Bicycle Mechanics - Seatpost Bolt Broke
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05-11-11, 09:56 PM
The seatpost bolt (8x1.25) on my 88 Bertoni broke for the second time. The first one lasted twenty years, the second one I found at a hardware store lasted one year. Just riding along and snap! the saddle is hanging, and I'm riding home standing the whole way.
If I went to a higher grade bolt, say 12.8, would that help the issue? I couldn't find one at a bike store, but I could get a 12.8 at Mcmaster Carr.
Thanks for any comments you have,
It is definitely more difficult to break hardened bolts. I don't know what grade 12.8 is, are you in a country other than the US? If I couldn't get a seat post bolt, I would use a grade 8 bolt or better would be Stainless 18-8.
05-12-11, 08:58 AM
I'm here in CA. I should have said 12.9. I think it is the class of bolt in metric. I will check out the 18-8 stainless. Thanks
05-12-11, 09:24 AM
"18-8" stainless steel is Grade 304 stainless. It's very corrosion resistant but not strong and not heat-treatable.
I'm surprised you broke even a standard strength bolt as an 8 mm bolt is very strong even is made from plain carbon steel. How tight are you torquing it?
05-12-11, 09:33 AM
are you talking about the bolt that holds the seat to the post or clamps the post in the frame?
05-12-11, 10:01 AM
shouldn't take that much force, is there an excessive gap
between frame and seatpost?
05-12-11, 10:20 AM
I'm talking about the bolt that hold the saddle to the seatpost. I don't feel that I over tighten it. I did find a class 12.9 at McMaster Carr. It is rated at 170,000 psi compared to 100,000 psi for a standard bolt. I would have never thought it would break but, it's happen twice now. I'm sure the original bolt better than the one I replaced it with.
05-12-11, 10:30 AM
I had the same issue on my mountain bike years ago... snapped about 5 hardware store bolts before picking up something hardened (I really don't remember the grade, sorry... I was 16 at the time) and that solved. Still using it today, some 10ish years later.
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