only in 1st gear (the accel won't be 31% better, only the power loss due to speeding up the crank will be better).
You can also see from this that 1st gear is far more affected than 4th, by 535% (2.52^2 ÷ 1.00^2), which is why cars that race in high gear don't feel much different.

You can stay w/your original crank,but I believe the crank w/12 counter weights will run smoother.

MBHD

Trump, as MBHD has stated we used them in circle track racing for acceleration off the corner; as also stated the heavier counterweights smooth some of the torsional effects of the crank....if you are building a primarily street engine then I would use that crank in conjunction with the small 153 tooth flywheel found in passenger car apps(or an original 26# Z-28 flywheel if you can find one)....I have been told that the 802N indicates a nodular iron crank (higher graphite content) but I've never seen any factory documentation to support this....fats