randomgear

All, I have an old steel crankarm, drive side, with stripped pedal threads - it's a German made, two piece crank with a cottered left arm and on a cheap bike. I'm thinking of using JB Weld to permanently attach a new pedal. The threads on the pedal side are stripped, the frameside threads are OK. The JB Weld would provide additional area to support the pedal. The intention is to simply get this bike on the road cheaply, realizing that the crank/pedal will no longer be serviceable without tossing the crank pedal assembly.

I realize the correct solution is to replace the crank, but it just can't happen on the budget I have for this.

So what is your verdict; is this a reasonable fix?

trailangel

Don't know. How about Teflon pipe tape?

gsa103

If you can get the pedal threads to catch some it would probably work. If they're too stripped to thread the pedal in, it's unlikely that epoxy will hold much better. The main thing is making sure the pedal is flush with crank arm so that it is properly oriented.

HillRider

JB Weld, despite the name, is not the same as real welding and not that strong. If some of the threads have survived it might be adequate. Another possibility is a Helicoil but that would probably be more costly than you wish.

SquidPuppet

It might just be me, but JB Weld failed on everything I ever tried it on.

gearbasher

I used JB weld to repair a cracked oil pan on my car. I did it as a quick fix for the winter, I got another pan and planned to replace it when warmer weather came. That was 3 years ago and it's still holding.

Apologies for semi-hijacking the thread.

ltxi

Chase the threads or heli-coil if necessary. JB Weld is great stuff for the right applications(s) but I wouldn't trust it on a stressed, critical part.

ThermionicScott 

If there is some remnant of the thread in order to at least guide the pedal to where it should be, would it be possible to tack-weld it to the crank?

AlexCyclistRoch

Just hit it up with a TIG welder....It's already ruined, anyway.......

randomgear

So, I should have posted a pic first. I should know better...


As you can see, the threads on the pedal side are wallowed out, there are 3 or 4 threads at the frame side of the crank that are still usable. My plan is to fill it with JB Weld and thread the pedal into the crank and tighten as much as I possibly can. I'm hoping that the JB weld will fill in the wallowed portion and keep the pedal from further wallowing.

Helicoil, may not be enough steel around the hole after it is enlarged.

I don't think that my Bernzomatic propane torch can heat the crank enough for me to lay down a layer of silver filler to me to get rethreaded.

ThermionicScott 

Hmm, so it has had ample time to rust after the threads were ruined. I don't think there's a safe way to JB-Weld it, and I don't think brazed filler would be strong enough for pedal threads.

dedhed

Looks like a tack weld job to me.

Reynolds 

+1.

Rollfast

Find a new crank. Unless you are serious about keeping it stock for historical reasons, you can do much better now.

altenwrencher

JB Weld may not have the resistance to pressure that this situation demands. I used it, with success, to seal cracks in the engine block of a Toyota truck. That application involved slow changes in temperature, but not the stresses of a pedal against a crank.

The picture shows a few inner threads in the crank, into which the pedal may mate. The missing outer threads will allow the pedal to move up and down. That is the problem. You need some material in those outer threads that will support, not yield or crumble, under the pressure of the pedal.

You might wind some brass shim stock around the inner half of the threads of the pedal. That may be enough support for the pedal, relying on the inner threads of the crank to draw the pedal firmly against the crank arm. The shim stock will fill the region that is missing because of the void in the crank arm.

If this doesn't suffice, you have other more undoable options.

okane

I'd try a local co op or bike show or friend with a lot of parts or CL (our local one has usually has someone selling a junker bike for $10 that has usable parts or someone advertising they have a lot of parts)

texaspandj

Do Not use jb weld. It's just glue, its helpful for some things but definitely not for threads that will be stressed. If you can't afford another crank arm set check out the paying it forward thread. Or save a a dollar a month til you have enough money to purchase another. Either way, if you don't want to "get racked" then don't use jb weld for that type of problem.

bulldog1935

I would find a local Frankenbike, swap meet, urban bike repair shop, etc., and buy a used crankset. I had an old cyclotouriste left crank arm (ebay crankset with unknown history) fail on me on the way to a frankenbike meet, and pedaled home on a $5 crank arm.
There are a lot of good used parts out there just taking up space.
maybe this wasn't the best example, but it was still part of a good project (and the drive side is sound)

drlogik 

Tack weld it. JB Weld will most likely not work for long.

RubeRad

I've never dealt with a cottered crank, but is it possible to install the crank backwards, so your few good threads are outboard?

Booger1

Shim the pedal to fit tight...Then JB weld it in.....It will hold BUT you have to get the pedal so it doesn't rock much first.

Or clean the rust out,fill with JB and recut threads.....JB works good under compression.

3alarmer 

...find someone with a set of Unior pedal insert tools (combination of a reamer and a tap) and pay them to put in one of the inserts. Make sure you set in in there with red Loctite or one of the Loctite bearing beading products. Don't just ream it and put the insert in there dry or it will eventually loosen up.



https://www.amazon.com/Unior-Pedal-T.../dp/B001PTCKJO

If it's the RH threaded arm (drive side), you can use a spark plug helicoil (which is the correct size and threading), but nobody makes a LH helicoil for this any more. The steel crank arm is a little more time consuming to ream and thread for the insert, but I've done a couple for people who were trying to stay with the original cranks. But it will still probably end up costing you more than a used replacement crank arm.

JB weld is not the proper stuff for this......if you're going for something you can just squeeze in there to fill up the space and lock the pedal in place you're probably better off with a bearing bedding compound...but it wont work with all that rust on the bonding surface.

Dave Cutter

I'd guess the fastest, easiest, and cheapest, fix would be doing it right the 1st (and only) time. Replace the crank with another similar old crank. Find a junk supply, look through some apartment dumpsters, or find a bicycle co-op for cheap/free parts and help if needed.

And while you're at it put a little grease everywhere that uses grease, and oil in the places that use oil. A few short hours of cleaning, lubing, adjusting, and replacing damaged old parts with good old parts can make and old bike near new-like.

ArmChairRider

What Dave said.
Or at least don't get JB-Weld on the outsides that way you won't be whittling and filing it back off before you go to the nearest mechanic/body/paint shop to get it tack welded on. I'd see if there is one of those shops that are mixed in with a residential area. They usually do that sort of thing for free as a neighborly deal.

fietsbob

# 22 is showing the professional approach..

Add a Layer of Molten Brass and re thread the brass , the chrome will burn so dont breathe any in.