Broken small block Ford Scat crankshaft

[thatdarncat]

My racing buddy Brad M. drag races a 1967 Mustang with a Ford 351W based stroker. It’s a Dart block with AFR heads, and uses a Scat 4.100” forged crankshaft with a ATI balancer. It makes about 705-714 HP, depending on carb used. The car weighs just under 3000 lbs and runs in the 9.60’s to 9.80’s and uses an aftermarket PG automatic. The engine was originally first built and run in 2016, and freshened up in 2019. Earlier this summer he was on a pass when the car made a big bang around the 300’ mark, and he shut it down as quickly as he could. They towed it off the track, and were checking it over in the pits trying to figure out what happened, checking the usual stuff, like for a sheared distributor gear pin, etc., and not really finding anything. Then they noticed the harmonic balancer was hanging down at an angle! Brad brought the car home after the weekend, and we pulled the engine soon after. Inspection showed the crankshaft had indeed broken right in front of the first main journal. Unfortunately right at the lower crank timing gear, which stoped the timing chain & camshaft. How the balancer basically stayed in place by only the crank seal, and didn’t go bouncing down the track is amazing. We brought the engine into the builder, and the full tear down and inspection showed the only damage done was the broken crankshaft, and all the valves had been tagged & bent, with some valve guide damage too. The engine has been rebuilt, with a new Scat crank, new AFR valves, and the ATI balancer rebuilt, etc., and today we stopped out at the builder and dropped off the headers & misc parts for the upcoming dyno session. I took some pictures of the broken crankshaft, and thought the group here might be interested in what it looks like. Certainly a strange failure, and hopefully a fluke. I know a few of you do failure analysis, feel free to weigh in if you see anything. The only thing that bolts on in front of the harmonic balancer is a pulley for the alternator belt, and a mandrel & pulley for the vacuum pump. Here’s some pictures:

[WConley]

The line at 2:00 in the pictures, along with the "beach marks", tell the tale of a long slow fatigue failure.  There appears to be an inclusion (flaw) near the surface at the end of the 2:00 line.  That started a crack that spread around the perimeter and slowly made its way toward the center.  The progression of the circular crack is shown as each beach mark ring.  Finally there wasn't enough material left to resist bending and the center failed violently.

Cool stuff!  I'm glad the engine was mostly salvageable.  That is a very unusual failure for a non-blower engine.  You don't usually see that kind of load on the crank snout of a race engine.

[GJCAT427]

I would bet that the crank started cracking when the key was pushed into place. I don`t remember a small block having half moon keys cut into it. Straight keyways yes , but not half moons. Also I would say it was cut too far back  to the bearing, which would promote a tension stress from the tighting of the assys pulleys. Another question is why 3 key cuts? Big blocks use shallow square keys with now problems. My 2 cents

[blykins]

I would bet that the crank started cracking when the key was pushed into place. I don`t remember a small block having half moon keys cut into it. Straight keyways yes , but not half moons. Also I would say it was cut too far back  to the bearing, which would promote a tension stress from the tighting of the assys pulleys. Another question is why 3 key cuts? Big blocks use shallow square keys with now problems. My 2 cents

All of your aftermarket cranks, including FE cranks, usually use woodruff keys.

[frnkeore]

Does the propagation of that crack, align with the Woodruff key?

[gdaddy01]

well if were a log to go on my sawmill , I would say the pith was off centered .

[Stangman]

I agree with wconley . The last little bit just off center was the only thing holding it together. Was most likely there in 2019 at the rebuild but unless you were looking for a problem like that its easy to miss. We had some Sauber trailors with 7000 lb axles that the spindle cracked at the base of the backing plate. In this case it was the grease hole in the rear was drilled to close to the end from the factory so it made it week. We had to change all the axles in our yard. We have10 trailors but the company has about 125. Glad he got it back together already and sounds like a great car would like to see it.

[pbf777]

      It perhaps would be interesting to have the sectioned snout, near the failure, hardness tested; both a surface 'Rockwell' Hardness testing and a 'Case Depth' hardness inspection.  And although this may prove very interesting, but the issue becomes what values do you have available to compare to in order to draw any conclusion?  And this truly is one of the biggest problems with the import materials, no real specifications with a lack of consistency in the metals and heat treating effects as witnessed.   

      Now, there are always dissenting opinions if one asks for enough of them, so if I may opine:   

           I believe the initial crack failure started at approximately the eight o-clock position    ; this due to the more 'ragged' edge perpendicular to the surface, this caused by directional changes in the migration of the crack, this giving the impression of a slower migration process nearer the surface and proceeding about the perimeter, as presented in this hemisphere.  As we observe to the opposing side the migration is smoother, over longer distances, with what would seem more directional intent, in other words moving faster, this as the deflection becomes greater and so the influence.  Also the greater sum rubbing and working or mauling of the two surfaces in this area to the left erasing the crack migration evidence vs the opposite side with cleaner more visible evidence indicating fewer motion events, less deflection and interaction of the surfaces over the time period of the failure process.  The "inclusion" at two o-clock, is as I believe actually the point where the crack proceeding about the circumference both from the left and right (clockwise & counterclockwise) met, just out of alignment, this resulting in a step, (the smear of metal here took place after separation) thus this was actually later in the process, not early.  Also note the discoloration of the crankshaft face, this generally to be referenced as heat effect, and working of the surface where the timing chain gear loads this surface; this indicating to me that the timing gear with a reduction in clamping load due to elongation of the distance of the snout over the stacked clamping distance on this side allowed the gear to work this surface vs. the opposite side (@ 2 o-clock) which is cleaner indicating perhaps less time exposure to this event.   

           Yep, just another opinion    , as I believe it is all supposition at this point.   

      Scott.

[70tp]

I may have missed it, but , was it internal or external balanced?

[thatdarncat]

I may have missed it, but , was it internal or external balanced?

I’m pretty sure it’s internal balance, but I can verify when we go out for the dyno session in a week or two.

[Cyclone03]

Peanut gallery comment.
Is it possible that the small nick above the end of the key in the first picture created a stress riser in the hardened surface that continues into the center?

[jayb]

Quite a while back I contacted Scat and asked about this very issue, because somebody on the old FE Forum had a crank that broke in the same spot, and I was getting ready to buy a Scat crank.  Scat told me that the only time they ever see this is when the engine is using a Fluidampr.  In that case the broken crank had indeed been fitted with a Fluidampr.  Obviously that is not the case here.  Might just be Scat's standard answer to a broken snout...

[pbf777]

     Make sure that the relief angle of the timing gear is adequate to allow the back-face of the gear to contact the crank face stop without gouging into the snout to stop face radius, as otherwise this will create the potential for a failure that looks just like this one; this no matter who's damper may be (or was ) present!     

     Scott.

[cammerfe]

      It perhaps would be interesting to have the sectioned snout, near the failure, hardness tested; both a surface 'Rockwell' Hardness testing and a 'Case Depth' hardness inspection.  And although this may prove very interesting, but the issue becomes what values do you have available to compare to in order to draw any conclusion?  And this truly is one of the biggest problems with the import materials, no real specifications with a lack of consistency in the metals and heat treating effects as witnessed.   

      Now, there are always dissenting opinions if one asks for enough of them, so if I may opine:   

           I believe the initial crack failure started at approximately the eight o-clock position    ; this due to the more 'ragged' edge perpendicular to the surface, this caused by directional changes in the migration of the crack, this giving the impression of a slower migration process nearer the surface and proceeding about the perimeter, as presented in this hemisphere.  As we observe to the opposing side the migration is smoother, over longer distances, with what would seem more directional intent, in other words moving faster, this as the deflection becomes greater and so the influence.  Also the greater sum rubbing and working or mauling of the two surfaces in this area to the left erasing the crack migration evidence vs the opposite side with cleaner more visible evidence indicating fewer motion events, less deflection and interaction of the surfaces over the time period of the failure process.  The "inclusion" at two o-clock, is as I believe actually the point where the crack proceeding about the circumference both from the left and right (clockwise & counterclockwise) met, just out of alignment, this resulting in a step, (the smear of metal here took place after separation) thus this was actually later in the process, not early.  Also note the discoloration of the crankshaft face, this generally to be referenced as heat effect, and working of the surface where the timing chain gear loads this surface; this indicating to me that the timing gear with a reduction in clamping load due to elongation of the distance of the snout over the stacked clamping distance on this side allowed the gear to work this surface vs. the opposite side (@ 2 o-clock) which is cleaner indicating perhaps less time exposure to this event.   

           Yep, just another opinion    , as I believe it is all supposition at this point.   

      Scott.

The above is a thoroughly professional evaluation. I say so as one who spent uncounted hours first creating samples of steel cut from parts to be evaluated, and then sitting peering through the optics of a metallographic examination installation. Cut out a sample, mount it, polish it, etch it, do the examination, photograph it, Write a report.

Do it all again.

And sometimes pull a pool car and drive it over to the Gate 4 Lab at The Rouge for a discussion that takes an hour and a half and gets about ten minutes of real work done.

The corporate way of doing things.

KS

[cjshaker]

All of the analysis is interesting, but doesn't address the root cause. The weak point should have been right at the edge of the keyway cut, but there is a space between the cut and the actual break. A small space, but a space nonetheless. I've never seen a fatigue failure that didn't occur right at the point of a machined cut (the weakest point, and a stress point). There seems to be a slight surface distortion right at the 2 o-clock inclusion, but that seems to be more a cause of the final failure point, where the metal would have been twisted as it reached its final breaking point.

An ATI balancer doesn't use any weights or fluids that can cause an off balance situation, such as a Fluidamper or Rattler design would. I could see the mandrel for the vacuum pump causing some serious stress if it were to become off balance, but again, the weak point of failure should have been right at the edge of the keyway cut.

To me, there seems to be a moon shaped crater right below the surface at the 9 o-clock position. In the shot with the snout in the picture, there seems to be a more visible inclusion (crater) in that same area. Kind of hard to see with no close-up of the snout, but it's more visible on the snout than the crank, in that picture. That would suggest an inferior metal, or sub-surface particle that started the whole process, much like the fan failure in the Des Moines airliner crash that happened so many years ago (1989). I'm not implying that Scat is an inferior product, there's enough of them in use that something is bound to happen sooner or later. Or to quote the brilliant Forrest Gump, ""It" happens". 

So, any ideas as to why it broke in the first place?