Shimano 739 crank

[wolfman]

Hi everyone!

I have a minor (?) problem with my oldschool FC-M739 right crank arm that I've bought as a used item. Having installed it I've heard a strange creaking noise during use. I also bought a set of chainring, because it's own chainrings were in terrible condition. The seller of the chainring set told me that the 739's typical failure are usually some cracks on the mounting surface of the biggest chainring (wich carries also the other two by the 739's).

Having disassembled I've realised that the crank arm has the cracks that the chinrings' seller noticed.

Fortunately I've had an another set of crank arms wich I installed with the new chainring set. But I'm afraid that my other crankset will have the same failure, so I just keep on listening if there is any strange noise...

Unfortunately I don't know what could cause such failure by the 739's crankset. Do you have any idea about the source of the problem? Too much or too little fasten torque at the biggest chainring...? (Unfortunately I haven't got any torque wrench so I can't check it, but I think I could check it, if I knew the correct measure.) Or just a simple fatigue in the material wich cannot be avoided...?

I have some photos about a damaged crank arm. There can be seen the cracks I've mentioned.

Thanks for any help

 

[wolfman]

:shock:

 

[tonedeaf101]

Ooh nasty. I'd say lock nut was not tight enough and chain ring was twisting during loading. Maybe, I'm no expert ;-)

 

[Thias]

Nah! That looks like a typical material failure. Nothing you can do about it.

 

[wolfman]

That's what I was afraid of... But if it's a typical material failure, I have to expect the same failure my other crank arm - and sooner or later on every 739 arms...

 

[wolfman]

Ooh nasty. I'd say lock nut was not tight enough and chain ring was twisting during loading. Maybe, I'm no expert ;-)

I tought the same for a moment but if the rings had been twisting on the arm, there would be some traces (scratches, deformity etc.) on it, wouldn't there?

 

[wolfman]

(sorry for grammar mistakes, I'm just learning English, so plesae correct me if I made any)

 

[one-eyed_jim]

To me it looks like a fault in the forging. The two voids (circled in green) must have been present at manufacture. That might be explained if the crank is formed from two cast blanks that are then forged together.

I don't think over-tightening or under-tightening the lockring can be to blame, or pedalling stresses. As you say, you would expect the damage to be elsewhere if that were the case.

 

[02gf74]

My vote is the crank arm flexing. Either a bad design, bad casting or simpky failure due to old age and much usage.

 

[wolfman]

So the question is: is it a unique, or a costruction problem? It would be good if it were only unique, but my crank arm has got the same cracks, yet they are minor than these in the photos...

I tought that 739 XT-s are one of the best - until I've realised that this could be a costruction problem....