Brompton left crank arm loose - do I just need a new bolt?

The left crank arm came loose on my Brompton. I didn't ride on it loose so I have good hope I haven't damaged the spindle, crank arm or threads. I suspect it was a result of vibrations of lots of long commutes possibly added by it dropping once during Sunday's winds in London.

I lost the black thing that goes with the crank arm bolt though (spacer/washer?). Do I need this to tighten it back up properly?

See pictures - it doesn't look like the crank square or threads or anything is damaged. I'll get it tightened properly at a shop but you do reckon I need a new bolt too (or somehow a crank arm or bottom bracket even though none of this looks damaged)?

Any thoughts appreciated.



Get yourself the right size Allen key and carry it with you. This guarantees that it doesn't come loose again. Leave the Allen key at home and it will work loose on your next ride.

I like Skol

Hold my beer and watch this....
You might be lucky but once these work loose during use it is normally terminal for the soft alloy crank arm! It's difficult to see any damage in your picture as inside the square hole is blurred. Any damage will not be visible from the front as the axle doesn't (shouldn't) protrude that far, look at the square from the back to see any damage.
Couple of things that might help save the arm. 1 put some locktite on the axle before refitting the arm. 2 tighten the bolt a bit tighter than the recommended torque. 3 tighten it again after each ride to make sure it doesn't loosen off. This may help creep the arm onto the tapered axle enough to make the square reform good enough to grip the axle.

The bottom bracket axle will not be damaged, it is as hard as iron!
I used loctite and had it tightened at a bike shop. Seems good. Hope it holds.

What size allen key do I need to buy to do it myself? 8mm or 10mm or something else?


Pretty sure it is an 8mm allen key.

If on the road and it works loose, using a combination such as a 3mm with a 5mm allen key together, will get you home.


Legendary Member
They sell the LH crank as a spare part.

Incidentally, the LH one always works loose first. The RH interface has an easier life because it's in line with the chain.
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