rcbike

( Heading should say Nut not bolt )
BB axle nut on the right crank comes loose repeatedly.
Went to the LBS for a new nut since the thread on the axle looks good and the nut looks little worn.
Tech says that this is a recurring problem and that I'll need a new bottom bracket and gave me the requested nut to try out. He advises that there is a design flaw in many of these three part BB's axles in that there are not enough threads to really secure the nut to the axle. It has happened to many of this customers.
Bicyle: Diamond Balck "Ascent" model, 26" MTN, maybe 1990's.
Has anybody have similar and how was it resolved?

JanMM

Guessing that the problem is that the square taper of the axle is worn, and not the fixing nut or axle threads. Original bottom bracket?
Is it a traditional style BB? Would have to be if it has a nutted axle. If so, a new axle could be the fix.

terrymorse 

If your bike has square taper cranks and you ride it while the crank is loose, the crank is ruined and will no longer remain tight. Rarely does the stronger steel spindle need replacement.

https://sheldonbrown.com/brandt/installing-cranks.html

JanMM

That's what I meant to say. Correct it's likely the aluminum crank and not the steel axle.

rcbike

The square taper doesn't look at all worn, it shows that same metal finish as the rest of the axle.
It seems to me that this is the original to this bike. The ring gear is labeled: Shimano Biopace-SG and the right crank is labeled: EXAGE 400L X. Haven't taken the BB apart yet.
How can I determine what replacement BB will fit? I just got this used bike and like it very much.
Thanks

rcbike

The aluminum crank square does seem worn, somewhat "opened up" as if has been ridden loose.
(The above responses were received while I was typing mine.)
So can I buy a replacement right crank of do I need both?

Jeff Wills

Left crankarms tend to be a little more available than right since they are what usually work loose. Any good bike shop can get either, but it may be easier and cheaper to replace the whole crankset.

Kimmo

This isn't an inherent problem of square-taper cranks; sounds like your crank arm has been ridden while loose and is now cactus.

You need to match the length stamped on the inside of the arm, most likely 170mm.

Also, you won't be able to match the old crank with a new part, but you may be able to find something very similar second hand... and at second hand prices, you might as well get both arms if you can't find a match.

This is why it's a good move to fill out the location field in your profile - try here: https://www.magiccitybike.com/

rcbike

Thanks for you generous answers. I'll set to work with your info.

HillRider

One more thing. When you replace your crank, be sure to tighten the nuts to full torque (typically 300-400 inch-pounds), which is much tighter than you would think. The most common cause of the kind of damage you just suffered is the crank bolts/nuts not being tightened enough initially. If the "Tech" you talked to sees this problem a lot, he doesn't know how to install these cranks correctly.

As noted by the above posters, once a square taper crank is ridden while loose, it will never work properly again.

ksisler

OP/rc; Far better to just get a new one;
-- Count the teeth on your current crank and look on the back of the arms for the number indicating the lenght (such as 170, 172.5, etc.).
-- Then get on amazon and search in the outdoor sports | cycling area for Crankset with bottom bracket. If you have a double now, look for a double. If you have a double and just decided you want a triple crank they you have more to figure out and buy which I won't cover here.
-- You will find lots of square taper cranks and they are the cheapest ones...starting at about $29.99 or so complete, easy. If you stay with square taper and keep it tightened to spec, it will hold up fine for decades of use. Don't think that a $30 crank won't be good enough...they will do fine and are better/stronger than many mainstream products from 10 year ago.
-- If you are changing both the crank and bottom bracket, you might want to just make the leap to a more modern system, such as a MegaEXO, ISIS, etc., several versions and it matters not a bit, as long as the crank and spindle are the same.
-- For grins, I would recommend looking for SORA or Truvativ as brands. Both are really good and dirt cheap.
-- Remember to match up what looks good with the crank arm length, etc., information that you wrote down earlier.
-- Once you decide exactly what you want, check to see if you have tools needed to do it.
-- Most external bearing type brackets require as special splined tool to tighten the bearings and a socket handle to turn that.
-- A removal tool may or may not be needed.

- Repeat the search on ebay. Recommend ignoring ads for used cranks as you can get new for same or less money.
-- Lots of SORA's and Truvativ cranks there.

- If you want to PM or post to here on what you find before you buy, thats probably a good idea to preclude an expensive error.

Hope that helps
/K

cny-bikeman

Yes, please double-check before you buy. Not all cranksets will be compatible with your BB spindle. From a quick search it appears the Impel most likely takes a 110-113 length spindle (not counting the threaded stud, as yours takes a nut, not a bolt). That means you need a crankset compatible with that length.

DannoXYZ 

Interestingly enough, the lower-end cranks on cheapo bikes tend to stay on better than the higher-end boutique stuff (bell-curve of the groups from by 10-years as working in a shop and 10-years racing bikes). The cheaper stuff like SR/SuginoVP that is on 80% of all low-end bikes from 1975 onwards, has more material than the nicer stuff. More metal results in less flexing and working loose for the same thread-tension on the axle. The lower-end bikes also tend to use serrated flanged-nuts, which stays on better for the same torque on crankarm fixing nut/bolt. Most mechanics tend to install the crankarm nut/bolt to only 1/2 of the required torque because they're using itty-bitty 6" wrenches or allen-keys.

The required 25-33 lb-FT is among the highest amount of torque used on any fastener on a bike. You really need to use an automotive-style ratchet-wrench with socket to get the nut/bolt up to the manufacturer-recommend torque. I prefer to use a dial-type torque-wrench on my bikes, motorcycles and autos.



New ones are digital!


Even an old-fashioned clicker type will work:


No worthwhile pro mechanic will claim that they have a "calibrated" hand/arm that can get it right without a torque wrench. Numerous studies have already been done on these "calibrated" arms and most of these organic gauges tend to be off by over 50%. They also tend to over-tighten small fasteners and under-tighten large ones. With the most basic of torque-wrenches, even non-pro mechanics can get the crankarm-bolt closer to the right spec than the most "calibrated" arms.

And you have only ONE chance to install that crankarm nut to the correct tension, and that's the very first time. If it comes loose, it's possible to mangle and damage the square hole in the arm if you don't stop riding right away. Do a search for "loose crankarm" on this board and you'll find how common this is when the bolt/nut's loose.

fietsbob

Fixed with a Nut means its a solid axle cup and Cone BB , Bearing races may be potholed by now,
past due time to tear down , clean and look.

Bolt types the Axle is Hollow, bolt has more threads engaged , and Torque stretches better, to not loosen.

HillRider

Even the less expensive beam-type torque wrenches are very suitable for bicycle use since they are always in a visible location. My 3/8"-square drive Craftsman beam-tyle torque wrench is calibrated to 600 lb-in (50 lb-ft) which is plenty for any square drive bb fastening nut or bolt.

And, yes, you do indeed get ONE chance to install crank arms correctly as once the get loose they are throw-aways.

rcbike

Wow! This is really good information, it gives a thorough understanding of why it happens and how to prevent it. Thanks