straight square taper cranks and m5 threads

[Vladimir]

Hi guys,

This weekend has seen the inauguration of project "pimp my bike part 2"

I've fiddled with it to a certain extent; a new rack was in order as the old one didn't have enough support for the bag (some spoke fouling, not good!) and in the process either me or my old LBS managed to partly strip the thread on the lower attachment point. My solution has been a bit of ptfe tape on the thread and back in it went! Now I'm not certain of how well the new rack will hold under load... what do you guys think?

Next question; my cranks did not feel like they were on "straight" and they also probably weren't tightened properly; I am now in the posession of a torque wrench and all of the bits for it (park tool are bloody thieves, but that's another story) so I will be remounting tomorrow (it's getting late). Two questions here; 1.what is the correct torque? (BB = UN55, chainset = alivio)
2.how does one ensure that the cranks go on "straight"? Will tightening properly (to the correct torque) serve to straighten the cranks on, to an extent?


I'm very sensitive to the angle of the cranks and want to get this bit right! I rode the bike over the last week, and the cranks felt "not right" to me. wierdly enough, when I am clipped in; the sensitivity is alleviated, almost to the point where if I am clipped in, the pedals and cranks feel perfectly perpendicular to everything - pretty much perfect. (time pedals). The solution seems simple, just stay clipped in and all will be all right, and I would do that, if it wasn't so blooming hard to clip into the pedals (I also ride through london, so clipping out is very common.

this leads me nicely to my next point; my new time pedals - time all road grippers - are very hard to clip into. clipping out is easy enough, but clipping in is a right chore! certainly nowhere near as hard to clip into as my time roc atac pedals. I have tried setting the tension to the lowest setting, and this did not help at all - in fact the pedals were already at the lowest setting (as the instructions confirmed), I dread to think how tough it must be to get into them at the higher tension setting. The question is, what can be done about this? I've been toying with the idea of taking them apart, and filing down the spring or the tensioning pin, where the two come into contact. Is this a sound idea at all? I imagine that it will have to be done very precisely...

Thanks for reading, any advice appreciated.

Vlad

[Brucey]

if a thread is stripped, its stripped. PTFE tape will hold it if it sees no load. Try using a nut and bolt.

Crank torque here;

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Alivio-Acera/FrontDriveSystem/SI-F920A-En_v1_m56577569830612827.pdf

If the chainrings run true then the chainset is fitted straight, on one side, anyway.

Can't really help with the pedals, except to note that sometimes the shoe sole fouls on the pedal and causes troubles of the sort you have encountered.

cheers

[Vladimir]

if a thread is stripped, its stripped. PTFE tape will hold it if it sees no load. Try using a nut and bolt.

Crank torque here;

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Alivio-Acera/FrontDriveSystem/SI-F920A-En_v1_m56577569830612827.pdf

If the chainrings run true then the chainset is fitted straight, on one side, anyway.

Can't really help with the pedals, except to note that sometimes the shoe sole fouls on the pedal and causes troubles of the sort you have encountered.

cheers

Thanks for the reply, Brucey. Nice to know I'm not the only one up at this hour.

Nut and bolt, eh? Blooming genius idea! Shall hope for two nicely matched nut and bolt combos when I root around the spares box tomorrow...

[meic]

The other option is to tap it out to a 6mm thread but that requires resources, skill or money.

It is quite possible that no amount of torque setting will make your crank sit straight.
I had a crankset that must have been deformed in some way and it will not sit square on the taper, any taper at any torque.
This is tested by trying it at different rotations on the taper and the "tilt" turns along with the crankset rather than staying with the axle.

[Vladimir]

Thanks for all the replies, guys.

I'm not going to be tapping the thread out to M6 anytime soon; I haven't a tap and last time I used a tap, it didn't go well and I'm not keen to repeat the experience...

so I did something a little bit "thick" today. I found some longer M5s in the spares box; and proceeded to thread them all the way through the seat stay. there is a small hole on the other side of the stay, and I assumed that logically the holes were lined up, and this would be the site where the bolt would exit.

Turns out that this is not the case; I was threading the bolt through using a socket wrench - and at points the going was a bit tough, but it didn't feel THAT tough, I just assumed there was dirt in the treads, etc. Aaaanyway - long story short, I punched a very neat hole straight through the other side of the seat stay, about 1.5 mm above the hole that I assumed the bolt was going to come out of. The area around the hole that I punched with the m5 bolt is raised, and the bolt is just protruding through, not way to put a nut on it, as only a thread or two are protruding...

What do I do now? I have the instinct to take the bolt out, flatten the area back to its original shape with some pliers, and put the bolt back through, using a nut on the other side to give it a bit of stability...

should I get a reamer or de-burring tool to make the edges of the punched hole nice and smooth, in order to avoid the stress focusing in some of the sharp micro-cracks that have been invariably created? (I think I might have answered my own question here....)

anything else I need to know? I don't need to throw my frame away, do I?

The whole process (assuming it's safe) might well need to be performed on the other side, as I was tightening the bolt, it was starting to feel a little buttery, at which point I promptly stopped tightening... I suppose I need to do the nut and bolt job on that side too?

[shotaway]

If you think it might have been the LBS take it back they will be able to fix this using a heli-coil this is the best way to fix damaged threads.. you can of course buy helicoils and do it your self but it a lot for a one off job.

http://www.helicoil.in/helicoil.htm

just found a kit on ebay for 17 quid... not the real mac-coy helicoil tho...

believe it or not it is actually a improvement once you helicoil it.
Normally Ali is always heli coils'ed for extra strength, dunno if the bike industry does this tho it is cost cutting not to use them..thats is if you have a ali frame but still suitable for any frame material even carbon fiber i would suppect.

[Vladimir]

If you think it might have been the LBS take it back they will be able to fix this using a heli-coil this is the best way to fix damaged threads.. you can of course buy helicoils and do it your self but it a lot for a one off job.

http://www.helicoil.in/helicoil.htm

Thanks, I might investigate that in the future.

In the meantime, what do people make of this hole that I (accidentally, honest) punched in the seatstay? Is it safe to carry on as is?

Cheers

Vlad

[Brucey]

Oh blimey... is the frame steel or aluminium?

If the fitting was stripped, how did you manage to do this?

cheers

[Vladimir]

Oh blimey... is the frame steel or aluminium?

If the fitting was stripped, how did you manage to do this?

cheers

aluminium (7071 or some such). I think the distal threads were "kind of" stripped, whilst some of the deeper threads were still intact. The previous bolt might have been a little too shallow for purpose...

One way or another, is it going to be all right, doc? Can I flatten the area, put a nut on the other side, and just carry on?

Thanks

Vlad

[Brucey]

very often such fittings are 'nutserts' which have no threads at all near the very top.

I wouldn't bother with a nut and bolt on this type of fitting, I'd just change the nutsert, if it needs changing.

Otherwise just use a screw of the correct length.

Re the frame; whatever you do now there it is possible that the frame could crack later on, so you need to get in the habit of examining this region for cracks if you are going to continue to use it.

This learning-by-experience business doesn't always come cheap, does it....

cheers

[Vladimir]

very often such fittings are 'nutserts' which have no threads at all near the very top.

I wouldn't bother with a nut and bolt on this type of fitting, I'd just change the nutsert, if it needs changing.

Otherwise just use a screw of the correct length.

Re the frame; whatever you do now there it is possible that the frame could crack later on, so you need to get in the habit of examining this region for cracks if you are going to continue to use it.

This learning-by-experience business doesn't always come cheap, does it....

cheers

not quite, no...

I shall experiment with different length screws... Hopefully I can find something that will fit and hold.

The good thing is, my best man (to be) has a spare MTB frame - which he has been looking to get rid of. the only touble with this frame? - is the lack of forks, and I will have to buy those and a headset (not the same as on my current bike unfortunately; the current bike is quill stem and his frame is A headset.

Ah well, I will get to rebuild the bike over again, which has to be fun, right?!

[Brucey]

well you never know, it might go on for years like that... there were several holes in that region already, right?

Only one way to find out....

cheers

[Vladimir]

yes. there were holes there already

What concerns me, ever so slightly, is that there are now two 5 mm holes withing 2 mm of each other...

shall ride and see! I will de-burr it first, of course.

there is no way that the aluminium will fail critically, is there? If it cracks, I will have some warning that it has cracked and avoid some kind of catastrophic incident?

[Brucey]

well it could fail suddenly through overload, but unless the design was marginal to start with or you are in the habit of jumping off things this probably isn't very likely.

It is more likely that the damaged region could crack and that the crack will propagate over time until the part is weakened enough that it fails in normal service, hence my suggestion re inpsection.

cheers

[Vladimir]

well it could fail suddenly through overload, but unless the design was marginal to start with or you are in the habit of jumping off things this probably isn't very likely.

It is more likely that the damaged region could crack and that the crack will propagate over time until the part is weakened enough that it fails in normal service, hence my suggestion re inpsection.

cheers

makes sense. no, I don't jump off things - I simply am heavy, put a lot of weight on the saddle (on that particular bike cause of it's set up) and always ride with panniers, which I'm attaching through said region...

I shall inspect it every other day during my routine checks...

Thanks for your replies Bruce