MTFCA Forum

Good morning all,

I just found a new member of "the broken crankshaft cub".
Just wondering why it is broken on this place just behind the first main bearing. Photos in attachment.
Was this just bad luck or are there some item to look for?

I hope not finding any other problems in the engine. It was rebuild in 2022 and have about 300 miles since rebuild.

Andre
Belgium

Hard to tell by looking. In my opinion, most breaks are caused by misalignment, incorrect grinding or finishing, abusive handling or operation, and perhaps very high accumulated hours. Did someone take a hammer to that front main cap? If so, why?

An old racer friend told me to leave room between the cap & crank throw for the crank to grow @ higher rpms.
I don't see any such clearance here.

Lack of oil to front main, bearing locked up and broke crank. Did it happen going up a hill?

New pistons, bronze rod caps with inbuilt dippr
ers new alloy timing gear. Lots of good stuff going on there. Was the crankshaft subject to crack testing prior to machining/fitting? To break within 300 miles would suggest the may have been an existing crack.

Allan from down under.

Allan, It was and I didn't see it at that time.
Yesterday I looked at the photos of that area I took in 2022 to show the new crank gear. Made it 10 times bigger and framed it to that area.
I think I can see a starting crack that was there before the rebuild.

Andre
Belgium

There does appear to be some staining on the back edge of the break which may indeed be the beginnings of your crack. Closer inspection of the break may confirm this. Bummer! At least the damage appears confined to the crank only. Another ground to the same specs may be an easy replacement.
Best of luck.
Allan from down under.

Assembled with grease? White lithium?

You should check number 1rod close also.
Blow up picture and look at rod thrust area.
One side way thicker looking than the other.

Even a small scratch or nick or dent could lead to failure. There is a process known as shot-peening which might be of benefit to used crankshafts. Surface scratches, gouges or dents from hammer blows or having been dropped can all lead to fractures.

IMHO- The installation was apparently too tight front to back, OR, the shaft was bent when it went in and pulled square by the caps, OR a combination of both. Pulling in a bent flange area or misalignment, then having the tranny hanging on it is pretty much akin to swinging a water bucket in circles and trying to defy gravity...

As far as pictures, they are always tough to discern what really went goofy...however...when you pull the crank out take a close and detailed picture of the surface of both sides of the break...then the fun on the 'how' will begin. In collaboration with the late Ralph Ricks, I use to have a 'send me' offer for me to do a bit of visual 'crank whisperer' on. Then it got up to over 100 cranks in my garage, all broken, no one wanting to return, and...pack rat sat on them for years! So sorry in advance, no hands on looks. Get clear pictures, I'll offer an opinion, as to the 'how' and if it has not hammered itself smooth, can usually predict where it started! I'm actually pretty good at the forensics of failures, as I spent years understanding the how's and why's on all steels...then went and designed A-649 a specialty steel that gets deep hardened to Rc 60 or so by induction. It's an alloy with high carbon and very controlled amounts of other additives that when used can rotate at high speed as a pressure loaded forming roll with a crown of about 1/32 over 144" bearing centers and still survive billions and billions of cycles years upon years with nary a slight check crack! Not really a commodity grade so unsuitable for cranks due to high cost just to make it in the first place as there are only a few foundries who can even do boutique melts of that order.

George,

Thank you for sharing your experience.

Out of all of the broken cranks you forensically inspected, what do you feel was the most common cause of the failures?

Whats with all the white grease in there? Never seen an engine assembled with grease before. Correct clearances and oil is all that is required. Could the grease have impeded the normal flow of oil?

Ed fuller,

The vast majority were the result of a tight radius on the pins. Next in line were pin failures also, but with good radius yet the crack started in the radius- my conclusion on then was too rough of a stone/stone needed dressing. All it takes is a scratchy and thing can go wonky from there. On the throws…assorted….some were due to insufficient parting line flash removal (looked like a gator chewed the flash off), very few voids.

Although I did not quantify…the pin areas ‘beeched’ about 2/3 of the cross section, then the final 1/3 failed in torsion…its safety factor had been used up. Throws have some ‘beech’ but they typically just cleave like a chunk of hard sugar.