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Ha! Of course I'm not sure, John, but neither is GP. Spouting conventional wisdom, attaching it to what you heard some people say and thus drawing broad sweeping conclusions doesn't qualify as a valid observation to me. Not that I really care about people bagging on BSA's since that reputation as grenades is one of the things that, I think, keeps them as great bang for the buck (I think I made a pun). I mean maybe it's true...I've certainly blown up more BSA's than Trumpets or Notruns. Then again, I haven't TRIED to blow up any Trumpets or Notruns... grin


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Just for the record. The broken crank were never regrind, and it is a Lightning.

Best regards


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Originally Posted by Ola
Just for the record. The broken crank were never regrind, and it is a Lightning.

Best regards


AH ha a Lightning you say....(two carbs -- theory get a positive stroke).


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So highway, you are telling use that the fillet is radius is .120" radius (.304cm) and the journal is 1.685" (42.387mm)?

Looking at the reliefs around the oil feed holes on the journal which look like the crank has been ground and what appears to be less than the needed radius you could have fooled me. Ah, pictures can be deceiving!
HTH

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The journal was ground by bits and debris after the breakage I suppose. Remember, the engine was still running, and the owner drove it home...
I measured the LH journal, and it has original diameter.

Last edited by Ola; 08/02/10 9:09 pm.

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Quote
I have to say that over the years, on many forums, I've heard of more catastrophic BSA failures than Nortons, and more Norton failures (90% Combat related) than Triumph.

Well considering BSA's out number Notruns by something like 20 :1 one would expect to hear of more BSA/Triumph failures than Notruns.

And yes John definately looks like it started at the journal radius but not having seen as many as you could not comment about the size without measuring it.

But I can identify a brittle fracture from 100 meters.
A stress fatigue fracture can be initated by bad machining along with 100 other different causes.

To qualify for a fatigue it just has to happen over repeated stress cycles


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Is there a calculation formula with shaft diametre and maximum transmitted torque (or hp per rev) to determine the correct radius?
Or do machine builders use the rule of thumb, passed on to them from father to son?

I think this was one of those moments I did not pay attention at marine engineers school. blush


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I know this is an old thread, but I had one disintegrate, took out the cases,lifted the barrel studs clean out of the crankcase, smashed up all of the left hand side of the barrels, a short section of rod found it's way out the back of the cases through the gap and into the gearbox. Witness marks on the inside of the piston skirt and the fact that there was just a naked gudgeon pin indicated that the top came off left hand little end. This happened while touring at about 60mph, I owned the bike from new, it had about 9,000 mile on it and had always looked after the oil changes and other maintenance, never rode it particularly hard. So yep they do let go.

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**ck me Lemans never seen one go bang to the point it bent the frame . how many revs were you pulling when it let go?


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leemans that's pretty much exactly what my engine looked like, but I've only got a small ding in the frame tube.

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Put me down as another that's had the l/h rod give out.
It was on my A65LC which I had no idea what I had back in the day.
Looking at some of the other pictures in this thread I was (if you can be in this situation) reasonably lucky.
No massive holes, but cracked through the engine serial number and around the front and on to the timing side stopping somewhere between the main bearing and engine mount.
Broke the rear base flange of the barrel.
I was only 19 at the time. Pushed it some 6 or 7 miles home!


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Originally Posted by Ignoramus
**ck me Lemans never seen one go bang to the point it bent the frame . how many revs were you pulling when it let go?


gladly it is not my bike, but the bike from a belgian bloke.
I was searching the internet for a drawing of an A65 engine, a so called "exploded view" when google presented me that site.
but it is an impressive malfunction

I once had a conrod in a 6 cyl-diesel-engine go in a spectacular way, ended up with a 3 and 2 cyl section.
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With seven more years of significant forum participation and observation, I'm revising my PERSONAL OBSERVATION results to:

Approximate percentage of blown engines between top 3 BritBike brands

Norton - 50%

BSA - 35%

Triumph - 15%

Maybe it's because Norton owners are proud of their accomplishments, Triumph owners are ashamed, and BSA owners don't care one way or the other?

Without a doubt, BSAs seem to blow in spectacular fashion, including entire sections of cases AND cylinders flopping about, bending frames, etc. Nortons tend to blow large pieces of cranks out, sometimes bending frames at the lower crossover tube. Triumphs just blow neatly, and leave it at that!

Last edited by GrandPaul; 06/22/17 4:01 pm.

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Grandpaul, would the large amount of Nortons blowing up be down to the cast iron flywheel on the commandos ?

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Originally Posted by bon
would the large amount of Nortons blowing up be down to the cast iron flywheel on the commandos?

All of them have cast iron flywheels, Nortons just seem more susceptible.

Of course my personal experience is anecdotal, as noted...


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Originally Posted by GrandPaul
Originally Posted by bon
would the large amount of Nortons blowing up be down to the cast iron flywheel on the commandos?

All of them have cast iron flywheels, Nortons just seem more susceptible.

Of course my personal experience is anecdotal, as noted...



Are you sure of that ? I have never heard of triumph or bsa flywheels going, and always thought they were steel.

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Originally Posted by bon
Originally Posted by GrandPaul
Originally Posted by bon
would the large amount of Nortons blowing up be down to the cast iron flywheel on the commandos?

All of them have cast iron flywheels, Nortons just seem more susceptible.

Of course my personal experience is anecdotal, as noted...


Are you sure of that ? I have never heard of triumph or bsa flywheels going, and always thought they were steel.


Okay, I was wrong (as to all being cast iron), but the argument remains the same, relatively speaking...

From the factory workshop manuals:

Crankshaft-

Norton - EN16 (high tensile steel)
Triumph - Forged Steel
BSA - Forged



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As a good friend of mine pointed out, the most common Notrun with a blown crank is the rubber mounted Commando.
With the rubber mounting set up properly the rider is isolated from an engine leaping up & down screaming for mercy just before it grenades.
OTOH on our BSA your eye balls have already been shaken to the back of your head, you nuckles are white and your fillings have fallen out of your teeth well before the crank goes bang.

Last edited by BSA_WM20; 06/27/17 10:27 am.

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Originally Posted by BSA_WM20
As a good friend of mine pointed out, the most common Notrun with a blown crank is the rubber mounted Commando.
With the rubber mounting set up properly the rider is isolated from an engine leaping up & down screaming for mercy just before it grenades.

Makes sense...


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Uhm, Bon?
I have a long time friend in SoCal, He was riding an A-10, and the flywheel broke into three pieces.

I will have all 6 cranks I'm building engines for magnafluxed.

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Originally Posted by Rickman
Uhm, Bon?
I have a long time friend in SoCal, He was riding an A-10, and the flywheel broke into three pieces.

I will have all 6 cranks I'm building engines for magnafluxed.


And get them nitrided as well.
Strait nitriding will not make them harder or stronger but will increase the fracture toughness by many orders of magnitude.
So you will hear them going bang bang bang for a long time before they go boom.
It should be something like $ 10 a hit.
Note CARBO-Nitriding is a case hardening process and that will increase the hardness as well as the toughness but in that case, the one tends to cancel out the other so the net effect is a harder journal but the crank only gets a marginal increase in fracture toughness.
FWIW, fracture toughness is the resistance of the crank to propagation , not a measure of the resistance of the crank to crack in the first place.


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