What do do about stretched binder bolt lugs

Yep, if you have somewhere local, could be worth asking them. It's a lovely bike and you've owned it from the beginning, so well worth a bit of time to work out what's going on before getting the hacksaw out!
 
I'm also fairly sure those lugs were angled rather than parallel from the beginning,

They wouldn’t have been when the frame was built. They come as one piece which you braze (or weld) on and then cut the slot.

To me this would suggest the wrong size post was fitted by the manufacturer.
 
I'd also say that they wouldn't be angled in towards each other, my 531c steel roadie with similar lugs are parallel. If the correct size of post is fitted, when the bolt is tightened the gap should not close up very much, that it has is an indication, as has been said, that bolt has needed to be tightened too much to grip post. The less movement to lugs means bolt isn't put under a large bending strain, and consequently breaking. I'll change my mind again to now agree that the wrong size post has been fitted from new.
 
Back
Top