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Old 03-30-09, 01:45 PM   #1
TallRider
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some pedal threads stripped out of crankarm: still safe?

I put some pedals on one of my bikes, and hand-threaded the drive-side pedal in a little bit, got distracted and forgot to tighten it the rest of the with a wrench. So, when I stood on the pedals to accelerate away from my first stop of the ride, the pedal ripped out of the crankarm.

The crankarm was at about 4 or 5 o'clock when I stomped on the pedal, and the threads that were on top (above the pdal axle) were the ones that got stripped. It's only the first two or three rows of threads closest to the pedal.

Now, I think it will be okay to continue to use the crank, but wanted to get feedback on this. The reason I think it will be okay is that the threads function to hold the pedal from pulling out horizontally (along the axis of the pedal axle) and most of the crankarm threads will still engage the pedal threads: 7 of the 9 or 10 threads are still intact, and the threads furthest from the pedal axle are probably slightly more important anyway. The place where 2 or 3 threads are stripped is only on one side, and the pedal threads will still seat against metal in the vertical plane at that point.

Interested to hear what you think. Here are pictures: full-on the stripped part, half-on the stripped part, and finally a photo of the threads on the bottom (with the crankarm at 4 or 5 o'clock position) that didn't get stripped.
Attached Images
File Type: jpg MG_7000--busted threads.jpg (74.2 KB, 10 views)
File Type: jpg MG_6998--mostly bad threads.jpg (67.1 KB, 8 views)
File Type: jpg MG_6999--mostly good threads.jpg (69.7 KB, 4 views)
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Old 03-30-09, 02:15 PM   #2
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You only need 3-4 turns of engagement to to maximum strength from a thread. Screw the pedal in all the way, then count the turns as the pedal is unscrewed.
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Old 03-30-09, 02:20 PM   #3
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there are 7 rows of intact threads, and I think when the pedal is screwed in all the way, it contacts 6 of them. so I should be good, based on the 3-4 turns criteria.
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Old 03-30-09, 03:16 PM   #4
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it looks like the threads are just mashed flat - can you chase them and clean them up a bit?
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Old 03-30-09, 03:32 PM   #5
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they were basically ripped out, gripped by the threads of the (steel) pedal axle when it pulled out. chasing wouldn't matter. the threads are missing and there's no getting them back, it's only a question of whether lack of those threads is going to be a problem.
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Old 03-30-09, 03:36 PM   #6
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Put the pedal back in and try it.
If it goes in and tightens you should be fine.
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Old 03-30-09, 03:43 PM   #7
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You should be fine if the pedal-axle reaches in far enough to grip 3-4 threads. You may need to thread a bolt through from the back side all the way to chase the threads as the 1st usable thread may not be perfectly formed.
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Old 03-30-09, 04:58 PM   #8
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Thanks for replying, Danno - I was going to send a PM to you and HillRider to ask your thoughts on this.
Chasing the threads from the backside is a good though, I'll get on that.
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Old 03-30-09, 06:38 PM   #9
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Originally Posted by timcupery View Post
Thanks for replying, Danno - I was going to send a PM to you and HillRider to ask your thoughts on this.
Chasing the threads from the backside is a good though, I'll get on that.
I agree with Danno and DaveSSS's assesment that 3 or 4 thread engagement is sufficient.

However, you probably won't find a "bolt" to act as a thread reformer since pedals are an oddball 9/16-20tpi and that's a very uncommon size. Try threading the pedal itself in from the back side of the crank arm. It should reach the damaged threads and iron out as much of the damage as possible. Maybe find a pedal with a particularly long threaded section is your current ones won't reach all the way. Or, see if an LBS in your area has a set of pedal thread chasing taps and run the correct one through from the back side.

In any event, when you reinstall the pedal from the correct side be very sure the threads, or what's left of them, engage evenly.
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Old 03-30-09, 07:51 PM   #10
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^^^ Yes, that's probably the most important point in all this: when you do re-install the pedal, the hard part will be getting the pedal spindle to engage the good threads without cross-threading because the entry point will no longer be perfect. Take special care that the pedal spindle is perpendicular to the crank in all directions. It will take some judgment to decide whether balking is due to cross-threading (in which case stop torquing and re-orient) or just because the thread is damaged and offering more resistance (in which case torque just a little harder!)
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Old 03-31-09, 09:27 AM   #11
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Thanks for the advice all. I used an old pedal to chase the threads from the opposite side, but it went in quite easily so it appears that where the threads are damaged, metal was just ripped out, as opposed to squashing the threads which would create resistance to a pedal threading in. Pedal went fine from the normal outside of the crank, engages after a few turns, and then threads the rest of the way in. I did have to be careful to make sure it engaged straight (don't want cross-threading) and everything was okay. But I did have half of the threads (on one side of the opening) still there so they guided the pedal threads well enough.
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Old 03-31-09, 06:52 PM   #12
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Yay! Glad it worked out for you. Boo-boos like that are all too easy to do; they've happened to all of us "experienced" riders at one time or another.
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Old 03-31-09, 07:23 PM   #13
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I would want more than 3 threads ingaged. The photo looks as if you have 9 so go for it. Fasteners have a nut that is as long as the diameter of the bolt. That should be the number of thread engaged, in this case 11.
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Old 03-31-09, 08:03 PM   #14
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Except that the majority of stress on fasteners are in the first couple of threads...
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Old 03-31-09, 09:21 PM   #15
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If it ever does fail, some shop will have a heli-coil kit to fix it and save the crank.
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Old 04-01-09, 06:49 AM   #16
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Except that the majority of stress on fasteners are in the first couple of threads...
Danno, do you mean (in my case) that when the pedal is all the way threaded in, the majority of the stress is going to be on the first two or three intact threads? My mental "modeling" doesn't look like that. Rather, forces in the vertical plane are greatest toward the outside of the crankarm. Are those stressing the threads? I guess since this is all torque, any vertical-plane force would be rotational, which would translate to horizontal force against the threads...
Can you explain more?

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