full counterweighted crankshafts???

[jdperform]

I broke in 2 a crank behind #4 rod. There was no oiling issue, many of the bearings could be reused and show no sign of detonation. Upon magnaflux inspection there were 10 additional fractures. This is a sprint engine SBC. 3.800 with 30 shows. I have multiple engines that have well over 100 shows and no issues.
I have many similar engines racing weekly and NEVER had an issue like this. This make me wonder about fully counter weighted shafts AND OR location of the Mallory metal. Should the metal be located equally among the existing counterweights (3 forward and 3 toward the rear) or is it permissible to put all the Mallory needed on just the 1 end counterweight? Rockwell numbers seem normal.

[Ken_Parkman]

The description sounds like a torsional vibration problem. The crank will have several modes where it wants to vibrate, and this vibration can be excited by the firing pulses. There is are complex combinations of these pulses that lead to exciting orders for the crank. If the engine is being operated in an rpm band where there is an exiting order that coincides with crank frequency you are in big trouble - it can be very destructive. A damper is designed to damp out this destructive situation.

Another sign of torsional vibration is the timing chain being beaten up.

The suggestion is the damper is not working for this crank/rpm combination. Not tuned right for the frequency. The problem is a lot of aftermarket manufacturers do not even know what a damper does, let alone tune it.

This engine much different operating rpm, high or low than the others? Different damper?

[SupStk]

Did the car ever get into an incident where the rear wheels were off the ground?

In regards to the previous post, are you running a gear drive and crank mounted water pump?

[houser45]

No damper needed because you would more than likely have a crank mounted water pump and the cam driven power steering/fuel pump drive on the rear.

[mag2555]

With any V8 Crank be it made with 6 counter weights or 8 ,the counter weights must be index properly and a race Crank like for a sprint car motor needs to have the balance done in regards to being 1" away from the center line , not the more common 3" distance, and not every engine shop that does balance work is trainded to do this , nor is there balancer set up on a concrete base thick enough to do this level of precision!

[Dave Koehler]

I broke in 2 a crank behind #4 rod. There was no oiling issue, many of the bearings could be reused and show no sign of detonation. Upon magnaflux inspection there were 10 additional fractures. This is a sprint engine SBC. 3.800 with 30 shows. I have multiple engines that have well over 100 shows and no issues.
I have many similar engines racing weekly and NEVER had an issue like this. This make me wonder about fully counter weighted shafts AND OR location of the Mallory metal. Should the metal be located equally among the existing counterweights (3 forward and 3 toward the rear) or is it permissible to put all the Mallory needed on just the 1 end counterweight? Rockwell numbers seem normal.

All HM on one end would be unusual unless there was enough weight already available on the other end for whatever reason.
So saying that it should always have 3 and 3 would be incorrect without knowing the whole deal.
Could be 3 on one end, 2 on the other or any other combination necessary to get the job done.
I would think with a solid chassis a experienced driver would have noticed an abnormal vibe.

H Balancer or lack of it doesn't come into play here.

I like center counter weights fwiw.

Maybe you just got one with a flaw that reared it's ugly head later.
Unusual with good stuff I know but maybe.

[Dave Koehler]

With any V8 Crank be it made with 6 counter weights or 8 ,the counter weights must be index properly and a race Crank like for a sprint car motor needs to have the balance done in regards to being 1" away from the center line , not the more common 3" distance, and not every engine shop that does balance work is trainded to do this , nor is there balancer set up on a concrete base thick enough to do this level of precision!

Did I miss a memo from the last 100 years? Is an in/oz no longer an in/oz?

[SupStk]

With any V8 Crank be it made with 6 counter weights or 8 ,the counter weights must be index properly and a race Crank like for a sprint car motor needs to have the balance done in regards to being 1" away from the center line , not the more common 3" distance, and not every engine shop that does balance work is trainded to do this , nor is there balancer set up on a concrete base thick enough to do this level of precision!

Did I miss a memo from the last 100 years? Is an in/oz no longer an in/oz?

I was still scratching my head on this too Dave. The part I'm trying to figure out is how the Mallory goes in 1" off the centerline on a 2 1/2" main journal crank.

[Newold1]

May we ask whose crankshaft, spec. and design. I am also assuming here that this is a high quality forging from a crankshaft maker with good sprint engine results.

I would for one recommend you also consider the 4/7 & 2/3 swap on the camshaft for the LS style firing order. A lot of science and data now shows the crankshaft harmonics are reduced with this firing orders and that can be transferred to the SBC engines.

I personally like properly configured and built center counterweighted crankshafts as they also tend to help reduce crank flexing under extreme loads and rpms. I use them in almost all my chevy V-8 builds. JMO

[jdperform]

I won't disclose crank manufacturer. Yes the car has been in 2 or 3 violent flips so it is plausible the rear tires hooked the ground loading and unloading the motor violently. The crank has 2 large pieces of Mallory in both #1 and #6 counterweight. I would guess and it would be only a guess that it would seem to benifit IF some of that weight was installed on #2 and #5 counterweights. I have never balanced a crank but I could see that it would be easier to just put the Mallory on the easiest counterweight and call it good. Thoughts ya'll?? For those not familiar with a sprint engine there is no crank hub or dampner, gear drive to turn cam, no flywheel just a yoke with splines that weigh about 3 or 4 pounds. titainium driveshaft etc.

[Newold1]

Yep, I suspect violent flips with load stall and release can eventually weaken and twist up any crankshaft! The perils of sprint car racing!

[Dave Koehler]

Try not to overthink it. That is not your problem. Tumbling, Upside down on fire might be.
The only time you see HM in the other counterweights is when there is not enough room in the outer CWs.
Usually it is a really long stroke with heavy parts that get this treatment.

[SupStk]

I won't disclose crank manufacturer. Yes the car has been in 2 or 3 violent flips so it is plausible the rear tires hooked the ground loading and unloading the motor violently.

My opinion that's the cause of your broken crank. I tell my customers after flipping a sprint, "We need to take that thing apart and mag the crank". More often than not, they are cracked.
No matter if a reworked stock forging or a high dollar billet, they all suffer the same fate
.
Concerning balancing, the only time I slug the inboard counter weights is for more correction.

Also much prefer center counter weights if given the choice. They are an asset at higher RPM.

[SchmidtMotorWorks]

Placement of heavy metal is not the cause of cracks if the bearings do not show unusual wear.

[Geoff2]

Problem may well be the harmonic dampener.

Situation: engine kept cracking the crank pulley, happened about 3 times. No obvious reason, no strange vibration felt, car was drag raced & street driven.
One day the owner started the engine, engine made a clunk noise & stopped dead. I got the rebuild job. Crank had broken near #2 main.
The engine had a dampener that was held together with socket head bolts. Half had come loose & this was the problem....