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by Mal Marsden - 06/16/22 7:00 pm
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Allan G, BSA_WM20, Chris the camper., gREgg-K, John Harvey, NYBSAGUY, reverb, Shane in Oz
Total Likes: 17
Original Post (Thread Starter)
by reverb
Hello; I am here debating about that.
For example, what is the point to have a tested crankshaft or connecting rod? if actually what you need is to know the fatigue of the materials and that is something that is not possible to be in the known.
Most use the same connecting rods only changing the bolts and nuts. May be doing a magnaflux test there could show plenty of "things to be worried" but those connecting rods are still in use on and on.

I am not talking about very suspicious parts but parts that look very nice and smooth. All with STD measurements (by the manual)

What is your opinion?

Liked Replies
by Magnetoman
Originally Posted by reverb
---alright; so is possible to know what type of internal "fissures" or "spider webs" are the ones that are a no no
The place you send it to have the magnafluxing done will know what to look for and will advise you on what they find. If they don't know what to look for, don't send it to them.

I have my own magnafluxing equipment so I taught myself what to look for. But, you will be paying a company for their expertise in interpreting the results, not for having applied a magnetic field for a few seconds.
3 members like this
by Magnetoman
Originally Posted by reverb
Hi; I am trying to say that if a great % that have these old bikes do a magnaflux; would occur that to be in "panic" due to you will find plenty of things that actually do not have a solution; like the crankshaft.
You're assuming just because the parts are old they would fail a magnaflux test. I don't think that's correct. It's a very small number of data points, but I magnafluxed the forks, crank and rod of my 1928 Ariel, and the same for my c1954 Alloy Clipper engine, and didn't find any cracks.

Stress is concentrated at cracks, so if stress initiated the crack in the first place it is likely to just get worse. The equilibrium situations are either no cracks, or a crankshaft destroyed by a crack(s) that grew. That would mean that if there are cracks in a crankshaft it's living on borrowed time, not that it's just one of hundreds of old, cracked crankshafts that will be perfectly fine to use.
2 members like this
by Allan G
Allan G
Magnaflux on a crank or other parts is worth while and whilst debating it, you could have had it done.

There’s a element of risk with any 50+ year old component, if you intend using it then you can do so blindly and take all your casino chips and place it on one square, or you can reduce your element of risk (the risk being here that you have the ultimate worst case scenario and the engine blowing up and totally trashing itself and probably the frame too) and have the magnaflux done. The hope is you have it done and everything is all fine, you might then think it’s a waste of money, but had you found cracks from the test then you’ve only lost the cost of another crank (and hopefully the next one tests fine). It’s saved you the somewhat unlikely but still possible potential cost of total destruction.

With rods, I don’t mess about. There’s lots of these available brand new, from the likes of MAP or Thunder engineering. (Plus some others), again their cost is quite small compared to the element of risk.
2 members like this
by Magnetoman
Unlike aluminum, the fatigue limit of the steel in a crankshaft would be essentially an infinite number of cycles were it not for crack formation. It's the stress buildup at cracks that is the practical limit on the lifetime. Basically, if you had a new crankshaft that magnaflux showed had no cracks, you could expect it to last as long as a used crankshaft that also showed no cracks.
2 members like this
by Magnetoman
Originally Posted by reverb
the Aluminum stuff only rely on the dyes?
Yes, anything non-magnetic (Al, brass, most stainless steels, etc.) has to be crack-tested with dye since magnafluxing won't work on them.

Originally Posted by reverb
most do not have the Magnaflux machine
To call it a "machine" is to give it more credit than is due. It's simply a hand-held yoke that applies a magnetic field between the two ends that are placed in contact with the object to be measured.
[Linked Image from]
Cracks show up with a field perpendicular, or at least at an angle, to the crack so two measurements are typically needed. Also, DC and AC magnetic fields have somewhat different effects, with AC better for revealing surface cracks and DC better for subsurface ones. Still, both are good for both types of cracks, just somewhat better for one than the other.
2 members like this
by Magnetoman
Originally Posted by reverb
what is the point to have a tested crankshaft or connecting rod?
parts that look very nice and smooth.
Magnaflux will reveal small cracks on the surface of smooth parts that you might not have noticed, as well as cracks under the surface you can't even see.

Spotting a small crack can be difficult on bare metal. However, Magnafluxing uses a fluorescent dye that makes even small cracks quite apparent under UV light.
1 member likes this
by quinten
I haven't read anywhere that Triumph crankshafts are a engine component that could be considered as being a potential failure item. 

its usually a ham-fisted owner that adds elements of failure to the crank .

good [Linked Image from]

or this [Linked Image from]
can lead to this
[Linked Image from]
1 member likes this
by BSA_WM20
Originally Posted by reverb if a Magnaflux machine cannot do an Aluminum test due to no magnetic field; the Aluminum stuff only rely on the dyes? Could not be good enough...
Very possible most do not have the Magnaflux machine and do these tests with the spray dyes.

Ultrasound and X-rays
However the fatigue life of all aluminium is very short so rods get replaced without question unless it is a bike I have owned since new .
1 member likes this
by BSA_WM20
The gear is not all that expensive when compared to a trip in an ambulance to the emergency department and towing for the bike
Even taxi fares to & from the physio .
Dye penetrant testing is not quite as sensitive and of course will not detect sub surface cracks that have not yet broken through
However what is often overlooked is the amount of over engineering that went into the bikes we ride.
I can not even count the number of bikes that were running quite fine , sent off for a journal grind & came back with a great big crack revealed when tested .

It is up to you and your own cost benefit analysis between how much you are willing to pay to reduce the chances of a very expensive ( & potentially harful ) failure in service .
1 member likes this
by John Harvey
John Harvey
Chris, you did say that and I'm not disagreeing with you, just pointing out that it's rarely (if ever!) the big end corner radius on a Triumph crank that's the main cause for crank breakage but the sludge trap area is known for it.

Interestingly there's a follow-up video that shows the source of the crack, it's exactly as you have described and was initiated by a sharp edge from the blanking plug machining running across the middle of the oil feed cross drilling.
1 member likes this
by Chris the camper.
Chris the camper.
Unfortunately the owner puts it down to metal fatigue rather than stress concentration.
1 member likes this
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