Whoops!

[Greybeard]

This is from one of my BSAOC mates, Dave Willmer. The bike continued to run for a few miles.
"Well I found the knock"

[Swarfcut]

 That's a good advert for the benefit of the big journal crank, with a proper central oilway. Also time to examine the grind profile of the radii on the surviving big end journal. Picture would be a good addition to the Great Mechanical Disasters thread. Well worth salvaging the flywheel, along with the retaining nuts and bolts.

 Swarfy.

[berger]

mine changed engine note going home from silverstone races in 1980 up the motorway and got me home . i stripped the engine and found this

[RDfella]

What engine is that, Neil? Can't be Triumph as conrod caps are alloy, yet the flywheel is bolted - thought the BSA bolted ones had the draw-bolt big end.
Edit: The early engines with draw-bolt had steel rods without caps if I recall, so is this an intermediate model?.
As for what caused the break - that's a substantial section there, and more that capable of taking the modest power of the engine. I've seen broken crankshafts, but those were on 6 cyl 200+bhp diesels, where the cause was torsional vibration. A twin cyl engine is too short to suffer meaningful torsional vibration. Because of the section, I'd be surprised if even wrong - or lack of - radius could instigate such a failure. Wrong heat-treatment would also seem unlikely (BSA were pretty good at material selection / treatment).  Too many revs?

[berger]

RD fella early A10 or A7 mine are the same bolt up flywheel

[muskrat]

G'day Fellas.
Yes been there done that Snapped a couple of cranks in the A7SS racer. The problem started with lifting the barrels off so fixed that by through bolting. The crank was the next weakest link. Small journal cranks don't like 7500 rpm and 14:1 comp
Cheers

[Greybeard]

What engine is that, Neil?

I believe this is the bike.

[bikerbob]

I thought I recognised that bike checked some old photos and I last seen it at the BSA camp in Northumberland in 2018. In regards to the broken Crankshaft that happened to my brother's 1957 A10  back in the early sixties he fitted a sidecar and did not alter the gearing it  lasted about a month, also when he had the sidecar fitted I believe it was a Swallow jet80 he was riding it home and going down a steephill he braked hard and swerved to to one side and put a lampost between the sidecar and the bike, I got the job of fixing the sidecar luckily he was going quite slowly at the time and no damage to the sidecar fittings or the bike just a huge dent in the sidecar. The reason he fitted a sidecar was his best mate had just had a nasty accident having written off his Super Rocket and lost his right leg I believe they took the crankshaft out of the Rocket and fitted to the A10 Gold Flash.