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Tried to fix a stripped pedal thread with a helicoil kit vs. just tapping it out one size bigger | Bicycle Mechanics - Librear

corab77•5mo ago

Tried to fix a stripped pedal thread with a helicoil kit vs. just tapping it out one size bigger

Had a vintage Peugeot come in last month with the left pedal crank threads totally gone... the owner wanted it saved. First, I spent like an hour carefully installing a helicoil kit, following the instructions to the letter. It held for about five minutes of test riding before it just spun out again. Then my buddy at the shop said to just tap it out to a 9/16" and use a slightly bigger pedal. Took ten minutes, cost the customer nothing extra, and it's been solid for weeks now. Anyone else have a 'fancy fix' that just totally failed compared to the simple old way?

3 comments

3 Comments

max2735mo ago

That "fancy fix" failing is a perfect example of over-engineering. A helicoil needs good material around it to bite into, and a vintage crank arm is often worn thin or soft. Tapping one size up uses the full, untouched thickness of the metal wall. It's not just simpler, it's actually stronger in this case because you're engaging more original material. The fancy repair assumed the base metal was still good, which it wasn't.

hugo6455mo ago

Yeah, max273 has a good point about the metal being thin, but it's not always about the thickness. Sometimes that old steel is just work-hardened and brittle from years of stress. A helicoil needs the threads it cuts into to hold their shape, and if the base metal is too cracked or fatigued, it'll just let go again. Tapping one size bigger often works because you're cutting completely new threads into a deeper, untouched part of the hole that hasn't been stressed to failure yet. The simple fix wins by avoiding the bad material altogether.

beth5594mo ago

Man, that's a great point about the base metal. So when you tap it one size bigger, are you basically just chasing the old, damaged threads out of the way to get to the fresh stuff underneath? Or do you have to drill the hole completely oversize first to make all new threads from scratch? I'm trying to picture the actual metal you're cutting into.