Cracked crankshaft

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Time Warp

Time Warp

.......back to the 70's.
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It must be the week for it after reading some of the threads with engine problems then I see this.

It sure does look like a crack and it is goes from the radius at the cheek to the rotor nut thread.
This can't be common ?
I would have thought any cracking would end up go radially and the shaft breaking ?

Cracked crankshaft
 
well, that pretty much baffles me if its a crack, you would think the crack would start at the keyway.
 
If it is a crack, it is the most strange place I had seen , and I had seen some..........!! try the dye penetrant test.
as mentioned by Don , it "should" start at the keyway and that part of crank is not hollow ....so, just cross my fingers and waiting for more investigations !
 
This kind of crack is attributed to internal pressure.

https://www.quora.com/What-are-the-causes-for-the-failure-of-shafts

As the shaft is solid, the reason could be high residual stresses from a previuos heat or surface treatment which would cause high hoop stresses. Or it could be an internal manufacturing flaw in conjunction with torsion fatigue.

The shaft appears to be shot anyway and you might as well give it to a material laboratory for closer examination.

-Knut
 
YIKES!

Never seen anything like it on many motorcycle forums...

...then again, I never claimed I've seen it all!
 
It would be interesting to determine what caused it but no matter it is now a paper weight. Time to buy a new crank.
 
Time Warp said:
It must be the week for it after reading some of the threads with engine problems then I see this.

It sure does look like a crack and it is goes from the radius at the cheek to the rotor nut thread.
This can't be common ?
I would have thought any cracking would end up go radially and the shaft breaking ?

Cracked crankshaft
Click to expand...



Sorry to see that.

It look as though it's on the cheek too, unless my eyes deceive me....






Cheers,

cliffa.
 
I will try and get some better pictures, its a real shame being such a nice crankshaft still on original journal sizes.

Cracked crankshaft


I will try and find a replacement drive side cheek and yes Cliffa the crack is in the machined portion above the radius, the crack along the bearing inner area and sprocket taper drops all the way down the shoulder to the parallel section for the rotor and continues on to the thread end.
Other than that things are going well.
 
I know I am daft, but not only do I wonder how you end up with a crack like that but whether you might get away with running it!



(Yeah, I know, too scary to contemplate)



:eek:
 
PM me with your contact, if the rest of my post appeals to you.

I have a crank that has new joining H/W and freshly turned down big ends (Mile High Crankshaft), it would come with a set of undersize big end inserts. It has some (now polished) scores on the drive side pin that, in my opinion, would make it questionable for use with a belt drive primary transmission, but would be fine for a lubricated chain system. It came out of a 1973 750.
 
What am I missing?

What is different about using a belt drive and a chain drive in this situation?
 
Thanks for the offer RoadScholar but a complete crankshaft to Australia is off the cards at this stage.
I will find a NDT facility next week to examine the cheek and report back.
No shortage of other things to do in the mean time.
 
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rvich said:
I know I am daft, but not only do I wonder how you end up with a crack like that but whether you might get away with running it!



(Yeah, I know, too scary to contemplate)



:eek:
Click to expand...

Who knows how long it has been like that.
There was no drama removing the sprocket or inner bearing race which I heated for some expansion ( hot air gun ) and it withdrew smoothly and without excessive force needed with the bearing puller.
Maybe the race and sprocket were acting like a band reinforcement, maybe removing them started the crack. (If that is what it is for sure)

I only ever work on wooden top benches and can see no sign of previous impact, as far as I can tell the engine cases had not been parted / split before.
It might never snap off, but if it did I am fairly sure any damage would not buff out.
 
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Time Warp said:
Thanks for the offer RoadScholar but a complete crankshaft to Australia is off the cards at this stage.
I will find a NDT facility next week to examine the cheek and report back.
No shortage of other things to do in the mean time.

Cracked crankshaft
Click to expand...
I'll bite, what is that? So moto/automotive!
 
arch said:
What am I missing?

What is different about using a belt drive and a chain drive in this situation?
Click to expand...


The difference is that a belt drive primary doesn't like oil, a chain drive primary thrives in oil. The drive side pin of the crankshaft I offered (for sale) has some concentric scores that MAY allow oil past the seal to a greater extent than an un-scored pin and new seal (which leak anyway). I am trying to be as unvarnished as possible; I'd rather exceed your expectations then get stepped on by them.

I knew that I would be installing the CNW e-start kit, based on a belt drive, so I purchased what I believe was the only NOS, still in the Berliner box, correct part number crank on the planet; I spend a day web crawling and calling all over the world and found the crank in New Orleans from a guy that purchased it on an auction from an estate sale of a former Norton shop owner in the Philadelphia area that had made the original purchase from Berliner in late 1972.

You can get new, forged one piece cranks without a lot of waiting for around $2000 USD, some more some less. My riding habits and ego no longer require a race prepared engine. I was able to capture the NOS crank for $500 USD.
 
cliffa said:
Who knows, that crank may have come out of the factory like that.

Cheers,

cliffa
Click to expand...

Yes it is possible, if that crack was already there I should have noticed it before but have to admit needing reading glasses these days.
Its not my eyes, its the small writing (and cracks in this case)

There should be a heavy package at 30000 feet somewhere over the Pacific.

Fast Eddie said:
Sunbeam lump innit?

Bloody big lump for a low tuned 500cc twin !
Click to expand...

The Sunbeam SOHC 500 parallel twin engine is a fairly compact unit given the automotive like alloy block .
I can't think of another 'Brit bike that has the cylinder integral with the block/case and rubber mounted engine from 1946.
Perhaps Dr. Stefan Bauer had a Sunbeam.

Cracked crankshaft
 
Time Warp,
Did you do a liquid dye penitrant Test yet? If you don't know where to source this then try some weld shops that do Pressure boundary welding.
Regards,
Thomas
 
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