Crank arm bolts coming loose.

On my dad bike, the crank arm bolts are loosening every few rides to point I can feel them be loose. The cranks is an old Sugino triple on a (I’m assuming, it was in the bike) Shimano square taper BB. I’m using these Allen key bolts.

I’ve greased the bolt threads, but that’s it. Should I be using loctite? I’ve never had this issue in the past and I’ve used square taper cranks on a lot bikes with these bolts and the non-Allen key head.

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Sounds like the cranks are on their way out. Once the crank has loosened and been ridden on - the square taper will deform more and more and as a result will start slipping again in short space of time. Take this opportunity to upgrade to Hollowtech II?

Hmm.

Well the crank arm seems solid on the BB, just the bolt is loosening. The BB probably needs to to be replaced as well.

Upgrading the cranks and BB isn’t on the horizon right now.

Well the crank arm seems solid on the BB, just the bolt is loosening.

Do they only feel solid when you tighten the bolt or all the time? If the taper is misshapen as per @Pifko suggestion they will feel solid when you do the bolt up but it’s just pushing a square peg into a round(ed) hole so it has no staying power.

If the bolts loosen and the arms are still nice and solid on the axle, how much can you tighten them? I can always get at least 1/4 to 1/2 a turn on the bolts on square taper cranks any time I try.

They feel solid on the BB spindle when the bolts are out. The BB spindle feels loose (it’s a sealed Shimano one, no idea of the age), so I’m wondering if that being a touch loose is making the cranks wiggle slightly and then loose the bolts. I’ll try and tighten the BB.

Whoops, realised I tagged the wrong person!

I’d be surprised a loose spindle was causing the bolts to loosen but maybe?

If the spindle is loose the BB will need to be replaced.

If you catch it early then a bit of loctite can sometimes secure a crank on a bb spindle if the interface is not too badly damaged - if it has not been ridden for too many miles while loose and if it has not been ridden while it is too loose in particular...