Replacement right hand crank arm

boxty

Hi,

Excuse my ignorance if this is a very silly question, but hopefully, some of the experts here can share some of their knowledge.

While out cycling with my brother yesterday, the pedal (a flat toe clip type) fell of his right hand (chain) side crank. Upon inspection, it appeared that the pedal had sheared the threads on the crank arm to the extent that the pedal couldn't be tighthened back on.

I've looked online and haven't been able to find a right hand side crank only. It's possible to get a left hand crank only? Or must he get a whole new crankset? This option seems like a bit of a waster since apart from the right crank, his chainset is fine.

In the event, that a new crankset is the only option, how should he go about choosing a replacement. His bike is a Felt F90 (he bought it 2nd hand but thinks its a 2006 or 2007). The existing one according to the original spec I found online is a FSA RPM (triple). The rest of the bike is sora/tiagra equipped. How do we determine the crank length- should we just measure or is it written on the crank somewhere. A truvativ touro crankset costs between €33 and €59, tiagra crankset is avail for €75 from crc (but it says no cups what are they?), a 105 crankset is €80.

Any advice appreciated.

Wez

Crank length should be written on the arm, might not be though, usually around 170-175. I've got a Truvativ Touro's on mine now, and I do like them alot I must say. I've got 165's, using a triple with one ring on it (was the most suitable in my budget).

I've gone through 3 set of cranks (on the one bike) because of this happening.. Really annoying, those little bits of thread inside your crank arm are pretty much the be all n end all for the crank arms..

Oh yeh, and you're more than likely better off replacing both cranks at the same time. Have a quik look around Ebay etc for the driveside one, but they are the most important, so tend to be rarer!

neiljb

boxty wrote: »

I've looked online and haven't been able to find a right hand side crank only. It's possible to get a left hand crank only? Or must he get a whole new crankset?

I've not looked too carefully, but I saw this :
http://www.bonthronebikes.co.uk/search?q=right+arm&p=1

I got a left-arm crank from them a few months ago, no problems.

D!armu!d

Happened to me a few months ago. Before replacing, try screwing the pedal through from the inside to see if this sorts out the threads. I thought I was going to have to replace my crank & took it to a shop but they sorted it doing this. They cleared the very outside few out, and the remainder hold the pedal fine.

kona

run a tap through the threads.

They are either 9/16 or 1/2 UNF thread.

Where did you buy the bike and how old is it?

Húrin

How does this thread-shearing actually happen?

Wez

Can happen from overtightening and things like that, even wear and tear eating away at the threads..

Kona, handy knowing those threads, but it seems near impossible to get to right sized tools to do the job. Even if ya manage to get them, there will not be much metal left to re-thread! They seem to turn to dust when friction begins..

A Helicoil is another option, but again, the equipment can be pricey!

I honestly reckon splashing around €35 on a brand new set of cranks and having them delivered in a couple days, is much safer than replacing/working the old crank arm. It's a bitch, but it's the best all round option..

kona

Wez wrote: »

Can happen from overtightening and things like that, even wear and tear eating away at the threads..

Kona, handy knowing those threads, but it seems near impossible to get to right sized tools to do the job. Even if ya manage to get them, there will not be much metal left to re-thread! They seem to turn to dust when friction begins..

A Helicoil is another option, but again, the equipment can be pricey!

I honestly reckon splashing around €35 on a brand new set of cranks and having them delivered in a couple days, is much safer than replacing/working the old crank arm. It's a bitch, but it's the best all round option..

Well 1/2 and 9/16th UNF are imperial threads so, you wont be able to get the taps in woodies or the like. Your best bet would be a engineering suppliers or VAR or Park Tools. I find that cheap taps are just so brittle(as they have to be hard) they snap, its best to buy the best you can.

When cutting the threads, its best done with the crank in a bench vice, so its level and not going to move. Then You use the tap front the back of the crank so it gets going easier, Using plenty of cutting fluid or wd40/gt85/finishline, you go two turns in and one turn back. I do it quite regular and ive never had a thread tun to dust.

With hollowtech your pretty ****ed as the arm is well, hollow, so if the threads go there is not much room to re-cut them.

A bike shop who knows their stuff will have a shot at this for at most e10.

kona

Húrin wrote: »

How does this thread-shearing actually happen?

Bike shops not Correctley PDI a bike. Bikes not serviced regular. Cheap pedals with threads cut bad, cranks with threads cut bad.