Factory cast crank experiences
 

Curious what issues or success stories folks have had with factory cast crankshafts.

I've seen one break in two pieces. It was the later style crankshaft that was in a 76 455 four speed Trans Am that I doubt was making 300 horsepower. The outer ring on the harmonic damper had slid back towards the timing cover and was rubbing against it. Generally, the car seemed like it had seen a lot of harsh driving. I figured the damper becoming non-functional and one too many power shifts led to the crankshafts demise. The engine was still running but made a bunch of noise and shook a lot. This was the only factory crankshaft failure we have seen first hand.

Attached is a video show casing that crankshaft

https://fb.watch/h-fC5Pks-W/


On the other hand, we have been successful using factory crankshafts. We've made close to 1100 horsepower using nitrous oxide. We've built belt driven (8-71) supercharged engines that have seen many years of service in the 800-900 horsepower levels. We've routinely shifted them over 7,000 rpm, buzzed them through the traps as high as 7600 and they have held up without any issues what so ever.

I'd like to hear other folks experiences.... good or bad?

I’ve pounded on cast factory cranks, my 2 bolt main cast factory crank turbo car has lasted forever, much of it is in the tune, but all my N.A. stick cars had cast factory cranks except one w a RAV forged .

I’ve never had a crank failure . I had one aluminum rod lose a fowl pin and cut into a crank but it was a forged RAV. All my cast factory cranks survived un hurt even at 960 Hp.

Rods are the weak link typically. I’ve throw rods spin rods, cracked a cyl . But cranks have been rock solid. . The filet area is important prep on a Pontiac crank.

I think it was in McCarthys book where he said he has seen less Pontiac cranks fail than he could count on one hand. Something like that.
I beat on my 30-30 455 crank for 30 years.
When I put a 4" crank into my 455 block it will be a PMD 428 crank. Steel crank will get me no advantage other than lightening my wallet.

Paul with how small the amount of failures to be read about on this ( and you have to search ) and how nice your success is with them when prepared and balanced right and run in a motor that's tuned right, I am very curious as to why you have even started this thread?

Actually this is a good thread. I will use this info for my next build. A 440””use a new forged or turn down a factory 428 crank.

I am not at all saying it’s a useless thread, as I asked I am interested in Paul’s reasoning.

Pretty sure he spelled it out in the original post.

I'd like to hear other folks experiences....

Seems pretty clear, no?

There was a guy on here w an aftermarket for fed brand new w a hole in the journal (not an oiling hole) I personally just bought a butler stroker forged crank for a build and it’ll be the first aftermarket crank I’ve ever used in my own engines. I’ve always used factory cranks and been very happy. Belive that aftermarket w the hole was from molnar but do a search and see .

I guess what I am asking is if he is starting to get a bit white knuckled at the power levels he has been safe with so far?

Both NA and Boosted based PMD cranks follow the Goodman Diagram. I think the NA powered Cranks last much longer.

My boated crank has been fine and I get cap walk

Paul the one in the current 455 in my 78 was in the same block different configuration racing NMCA with teh RAIV heads. Car had a jack under the front crossmember slipped off and caught on harmonic balancer. A couple of months later in the staging lanes balancer that was on the crank let go crank spinning it in pulleys not moving -probably cracked it. Crank still doing great and a lot of passes the past 20 years since then and more HP put to it. Just not a stock cast iron balancer.

As far as the cast cranks from PMD and failure in my own experiences, I've never run any crank in my Pontiacs other than a cast crank. So the only thing I've seen break one is when a rod breaks off, and the flailing rod gets between the block, and the rod throw. That includes from a 326, to a 400, which all use the same basic 3.75 stroke crank. I've also run a slew of the large journal cranks in racing applications, 428, and 455 cranks have not failed, unless an errant rod gets in their way.

I believe that the engineering on the Pontiac cast cranks was so over engineered that the sheer mass used in their construction made them far stronger than any other major manufactures cast iron cranks. Smokey Yunick eluded to this over engineering when he appealed to the engineers to reduce the size of the main journals of the 421, years later after his statement it's been pretty well proven fact that the cranks will still survive without the excessive overlap from the main journals, and the rod journals. The engineers had a formula that a certain percentage of overlap was need to assure longevity of the cast cranks.

Anyway I've run cast cranks in small main journal engines as well as large main journal engines for season, after season, on a dirt track without failures. Some were prepped, and magnafluxed, some were run just as the factory sent them out the door. The only Pontiac cranks that I've seen break under just normal driving were the 301 cranks, that cut too much mass out of the cranks it made them weak. The same engineers that engineered the large overlap on the 421 cranks would have been spinning in their graves when the 301 crank appeared.........:eek:

Anyway those are my experiences over more than 50 years of wrenching on them, and racing them. I hope that my experiences can give you some background on the information you're seeking.

One other thing I'm aware of in the cast cranks that GM used pertaining to the harmonic balancers. On the GM/Detroit Diesel 6.2 and 6.5 engines, which also use cast cranks. If the harmonic balancer fails (common problem) and you don't catch it quickly, or just ignore it, the cranks will fail, and break pretty quickly. Of course gas engines, and diesel engines have much different harmonics acting on their cranks.

GM would have been money ahead had they choosen the Pontiac V8 to convert into a diesel, rather than the olds gas engines, The early olds 5.7s diesels had crank snapping problems when they first debuted in 77, and had to re-engineer their cast cranks to withstand the diesel torsional twist.

Well, I think PMD cast cranks are stronger than PMD blocks so there is no reason not to use one with a stock block most of the time.
Its a good thread and topic.

I have had a couple check cracked after being in service a while. In my opinion they are very durable but are still a cast part with about no radius

I have never had a cast N crank break or an Armasteel crank break in my older 421 days. They have just been very dependable. Possibly just incredible good luck, I don't know. The N cranks we ran in the Grocery Getter drag engines in stock block 455's. Made around 1000,1100 HP spraying the heck out of them. Aluminum rods, extra wide fillet radius ground in, 2% overbalance. 5 billet main caps. We broke 3-455 blocks all in the same place up through the #2 main oil feed hole up to the cam tunnel. Never failed a crankshaft. I know they do break every now and then because I have seen pictures posted here.

I've ran a 428 and 455s with stock cranks. Probably around the 625hp level. Never had a crank failure. The only crank that I ever broke was attributed to a stock rod failure in a 467.

Mike, I think you have hit on something that will extend the life of PMD cast cranks: lighter reciprocating mass, proper balancing and a generous radius on all the crank journals. Any weight you can remove from the reciprocating mass lessens the stress on the crank. Proper balancing keeps the crank from beating itself to death. A generous radius on all the crank journals strengthens the journals, helping the journals not develop cracks which can lead to crank failures. I haven’t reground a PMD cast crank that I didn’t regrind the radius on each journal. 6.2/6.5 cranks are well. For some inexplicable reason GM wasn’t overly concerned about the radius on crank journals unless it was a Hi-Po BBC forged crank.

The wife fillet radius is how I’ve always made mine live . It makes a HUGE difference.

As I recall the only differnce w the SD cranks were the radius Vs the same part number regular 455 cranks??

Most the factory Pontiac cranks I have seen broke in the same place like in Paul’s FB video. Thru the throw just inside the RJ. If it was something with the dampener it seems to break on the front throw right behind the number 1 main.

It seems like takes a smaller RJ diameter to get a very good fillet there that adds much strength there. I think it would have still broke.


It seems like it is fatigue life that breaks them with a mix of street driving and racing more often than big HP racing. It is hard to catch them before they fail because they do not start small cracks that you can inspect or mag, they hit the fatigue limit and just completely let go. About all the examples I can think of that broke cranks were for street duty. Lee has broke a 400. Jim Hand lost one his first 455. Mchell broke one in his 455 not that long ago. I can think of a few that broke that had some welding done. Steve C. had one the broke. You don’t have to look very far, they are all on this board. Another member broke one like the one your working on Paul, but the crank snout and dampener ended up in a road ditch, along with the front block webbing.