Loose crank arm woes

My non drive side (square taper) crank arm seems to become loose after a week or so and I can't figure out what's up with it. I've done all the normal things. Remove bolt, remove crank arm using correct toolage, cleaned, greased and put it back on very tightly. Then after riding for a week it seems to work itself loose again. However, the crank bolt isn't loose...bit baffled by it.

Anyone had the same or got any sage advice.

Thanks
Andrew

My guess, is that the taper on the (alloy) crank is damaged, so no amount of tightening the bolt is going to keep it tight. If that is the case, you probably need a new crank.

I'll take it off again tonight and properly look at it. I guess if its come loose enough times the square taper could start to get rounded causing a poor fit which will mean it continues to comes loose/get rounded even more...

take some pics of the alloy taper section of the crank when you get it off?

Also is the tool you're using to tighten it got enough leverage?

sure will do.

Yeah I've got an extendable arm on a hex bolt socket set which has a good amount of leverage.

Thanks both

It is fucked and needs replacing.

My guess, is that the taper on the (alloy) crank is damaged, so no amount of tightening the bolt is going to keep it tight. If that is the case, you probably need a new crank.

This and

It is fucked and needs replacing.

This

might it also be the wrong standard, so the BB and cranks are not exactly right? I can't remember without looking on Sheldon, but JIS vs ISO vs something else? But most like what he^ and he^^ said because it's always the worst case scenario.

Some pictures:

As a very temporary bodge I've put a little shim in there from a can to see if that'll work for a week or so until I source a new arm...