Broken crankshaft on 327/350 shp

[Ron M.]

On a recent pleasure drive to the northern part of New Mexico I experienced a "rattle" in the engine compartment. Long story short; I had to have the car towed to a friend's shop for diagnosois and repair.
He discovered the crankshaft was broken in two pieces just back of the front main bearing. I have owned this car since 1980 and have always driven it in a sane manner and kept it well maintained. Any thoughts why this happened, how to prevent this from happening and how common is this anyway?

[Rich G.]

Re: Broken crankshaft on 327/350 shp

Was this a GM original or an aftermarket crank? My aviation connections make me aware of a number of Airworthiness Directives for crankshaft failures in aircraft engines traced back to improper hardening processes at the sub-contractor.

I have not heard of this before in our cars (which doesn't mean much). I have a 327/350 in my 66 that was rebuilt by the previous owner.

Rich

[Stuart F.]

Re: Broken crankshaft on 327/350 shp

Ron;

I'm sure the experts will "post in" with some good information - not that that will help you feel a whole lot better.

Sometimes, though, it is just a bad crank that finally gave up. Case in point, back in the day, I had three friends that each bought new 59 335 hp "W" engine equipped Chevy Biscaynes; two were police equipped w/3.36 open differentials and one w/4.56 posi. All three broke their cranks in exactly the same place (#2 Main) in three pieces (one being a wedge) and, ironically, all at about 8k on their odometers. They all sounded like a rattle in the clutch and were drivable with some evidence of a slight miss or roughness. Each one fooled us into thinking it was something else as a cause - you'd think we would learn. The first two with the highway gears got to 8k first and were fixed under warranty. The third was drag raced so it took longer to get to 8k, but we pulled it apart ourselves thinking it was a rod bearing. As we finally got to removing the mains, we saw the oil bubbles in the journal.

Stu fox

[Michael H.]

Re: Broken crankshaft on 327/350 shp

Any thoughts why this happened, how to prevent this from happening and how common is this anyway?

That is a VERY rare occurance in a small block with a forged crankshaft. Somewhat more common in a 350 with a cast crank and even more in the 400" small block engines but for a 327, it's almost unheard of.
Possible forging problem from day one or a balance issue that eventually created cracking/failure?

[Joe R.]

Re: Broken crankshaft on 327/350 shp

Ron,

Don't worry about that EVER happening again! Find a good used forged crank at a machine shop, turn it 0.010 and balance the assembly. And make sure your favorite machine shop checks and re-conditions all eight of your rods. Then the assembly will outlast all of us!

JR

[Ron M.]

Re: Broken crankshaft on 327/350 shp

Was this a GM original or an aftermarket crank? My aviation connections make me aware of a number of Airworthiness Directives for crankshaft failures in aircraft engines traced back to improper hardening processes at the sub-contractor.

I have not heard of this before in our cars (which doesn't mean much). I have a 327/350 in my 66 that was rebuilt by the previous owner.

Rich

I believe the crank is GM since I found the part number 3782680 cast on it with GM10 stamped in also. I was not the original owner but did talk to the previous owner back in 1982 when I bought the car. He made said the engine had been rebuilt, but made no mention of a crank replacement.
Thanks for your interest and response.
Ron

[Steven B.]

Re: Broken crankshaft on 327/350 shp

I had the same thing happen to an L-82 I bought new about 27-28 years ago. A friend was getting my car ready while I was at work so I could swap cams. The day we completed the swap I drove about 3 miles outside of town at ABOUT 60-65 MPH. I heard the rattle and shut it down immediately. When I pulled it apart I found a six cylinder crank and a two cylinder crank. Later Mick told me he raised the front of the engine after unbolting the front mounts, but did not loosen the rear. I "assumed" the stress from behind may have done it????

[Tom B.]

Re: Broken crankshaft on 327/350 shp

I was reading a hot rod forum the other day and there were some photos from a machine shop in Caifornia. The guy took 2 big block Chevy cranks, one cast and one forged, and mounted them in his press supported only on the ends. When he started pushing in the center to see how far they would deflect. The cast crank snapped easily. The forged crank bent into a "C" and is now on display on the parts counter in his shop. I don't think you could ever break a forged crank with lateral force from lifting the engine when it is secured in the block. I think that the bellhousing would take most of the stress anyhow.

Steve,

Did you get your trees cleaned up from the hurricane yet?

[Duke W.]

Re: Broken crankshaft on 327/350 shp

I believe the crank is GM since I found the part number 3782680 cast on it with GM10 stamped in also. I was not the original owner but did talk to the previous owner back in 1982 when I bought the car. He made said the engine had been rebuilt, but made no mention of a crank replacement.
Thanks for your interest and response.
Ron

Yes "2680" is a correct forging number for a 327 crankshaft, and I believe there is one other that is commonly seen (I recall "4577') but I don't know the difference between the two raw forgings. They are both machined to the same specifications as far as I know.

Failures of these forged 327 crankshafts as previously mentioned is rare, but it can happen. This is why all crankshafts should be Magaflux inspected as part of any engine restoration. Any original flaws that have caused a crack to propagate will usually be detected.

Previous advise to "tune the crankshaft .010" is ill-advised. This is typical machine shop practice - don't measure or think, just grind!

SHP crankshafts have a special Tufftride hardened surface, which will be destroyed if the crank is turned. Crankshafts should only be ground undersized if there are dimensional problems or journal imperfections that cannot be eliminated by polishing.

Most 327 cranks will Mag okay, and unless there has been an oiling problem the journals should be within OE spec and the crankshaft will be straight.

In these most common cases, a good journal polish is all that's needed.

Ground cranks can be re-surfaced hardened using various processes.

This is also an opportunity to upgrade the weak 327 rods even if they are the second design.

Duke

[Bill M.]

Re: Broken crankshaft on 327/350 shp

I would check the harmonic balancer too.

[Steven B.]

Re: Broken crankshaft on 327/350 shp

I was reading a hot rod forum the other day and there were some photos from a machine shop in Caifornia. The guy took 2 big block Chevy cranks, one cast and one forged, and mounted them in his press supported only on the ends. When he started pushing in the center to see how far they would deflect. The cast crank snapped easily. The forged crank bent into a "C" and is now on display on the parts counter in his shop. I don't think you could ever break a forged crank with lateral force from lifting the engine when it is secured in the block. I think that the bellhousing would take most of the stress anyhow.

Steve,

Did you get your trees cleaned up from the hurricane yet?

Tom, I only have a couple more to go! Stop by on your way to BJ and we can install an engine!

Steve

[Tom B.]

Re: Broken crankshaft on 327/350 shp

I'll talk to the pilot.

I have heard of balancer problems causing crank failure too. A look to see if the ring has slipped would be a good idea.

Tom

[Joe L.]

Re: Broken crankshaft on 327/350 shp

I would check the harmonic balancer too.

Bill-----


I agree. In fact, if it were me, I'd replace the balancer on the rebuilt engine. There are possible problems with balancers that are very difficult to discern with visual inspection. It's just not worth trying to re-use this balancer, in my opinion, especially considering it's likely over 40 years old. In fact, I'd use a NEW GM balancer and not worry about the lack of "fins" on the rear of the balancer hub. I'd also DEFINITELY have the entire reciprocating assembly, including new balancer, custom-balanced. NEVER put an engine together without doing that.

One curious thing here: both of the engines reported on here, the L-79 and L-82, have FORGED cranks, not cast. So, based on the small sampling of broken cranks reported on here, we have two broken forged cranks and no broken cast cranks.

Years ago someone told me that NASCAR engines used cast cranks because they were better able to hold up to the sort of stresses that NASCAR racing imposed on them. I never confirmed if that was true, though. Plus, this was years ago, so even IF they did once use cast cranks, that wouldn't necessarily mean they still do.

[Ron M.]

Re: Broken crankshaft on 327/350 shp

Thanks to all who responded. I appreciate the good advice and knowledge shared.
Ron

[Clem Z.]

Re: Broken crankshaft on 327/350 shp

Bill-----


I agree. In fact, if it were me, I'd replace the balancer on the rebuilt engine. There are possible problems with balancers that are very difficult to discern with visual inspection. It's just not worth trying to re-use this balancer, in my opinion, especially considering it's likely over 40 years old. In fact, I'd use a NEW GM balancer and not worry about the lack of "fins" on the rear of the balancer hub. I'd also DEFINITELY have the entire reciprocating assembly, including new balancer, custom-balanced. NEVER put an engine together without doing that.

One curious thing here: both of the engines reported on here, the L-79 and L-82, have FORGED cranks, not cast. So, based on the small sampling of broken cranks reported on here, we have two broken forged cranks and no broken cast cranks.

Years ago someone told me that NASCAR engines used cast cranks because they were better able to hold up to the sort of stresses that NASCAR racing imposed on them. I never confirmed if that was true, though. Plus, this was years ago, so even IF they did once use cast cranks, that wouldn't necessarily mean they still do.

NASCAR engines use all forged cranks but there was a time when some top fuel engines used cast cranks. a lot of after market cranks and stock production are cast nodular iron with rolled radius in the journal corners to add strength. when we raced 2 liter 4 cylinder cavalier engines we used the stock cast nodular iron crankshaft turning 7500 RPM and making 300+ HP.