Good day to everybody!
Today pedal fell from the crank on my wife’s uni during her ride.
Reason is slipped crank thread.
I think it may be because it got loose but not 100% sure because I checking it from time to time. Do not tight pedals really hard, just make it sit on the place.
Could you please give me settled advice how to avoid it in the future?
The most likely thing is that you have the wheel installed backwards (or the seat backwards).
The second most likely thing is that the pedal was cross-threaded when it was installed.
To avoid it in the future:
All directions are correct - cranks, wheel and seat.
Trying to turn pedals all the way through crank and only at the end tight with wrench. But… I think I remember some problems with this pedals. Now cross-thread looks like a problem 
Probably the pedal wasn’t tightened enough, so it had a tiny bit of play. Enough to slowly eat the thread. Is the uni a muni? Standing on the pedals puts more effort on the cranks. Pedals should be tightened, not a lot lot lot, but enough for them to really sit in place.
Also, the threads can be replaced at a decent bike shop. They will drill out, thread larger and put a helicoil insert in to fit the original pedal thread. This repair can be solid, or it may not last depending on the quality of the original crank, and the experience of the shop installing it. It is usually not expensive and worth a go.
These cranks are on 20" uni. I think that there was small play and edge of the pedal damaged thread with caused to cross-thread.
I need to check it more often.
I have no such work shop near me. Or far from me… Anyway, thank you for advice!
For anyone who doesn’t know, a Helicoil is a stainless steel helix with a diamond shaped profile. Both the inner and outer vees are exactly the same as the vee in a thread (FWIW in metric that is sixty degrees and 57 in imperial.)
The hole is drilled out to what is essentially the outer diameter of the original thread. Then a special non standard thread is tapped into the hole. It has the pitch of the thread being replaced but the diameter of the Helicoil.
A special tool winds them into place. They are fitted with a very strong Loctite thread lock compound so they don’t come out again.
So your aluminium part gets a very hard wearing stainless steel thread. It can be better than a new part. Indeed this kind of refinement is part of what makes high end engineering more expensive.
If I changed pedals a lot in expensive alloy cranks I would consider fitting them before the first use. However it must be remembered that the larger hole also slightly weakens the part. If there isn’t plenty of metal around the outside then it might not be a good solution.
In a used part note that the Helicoil would be compromised if the hole was already worn any larger than the diameter of the pedal thread at any point. Stripped threads in working alloy parts can wear to an oval very quickly. The part might not have yet come out but parts of the hole will already be worn beyond the outer diameter of the thread so the Helicoil results can be quite variable.
I have had a Helicoil kit for a very common spark plug thread for many years and have used it a number of times, especially on small motors on water pumps. Doing them on an engine without removing the head is an art, especially avoiding the swarf and snap off that engages with the fitting tool from falling into the cylinder.
Fitting them on a crank would be a cinch because you have access to both sides of the part.
Alright. Let me ask stupid questions then:
Should I pick M14 kit or M16? Which diameter are they indicating - inner of the insert or outer? I guess that inner diameter of insert but…
Is here any kits for left thread? This is left crank slipped.
Here is another thread. They said did they get left hand.
One poster says they shouldn’t be trusted on dynamic loading but another said they never had any problem in four years. As I said I have only used them in spark plug holes.
Having said that I can’t see why they would be any weaker than the original thread if fitted properly into a properly sized hole.
I expect they would be sized for the original thread. You have to get the special non-standard thread tap and inserting tool though. The tool for left hand could be different as they are designed to snap the tang when rotated backwards.
I image a left one is going to be premium price.
You don’t want metric. You need a helicoil set for bicycle pedals. The original threads are 9/16" x 20tpi. The right pedal is right threaded, and the left one is left threaded. The set is expensive, and not worth it for a single user. If you can find a used set it may be an option. The set we had at our bike shop was made by Eldi tools, and I think they were bought by Unior. So those two brands would be worth looking for.
Maybe for the cost/hassle of this repair you could just get new cheap qu-ax crankset?
I did not find any sets with left handed thread tap and inserts. I thank for the moment all I will do is take care about cranks more carefully.
I was going to check if this worth a go or not. Looks like it is not until I open me bike repair shop 
Another QA cranks were already installed by my wife - I am at work now and can’t help (her first cranks replacement).
Here’s a pretty good article that describes the process. (and it’s specific to crank arms)
Good luck!
