cooker

I assembled my Bike Friday today for the first time this season and went for a modest 20 km ride, and halfway home my pedal started to wobble. I had stripped the thread in the right crank arm of of the Tiagra crankset. Perhaps I didn't tighten the pedals as hard as I should, but I thought I did, and besides, don't they tend to self tighten as you ride? Anyway, what are my options - can I simply replace the right crank arm, and where would I get the part? The pedal thread looks intact, and when I reinstalled the pedal it carved out some of the damaged crank arm thread so it's obviously a lot harder. thanks

pennstater

I used a helicoil to repair the threads on an old alloy crank arm.
Depends on your mechanical ability and availability/cost of replacement crank arm.

mparker326

I had a helicoil installed for $15. Works like a charm.

Bill Kapaun

The fortunate part is it's the DS which has RH threads. Should be a simple fix at a machine shop to install a heli-coil
Or you could buy the kit. Probably not worth it for a one time repair though.

cooker

DS? Drive side? Helicoil sounds great! I will check if my LBS does helicoil inserts. Although...mostly I ride beaters and the problem may come up on another bike. If did it myself, what size is it? Thanks

Kotts

Odds are your LBS won't be equipped to install a heli-coil. You want to take the crank arm to a machine shop. They will have the tools to do it properly.

cooker

thanks

EDIT - I understand the 9/16" 20 TPI insert is pretty specific to bikes and is sold in specific kits to bike stores, so wonder if machine shops would have that.

N8N

9/16-20? Why not 9/16-18 (aka 9/16 NF?) what possible advantage is there to using a non-standard thread, other than making one buy more tools? I was just about to say that it's entirely possible to DIY with a drill, tap wrench, steady hand, and a little bit of Loctite, but I guess not.

Are there any other fasteners on a typical bike that can't be tapped/Helicoiled with standard tools (not counting the bottom bracket and headset, for which I understand the reasons why they're unique.)

cooker

^^I suspect the pedals have used that thread since long before Helicoil was invented.

Just to update the thread, for readers in Toronto, I called the Trek store, La Bicicletta, Urbane, Curbside and Bikes on Wheels. Both La Bicicletta and Urbane said they could do a Helicoil repair, but not right now - one loaned out their tap and are awaiting return and one has to order the inserts. They both said call back in a week. The others said I would probably have to buy a new crankset.

HillRider

Good pedals are not NF or NC threads and never have been. 9/16-20 pedal threads have been around for decades and nothing is going to change that. At least it is standard on most good quality bikes.

Ashtabula (One-Piece or "American") cranks are typically threaded 1/2"-20 tpi which is an NF standard so if you want to be able to use standard taps, you will have to use one of those cranks.

Several other bike threads are unusual. Rear derailleur hangers are M10x1.0mm which is not a common thread pitch in that diameter either. Common crank puller threads are M22 x1.0 which is also very uncommon elsewhere.

Jerseysbest

Similar thing happened to me on my ultegra crank arm. I looked into helicoil but I found a used replacement crank (same ultegra one) on ebay for $15 more than what a helicoil would cost. Mine were already used, so I don't mind too much, but if yours are brand new you never plan to change the pedals again, it might be worth it to go the helicoil route.

cooker

I thought you could still swap the pedals after helicoil repair.

Kotts

Of course you can. In fact, a properly installed helicoil is actually stronger than the original threading. They are often used in high-stress aluminum applications in aerospace, not as a repair, but from the manufacturer as a stronger way of putting threads in light alloys.

Jerseysbest

You can, and I guess saying I wouldn't is a bit extreme cause pedals aren't just gonna fall out if a helicoil failed for some reason, but I've never been 100% comfortable with helicoil repairs if the bolt is going to be threaded and unthread more than once. And for the price, unless you have really high end cranks, I'd rather just buy some used ones.

cooker

Well, a bike store I won't mention, because they say they will make it right, screwed up my Helicoil repair. I had thought Helicoil inserts were still coiled wired springs, but in fact they are now cylinders with threading on the inside to match the pedal and on the outside to match the hole one must drill and tap in the crank to accomodate them. The bike store drilled out my stripped crank to the new diameter, tapped the thread for the insert and began to insert it, and it jammed part way in. The helicoil's external thread did not match the tap. They couldn't screw it in farther and they couldn't extract it, so they filed it off. So I now have a pedal that screws into a crank that is only partway threaded. Farther in the hole is too big. They are ordering a new crank arm. In the meanwhile I will ride this. The helicoil material and the pedal stub are so much harder than the original crank it will probably work for the moment.

cooker

Update.

The partial helicoil insert has proven extremely satisfactory - I have ridden the bike for two seasons with no difficulty. Even though the hole in the crank arm is only threaded partway in, and then widens, the thread that is there has held the pedal securely even when I mash uphill with all 200+ lbs of my weight exerting torque on the insert through the 165mm crank arm. I'm told the pedal and insert are made of much harder metal than the crank arm, which must explain why I haven't had problems.

JTGraphics

Tools are only as good as the user! user error here. They should still replace it for you.
Those wire type Helicoil are very good also if inserted correctly. You can get both types inserts and coiled.
The coiled style are good for particular applications still.

cooker

They bungled it and didn't charge me, but it works, so I can't complain.

danceralamode

I just called my LBS to see if they'll do this repair, and they said that it would cost $35-50 and that it might not work, it's a last resort (they seemed really negative about it). My bike mechanic friends all say it's really simple. $35-50 seems a bit expensive. Solutions anyone? Should I trust my nice bike to an auto shop and get a quote from them? Please help, need to get riding again quick, I have a ride tomorrow!

JTGraphics

Well you can look around to see if you can get it done for less but they are making a money on it for sure, but to be fair the Insert kit cost them about $130 good for about 5 jobs.
Is the thread material gone or is just cross threaded? if its just cross threaded you can get a pedel tap set for about $35 and you can chase the threads youself and end up with the taps in you tool box.

danceralamode

The thread material is gone. Looks like I didn't get it properly threaded in originally, and when it came off mid-ride, it stripped the threads--chipping them off. Um, stupid question, what do you mean by cross-threaded?

JTGraphics

This is when you try inserting the pedal and it starts but gets stuck and the beginning first few threads get chewed up and the pedal wants to go in crooked and gets tight.
Not your case since your threads are gone insert is needed in your case. Hope that helps....

cooker

Cross threaded would be if you screwed it in crooked and carved new threads that cross the other ones.

Having had a bungled helicoil repair that still worked, I am now a convert. I think as Kotts pointed out, the helicoil insert is actually stronger than the original crank. So you might actually end up with a better crank than if you replace it with a new one.

danceralamode

Great, I'm going to try 2 different LBS tomorrow and see if I can get it done for a reasonable price (I don't think $50 is reasonable, considering everyone--here and elsewhere--is telling me they've had it done for under $20).

danceralamode

Thanks for help! I'll let you know how it turns out.