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Broken small block Ford Scat crankshaft

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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:

--- Quote from: GJCAT427 on October 13, 2021, 04:58:53 AM ---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

--- End quote ---

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

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

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