Where do cranks break?

[Silver 6]

We frequently have the debates on the safe usage of a N/A crank in place of one with the rolled fillets. But is the rod journal where the crank breakage happens?

The crank from my old T-Type broke in the thickest part of the arm just in front of the third main bearing. The break was in the rough cast area of the crank and did not touch a journal, rod or main.

So in what location on the crank did your crank break? You guys in the DOTC Club could give us some good information here.

If the cranks are not breaking on the rod journals, then the whole debate on if it is safe to use a N/A crank is just all of us having a good time typing.

 

[gn4u2c]

na crank is good for low 12's thats it the cranks break because of bolt streatch and bearing cap flex / cracking . also to much walk will break them.
A stock turbo crank in a stock block with steel main caps and arp fasteners will go in the 9's all day long.

 

[gnx7]

n/a crank good for only low 12's only....?

Why is it that quite a few are running 10's with no problems.....?

Stock GN crank with caps good for 9's all day long....?

Pushing a 109 block and stock crank past low 10's is a ticking timebomb and the phrase all day long should be used loosely.

I'd probably venture to say that stock cranks break because of detonation at high boost or simply cycle failure from being beat on over time. Cap walk doesn't help...

The crank physically flexes in the center #2/#3 bearings with high boost. Supposedly Buick Motorsports or someone else did some Xray testing and logged that. Forged cranks supposedly don't do this like the stockers.

-GNX7

 

[Nick Micale]

Originally posted by Silver 6
............If the cranks are not breaking on the rod journals, then the whole debate on if it is safe to use a N/A crank is just all of us having a good time typing.

Since I am not an expert on the structure of crank design, will give my opinion on this based upon the many Buick motors we have built, and dis-assembled.

First, the experts do state from facts the rolled fillet crank is "stronger" than N/A. What does this mean? I would think you want a crank that has the least amount of flex and distortion. That is why for high HP applications, forged or billet cranks are used.

The stock 109 block will move and flex even with caps and a girdle. We have seen this as cap walk is easily seen upon tear-down. Flex in the crank can be evidenced by bearing wear off to one edge. Using a forged or billet crank in a stock block greatly increases engine life in high HP applications.

A BIG problem in stock blocks is rod and rod bearing failure. Is this due to crank flex? We think most of them are. Will a better crank improve the failure rate? Again, that is what we have seen in ones we have done.

Another reason for bottom end failure in a HP engine we feel is the stock "balancer". This is not a real harmonic balancer, but a hub to mount the pulley. NHRA requires a SFI balancer at 10.99 and faster, we will not stand behind any of our motors running in this range without one. The BHJ [only one avail. w/interruptor ring] is a REAL harmonic balancer. This does work in minimizing crank flex.

As far as using a cast crank, rolled fillet or N/A, in a 9 or 10 sec. car and not having a problem, that is the owner's choice and he may get lucky? Castings like flex less than forgings or billets. If the casting is a good one and has had an easy life, it may work fine up to a point. If not, the result can be expensive.

If the rolled fillet was just as good as a regular crank, why would Buick have spent the extra $$$ for it in the turbo motors?

Also, I would like to have more info of the stock blocks running 9's all day with just caps?

 

[Silver 6]

Man you guys can dodge a question.

We have done a lot of posts on stronger, better, is it block flex, is it main caps moving, stress, block design, is it good for 10s, or 9s, or how long it might last........

I was just hoping we could take a look at where the cranks actually break. Do they break at the rod journal or do they break at some other location?

If you broke a crank, where did it break?

 

[Nick Micale]

Re: Man you guys can dodge a question.

Originally posted by Silver 6
........If you broke a crank, where did it break?

The last one we had broke, it was at the thrust journal.

One block is sitting here with a hole in the side where the rod went through and the crank is trashed. Probably cracked the crank, but not broken in half.

 

[ijames]

The two I've seen broke in the split throw between the #1 and #2 rod journals.

 

[sixgun86gn]

I have been beating a NA crank without caps or a girdle in my motor for over 3 years now.My best run was a 6.91 so far in the 1/8.I have only an 1/8 mile e.t. in my sig because that is all that is around here to race at.

Lets see...6.91x 1.55=10.71 or thereabouts.So the whole 12s thing is bunk.I have yet to break the crank,but when/if it happens I will let you know where it fractured.

BTW my NA crank is also cut at .010/.010.Also my entire rotating assembly has not been balanced.

 

[Nick Micale]

Not going too far off David's topic, but an 1/8th mile does not give engine loading like a 1/4 mile run. Most damage I have witnessed has been usually in the last 2-300'. The 2-3 shift is very brutal and imparts the most severe loading to engine and trans.

The drop in RPM and going into direct drive with no gear reduction, then pulling up to speed/RPM again is much harder than a stoplight-to-stoplight run, or letting off at 1/8 mile. [Unless you have 5.88 gears!].

 

[sixgun86gn]

You make a very good point.I have had it down the 1/4 many times,but most of my racing is done at the 1/8 tracks.I beleive this may just be the reason it has lasted so long.

Back on topic...I would expect the stock or even the NA crank to break on the split journals of the throws.Does anyone know if it has been tried to grind off all of the casting flash to reduce the chances of stress cracking?

 

[Silver 6]

Ok, we have four so far

Carl saw two broken between #1 & #2 rod journals. Nick and I account for one each near a main. Nick's sounded a bit like a ballooning issue behind the motor.

If it were to be at the joint of two rods, I would reason that point to be at the 5/6 joint. Aren't most knock issues caused by the leaning out of the rear cylinders? I don't recall many people posting that they blew out a head gasket around front two cylinders.

Any more broken cranks? Where'd they break? Come on guys.

 

[Nick Micale]

Re: Ok, we have four so far

Originally posted by Silver 6
........ Aren't most knock issues caused by the leaning out of the rear cylinders? I don't recall many people posting that they blew out a head gasket around front two cylinders...................

Off topic again, but you said it!

In my very large pile of blown head gaskets from many different cars, only 50% or less are from cyls 5 & 6. The block with a "window" and half a rod is in no. 3 cyl.

Balooning, Precision Ind. said that is not possible with their converter?

 

[750H.P.V6]

David,

I have a N/A crank or what's left of it sitting on my workbench as a paperweight. It broke between the #5 and #6 rod journals. This crank came out of a street car that was probably a mid 12 second ride. I also have a broken stock main cap out of a high 11 second car.

Personally I see no reason not to use a rolled fillet crank as they are still relatively easy to get and not that expensive. With that said I wouldn't want to try to run 9's with a rolled fillet crank but it has been done. The stock parts are good but pushing them way beyond their limits is a recipe for disaster.

Neal

 

[GDNF2ET]

Will a real BHJ harmonic balancer help my stock internal motor stay together longer ?...Where can I buy it and how much are they ?

 

[Silver 6]

My interest here is on "cranks"

What I am looking for is where cranks break. If you broke one, either a turbo crank or an N/A, please post the information on the break.

I was hoping that we could get an idea of what area of the crank is showing to be the weak point. I was thinking that the most likely area of weakness would be the point of breakage.

Information on both styles of crank would give us more to work with.

 

[Silver 6]

This is the type of information I was after

http://home.earthlink.net/~lstsimpson/

You can see from the pictures where the crank broke in what looks to be the thickest and stongest area.

The crank in my 83T broke in the same place, except only one break. And it still ran.

We always worry about the non rolled fillets being a possible weak and failure area. But if the majority of crank failures don't happen there, what is the issue?

 

[ijames]

But if you look at the chunk that came down past the girdle on Scott's spectacular parts redistribution, it contains the center two rod throws and the break line on each side goes into the rolled fillet area. I can't say where the crack/break started, but it looks to me like it could have let go at the rolled fillet on the main side of one rod throw, and then once that piece flexed it also broke off between the other rod throw and main. Just because it didn't break between the rod throws, at the split journal, doesn't tell the whole story in my (not so ) humble opinion. One of the cranks I saw that had broken between #1 and #2 broke cleanly across right at the fillet, like you set it in a saw lined up on the groove and cut straight across. I forget what the other one looked like - it's been several years since I saw that one. Scott's isn't as clear-cut but I think it is significant that the breaks go through the rod rolled fillet areas.

 

[Silver 6]

Humble or not, opinions are welcome and needed in cases like this to learn all we can.

I look at this break like this:

The initial break occured just in front of the #3 main, just like mine did.

The break happens on a bevel. As the crank continues to turn under power the bevel becomes a wedge between the two seperate parts of the crank. The wedge puts pressure forcing the two pieces apart.

This pressure is then felt at the #3 main and what is now a one sided crank arm for #3&4 pistons.

The pressure broke the crank arm a second time near the piston journals, where the crack is seen. It also broke the carnk arm in front of the rod jounals near the #2 main.

In my "humble" opinion, this crank failure started in the same place mine failed. It was in the thickest part of the crank right in front of the #3 main. But it doesn't make much sense for it to happen there.

 

[noslo6]

Originally posted by ijames
The two I've seen broke in the split throw between the #1 and #2 rod journals.

I ran a NA crank into the mid 11s for 5 years and it finally broke exactly where ijames suggests. Luckily mine must have developed a crack as it broke as I was cruising home at 2000 rpm. I was about 1/4 from home when it broke and I just coasted into the shop. It only broke the crank, no 1 rod and piston and nothing else. Since it broke at the front of the crank, it stoped the balancer and crank trigger and shut the engine off immediately, thus not beating the shemp out of everything.

 

[Red Regal T]

Sorry, David, haven't broke a crank yet, just a thrust bearing cap. I'm getting ready to throw a shortblock together. Yes, that's what I do. I don't care about going 10s. Maybe I'll campaign to get into the 11's someday. My car's not a weekend warrior. I drive my car all the time. I have a 10/10 turbo crank for it, but was thinking today, I could just save the turbo crank because I have a real nice NA standard crank. I'm thinking I might just suck it up, and chance the NA crank for myself. Sounds like, if your going mid 11s, it's just a matter of time that something breaks no matter what your running. When it does, I'll just be out hundreds, instead of thousands. I'd rather break, on a budget.