So I have been building up a fixie from a battered track bike from the skip. All seems to go well and I got a new bb and crankset. However, my left crank (onw without chainring) comes loose and I have to carry my large alen key around with me. I have tried hex bolts, and alen key bolts to keep the blighter on but after about 10 mins of riding it works its way loose and I get creaking. No matter how tight I dare tighten this in fear of snapping the bb, it allways comes loose after a while. What do you guys think I am doing wrong? Cheers.

P.S. this is the crank I bought, quite cheep but gets good reviews and seems to be ok. https://www.ebay.co.uk/itm/GREY-BIKE-...item415ba86f0a

Are you greasing the threads?

You may want to try cleaning the grease off, putting a drop of blue Loctite on the bolt and reinstalling. You'll still be able to remove the bolt if need be but the blue Loctite will help keep the bolt from backing itself out.

Probably time for a new crank arm. Guessing that the inside surface is all marred from "wobbling". It just takes a couple "wobbles" for the surface to get messed up, no matter how hard you tighten it, it will move a little when you pedal, slowly unscrewing the crank bolt.

+1

You might have rounded the cranks from all that accidental untightening. Also, I believe tightening the cranks involve a torque wrench (it has to be super super tight)

ok thanks guys, I hope to get some of that lockite stuff. The inside is not rounded as it is a brand new crank, only been about 30 miles max, but I am pretty sure that is not the problem. However the bb may be rounded (it is hard to tell because it is tapered as it is) Thanks for the help.

The crank is aluminum and the BB is steel. Steel wins over aluminum

Thanks for the help guys, it really helped.