BB Bolt coming loose, keep stripping crank arms
 

Hello, I am a new user here but i'll get to the point. I am a heavy rider.

I own a 6KU Single Speed bike and the crank arms are square tapered. It's a Neco Square Taper BB. It's been almost a year since I got my bike and I have stripped the square hole on 3 left-side aluminum crank arms and 1 BB bolt.
The drive side has never given me a single problem.


I notice the bolt loosens when I put stress on the crank arms to speed up, or go from 0 to cruising speed. The loosening of the bolt causes play on the crank arm, which then causes stripping.
Not to mention, only when I pedal too hard uphill I hear creaking, but on flat ground there is no creaking no matter how hard I pedal.
I suspect it's my own weight but first here are some other details.

The BB bolt tightens clockwise, so I suspect I am loosening the bolt whenever I pedal, which is counter clockwise.

The M8 threaded bolt has never been tightened to the precise torque by my bike service shop, they tighten it to hell and I can only tighten it so much with a hex key, which clearly will not get me near the proper torque.

I used blue Loctite, let it settle for 72hrs, and on the first ride the bolt came loose again. Again, I could only use a hex key to tighten it.


I thought that maybe the threads the bolt goes into were stripped, but upon examination they're fine.
So my question is, what is causing the bolt to loosen and my crank arm to strip?
Is it improper torquing, my weight, or the fact that the bolt is tightened clockwise while I pedal counter clockwise?
Do I need a new BB set?

I called my bike shop and they're suggesting a steel crank arm, but I have a feeling that's a temporary solution.

edit: the bolt is also in perfect condition, the crank arm is starting to strip.

Perhaps you should invest in a torque wrench so that you can adequately tighten the bolts; Your description indicates under-torqued fasteners.

In general, in the absence of a torque wrench, people tend to over-torque small fasteners and under-torque larger fasteners. There's no way on the planet that you'll achieve sufficient torque on the crank bolts with a hex key. Why only the NDS bolt is loosening is anyone's guess, but until you use the right torque, it's hard to consider any other cause. IIRC, the recommended torque on my square-drive cranks (Campag Chorus) is ~30 ft.lb, which is way beyond hex key territory

The issue is damage to the crank arm. If the arm was ridden while loose, there is a strong possibility that the square broach in the arm has been damaged. If this is the case, you cannot expect the mounting bolt to reliably secure the arm to the spindle. Replace the arm and install with the proper torque and it should hold.

The non drive side crank arm's fit to the axle sees greater loads and more back and forth loads then the drive side does. remember that the drive side arm is directly connected to the chainrings/chain. So all it's drive forces go through the chainring/arm construction and very little is seen by the axle/arm fit. Additionally remember that the vast majority of riders don't really pull up on the back stroke, so the other arm will drive the back stroke arm up via the axle fit. The non drive side sees both these forces with each stroke. The drive side sees mostly only the force to pull up the non drive side. Andy

As above, it's likely that you're not applying enough torque to the bolt. It has to be done when new or the crank arm will be damaged by normal riding forces.

They should be torqued to 300 to 400 in-lb.: https://www.parktool.com/blog/repair...s-and-concepts
This is darn near impossible to achieve with a normal hex wrench. I use an 8mm hex socket in a ratchet handle and use both arms to scissor the wrench and crank arm together.

If your bike shop is not using a torque wrench to tighten the replacement crank arms in place, find a different bike shop.

Yes, you are correct. I am 100% sure I have the right cranks for the BB because they're still OEM, I never changed either component.

This is the approach I was going to take before buying a steel crank arm, thank you very much for your help!
Additional thanks to dabac and JohnDThompson!

You rode it while loose. Now the crank taper-receivers are almost certainly damaged and you'll never get it to stay tight again. Doesn't matter if you locktite it or torque it to hell or not.

+1 on this point. I have seen customers who don't believe what I've told them. "I'll weld the dam nut, It won't move after that!" Well it's not that the nut moves but that the arm does via the tapered hole's changing dimensions/shape. Andy

Damn, it seems the issue may be far worse than anticipated. Thanks for your insight dude!

Not really worse than you thought. Bottom brackets are <$25 for just about any generic. Are there plenty for way more dollars? Sure. Yours? Probably cheap.

A cheap torque wrench is less than $30
The bottom bracket socket, (assuming cartridge type bb) is under $20

Boat trailer axle grease is cheaper than dirt. Grease all your threads.

Under $75 dollars in total to do it yourself.

As far as the crank arms go. That could be pricey, but it doesn't have to be. Square taper cranks are out of fashion. Hollow Tech II, GXP, BB30, etc... is all the rage now.

If it were me, before I spent a dollar on anything new, I'd clean the interface area real good, apply a very light coating of grease on the taper of the original bb, slide the crank arm on. Lay the bike on the garage floor, back up the bottom bracket with a block of wood and give it a good hard whack with a heavy hammer to seat it good. I'd use a piece of wood or something to avoid marring the crank arm. Then use a long allen wrench and torque the dickens out of it with my foot. See how long that lasts. If it doesn't you were destined for a new set up anyway.

Aren't the bolt holes drilled and tapped directly into the spindle? I guess I never looked close enough.

If it's indeed a case of initial loose fit as suspected, the BB itself is probably fine. Hard steel BB, softer aluminum crank arm, the BB wins and the arm loses. As for crank arm cost, switching to another technology would be expensive, but square taper cranks are easily had at any bike co-op. Depending on concern about weight balance and aesthetics, the OP could even opt just to swap that NDS arm, since the DS is single speed. Sourcing a single orphaned arm with no need to match it gets you into the "donation to the co-op" price range.

And +1 on the marine grease; it's water-resistant, and several LBSes around here swear by it. Available at any man-mall.

I think you have it right on the steel vs aluminum part. However I don't know what a stripped BB taper looks like since this is my first time
examining the area myself. This thread gave me a mountain of info and I really appreciate it.

https://www.sheldonbrown.com/brandt/...ng-cranks.html I use grease on the taper and a torque wrench and have never had a problem.

Okay, I recall decades ago, before the interwebzz, the "conventional wisdom" was that you lubricate the crank bolt/nut threads, but NOT the tapers. Has that changed? Was that bogus information then? What's the skinny?

Aluminum & steel don't play well together. Add to that the inconsistancy in the friction in the interface, galling of the surfaces and overcoming the friction of initial install & every time there after...When torquing bolts it's always best to know if it is intended to be lubricated or not...usually it has to do with "set & forget" or if it is a safety critical or field servicable item.

Square taper always fulfilled the safety sensative & field serviceable check boxes for me, so I always have. I've never had a problem & never gave it further thought. For others, maybe their concerns are it never be removed again. Or maybe they are ok not necessarily knowing if they overcame running torque to ensure it actually seated as far as it should. Maybe they feel the chances of galling is minimal, or have little concern for electrolysis, or whatever. Those people don't lube. In the real world, it probably has some oil or grease or something on it that was "left over" & is "good enough" to be ok. The real problems arise out of being truely bare & degreased. We are dealing with the worlds simpelist machine afterall. A speck of grease or not on reassembly is unlikely to cause a real problem either way.

In my field (aviation manufacturing) all fasteners have a specific torque procedure & nearly everything that moves is usually lubricated with grease in some manner. The rest of the structural fasteners are lubricated with cetyl-alcohol. Consistancy of execution is why a friction interface gets lubrication. So I do as I think the engineers at my work would expect.

I guess I didn't learn the conventional wisdom back then- we always greased up the bearings, put things together, adjusted the bearings, and torqued the crankarms. The tapers probably had a bit of grease on them, but we just wiped off any excess before delivering to the customer.