Left crank arm fell off -- seeking advice

[derndingle]

After Googling around a bit and reading through some similar threads here on this forum, I still wanted to ask a couple questions before I started ordering parts on-line.

I'm a very casual bike rider. I have a 10 year old Gaint Sedona DX 23" bike with a baby seat on the back. The other day, I was riding with my 40 pound two-year-old in the back when my left crank arm came off--luckily I managed not to crash, but I'm sure it was quite the sight watching us scoot all the way back home through the neighboorhood.

I wasn't able to find the bolt that held the left crank arm on. The crank arm is a SR Suntour 170mm arm. I see the number "846" stamped on it, but I don't think that's a SR Suntour part number. The square hole in the arm was damaged a little bit when it came off, but doesn't look too bad.

crank_arm1.jpg

I'm trying to decide whether I need to buy just a new bolt, or if should buy a new crank arm and bolt. Also, I'm not sure if this is something I can do myself or if I need to bring it into a bike shop. I've seen a couple of posts mentioning that cranks need specialized tools... would putting the arm back on require anything more than an allen wrench?

Thanks!

[bobotech]

Putting a crank on doesn't really need any special tools. Taking one off needs a crank puller.

To me that crank looks shot. Looks like there is a lot of wear in the square that will just be more of problem in the future.

Go to your local bike coop and get a replacement arm. Should be very cheap.

[davidad]

It's shot. Get a new arm and tighten it to 25 pounds feet of torque.

[fietsbob]

You should have not ignored your maintenance.. and rode so long with a loose bolt.
[begin guilt mode]
Its a cast part, so similar will be cheap enough.
all you need is to find one the same length.

online will not do the maintenance services..

[FBinNY]

Odds are the crank is shot, but you can always cross your fingers and hope for the best. The only tool needed to install a crank is a 14mm (99% of crank bolts) socket wrench. If after tightening it securely, it clicks as you ride, then it's shot and you need a new arm.

Cranks don't fall off without warning. They announce that they're loose by clicking with every revolution. If not attended to, loose cranks are rapidly destroyed as the square spindle works at the corners of the hole allowing movement that cannot be tightened away.

[Retro Grouch]

You definitely need a new crankarm.

If you aren't too picky about an exact match for the other side, your local LBS will have one for around $15.00 or $20.00. They'll have a replacement bolt too. Two things to look for:

1. A length will be stamped on the inside of your crankarm. It'll probably be something like 172.5 mm.
2. Yours is a square relative to the arm. Some are diamonds. Unless you want to learn a new peddling cadence, make sure that you get a square.

[IthaDan]

Time to visit your local bike coop!

[DannoXYZ]

The crankarm bolts require some of the highest torque on a bike, 25-33 lb*ft torque. You need an automotive-style 3/8" drive ratchet wrench to get that much torque.

Get a replacement crankarm and hex head bolt. Using allen-key bolt for crankarm is bad idea because it is tempting to use a short L-shaped allen key. You end up with only 20-25% of the necessary torque.

[gmt13]

While I admit that I would probably give this crank a try, I darn sure would not trust it much unless you ride less than a 30" gear and never go up hills. Mounting a new arm is easy since special tools are not required.

-G

[derndingle]

You definitely need a new crankarm.

If you aren't too picky about an exact match for the other side, your local LBS will have one for around $15.00 or $20.00. They'll have a replacement bolt too. Two things to look for:

1. A length will be stamped on the inside of your crankarm. It'll probably be something like 172.5 mm.
2. Yours is a square relative to the arm. Some are diamonds. Unless you want to learn a new peddling cadence, make sure that you get a square.

Thanks to everyone for the replies. Especially the part about watching out for diamonds! I went to the local bike shop today with the crank in hand and asked them if they had a replacement. She went in the back and came out with one and I was about to check out when I realized it was a diamond. She went into the back again and found another one that was almost an exact match of what I had. Cost me $11 plus another $3 for the bolt. I stopped at the hardware store and bought a allen head socket to fit on my 3/8th socket wrench and cranked it down as tight as I could get it. I don't have a torque wrench, but it feels pretty tight. I also tightened the bolt on the right side about half a turn as well.

It had been clicking and popping for quite some time, but I didn't attribute it to the crank arm bolt being lose. I'm a very big guy, and probably far too heavy to have any business on a bicycle--I thought the clicking and popping was just from the strain of my weight.

Thanks again!

[noglider]

The crankarm bolts require some of the highest torque on a bike, 25-33 lb*ft torque. You need an automotive-style 3/8" drive ratchet wrench to get that much torque.

Get a replacement crankarm and hex head bolt. Using allen-key bolt for crankarm is bad idea because it is tempting to use a short L-shaped allen key. You end up with only 20-25% of the necessary torque.

I did not know this, so I'm grateful for learning it.