Canti brake bosses missing - what do I need?

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Dirt Disciple
I recently removed some TriAligns from my old Bontrager Race frame with the intention of fitting V brakes. Front is no issue, but the seat stays have only a shallow bracket with no boss, and just a single spring hole.

What do I need in order to be able to fit V brakes to this frame? Thank you. IMG_20210414_172839778_HDR.jpg IMG_20210414_172847981_HDR.jpg
 
I need more than that. I need a method of adjusting the spring tension of my V brakes. One centred spring hole is inadequate because there is no tension. I also need to know where to source bosses, and the size of those bosses.
 
The bosses are standardised as far as I know- you can go on ebay and pick up a pair, or ask on here. Pretty sure I've got some spares hanging about somewhere if you want them. As for the single hole- that's all you get! Tension can be adjusted on the brakes themselves if need be...(please someone correct me if I'm wrong)...
 
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If you took the brakes off, they were attached to bosses so perhaps the bosses came off with the brakes? Does happen.
 
I'm sure the diameter is slightly smaller than standard. I found some bosses in my spares, but they don't screw into the holes.

The brakes I'm trying to fit are Avid SDs. Unless I'm missing something really obvious, I cannot see how the spring can be adjusted.
 
If you took the brakes off, they were attached to bosses so perhaps the bosses came off with the brakes? Does happen.

Nope. TriAligns were fitted simply with bolts directly into the frame. Those brakes had been on the frame from new.
 
What were they pivoting on? I've had some brakes where the bosses came off with the brakes and were a pain to free. As far as I know, all brakes fit on bosses of some kind, tri-aligns I'm sure do.
 
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