Where do cranks break?

[Lee Thompson]

break

Last summer (3 weeks after racing all week @ BG ) I had a BMS fordged crank break between # 1 and #2 rod throw on the highway. I did have it wide open in 3rd gear, broke the crank, cracked the block, bent 1 Carrillo rod in a 45 degree angle, put a hole in the oilpan, broke the front cover and some of the rods got into the cam. I managed to save the valve covers.

 

[S2V6RACER]

Re: break

Originally posted by Lee Thompson
Last summer (3 weeks after racing all week @ BG ) I had a BMS fordged crank break between # 1 and #2 rod throw on the highway. I did have it wide open in 3rd gear, broke the crank, cracked the block, bent 1 Carrillo rod in a 45 degree angle, put a hole in the oilpan, broke the front cover and some of the rods got into the cam. I managed to save the valve covers.

What about the intake?

 

[Dave Simpkins]

Crank Funerals

To continue the autopsy thread ... I'd like to know about them before they passed away.
were any of them magnafluxed before assembly
were any of them treated to a die grinder operation to remove casting risers
any of 'em nitrided or shot peened
did they all roll easily on the mains at assembly (caps torqed - rods off) with no bind
and what was assembly tolerances on the mains
were they internally or externally balanced
what bearings were in use at time of failure
what oil

Seems to me before al lot of conclusions about why and where they break, they have to be compared on equal terms.

 

[REDS HOT AIR]

http://www.turbobuick.com/forums/showthread.php?s=&postid=1005632#post1005632

read my other thread ...pic of my cracked crank in that thread and info on a second cracked crank I just removed from a customers engine

I plan to go eagle crank next time around but that wont be soon I have too much other stuff going on to build my car back up.

I have a eagle crank If someone needs one I may sell it
please email EVTB@cox.net if possibly interested

 

[national]

Originally posted by 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!].

Mine goes into 3rd before the 8th mile marker & I have stock gears & tire height unless you are talking when it really climbs high in the rpm's in 3rd.
Brian

 

[plowboy72]

this may be off topic some but my completly stock bottom end in my 87 GN went 10s all year last year. I probably put 50 1/4 mile runs on it without breaking anything. I'm having a girdle installed for next year with forged pistons to try for 9's. It did however have some minimal cap walk on #1 and #2. According to the shop.
10.67@127

 

[Drac0nic]

Originally posted by Silver 6

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.

No logic in adding material and in turn weight to something that's not stressed. The metal that was there had a reason for being there.

 

[turbo buicks]

Originally posted by Red Regal T
When it does, I'll just be out hundreds, instead of thousands. I'd rather break, on a budget.

but john, if your n/a or turbo crank goes, it usually takes out the rods/pistons/block etc so either way you would still be out more money than just the crank cost. o well, i vote you use the turbo crank and paint my car for free

 

[Ormand]

I believe I've posted this before, several times. The rolled fillets were added for FATIGUE strength, they do not add to torque capacity of the crank. A crank from an N/A engine will support just as much power as one for a turbo engine, only not for as many miles. Breakage from main cap misalignment, or loosened cap bolts or from a bad bearing, or any of a number of other things can occur at any place on the crank, not necessarily at the edge of bearing journal. The location of breaks is interesting to know, but unless it is combined with the cause of the break, it won't prove anything. If there are a number of failures of N/A cranks, at the fillet and much fewer failures of turbo chanks, under the same conditions, that would be good info. But there don't seem to be enough failures, especially of the well documented/analyzed variety, to make this analysis possible.

 

[Silver 6]

There you have it George.

It was my intent at the start of this thread to have people list the location of the crank breakage we have seen. It will help us look possible reasons for the break.

The posting of a machinist saying there was some bearing cap walk was a good one. If a main cap stretches it would leave room for the crank to flex at that point and likely the break would occur there. If the third cap is a weak point, the girdles become even more important.

We need more reports from the DOTC club.

David

 

[wbrophy]

6.91 1/8= 10.71 IN YOUR DREAMS

Originally posted by 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.

In the real world 6.91 in the 1/8 mile is more like 10.85 in the 1/4, trust me.

 

[TRYMY231S]

Re: break

Originally posted by Lee Thompson
Last summer (3 weeks after racing all week @ BG ) I had a BMS fordged crank break between # 1 and #2 rod throw on the highway. I did have it wide open in 3rd gear, broke the crank, cracked the block, bent 1 Carrillo rod in a 45 degree angle, put a hole in the oilpan, broke the front cover and some of the rods got into the cam. I managed to save the valve covers.

How did this happen? I'd cry until my girl killed me (which would be better than living without a turbo motor in the Regal). I can't afford to do this twice.

 

[V6RACER]

Originally posted by 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

Mine broke from #5 rolled fillet to the #6 rolled fillet. Many years of mid/low 11s, plenty of knock and rev limiter hits. Put a 70 turbo on it and started running high tens then it broke. Luckily I had a girdle and nothing else was damaged and re used everything but the crank. Now it has a BMS crank with a girdle and no problems..so far..

 

[SloGN]

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.

i just replaced a set of head gaskets that blew out in the 2-4 cylinders and it blew into the intake.

this was on my buddies blue -t ( sydwyndr) after he tried chasing down a hemi truck