Any experience out there? Doing a restomod on a Fuji S12S LTD. 650b so will have to check reach on those. I have too many bad memories of 1970's Dia Compes squealing and chattering

Those are nice brakes. Take the time to set them up properly and maybe get new pads, adjust toe in, and they should work great. Pay attention to your cable routing - replace if necessary. Whether they're long enough reach for 650b I don't know. Also maxing out the reach will reduce you mechanical advantage and introduce more flex into the system.

Here's the 650b conversion math.

700c diameter = 622mm
650b diameter = 584mm
diameter difference = 38mm
radius differece = 38/2 = 19mm

To have a chance for the brakes to work with either size, the brakes need to adjust over this range. The interwebs tell me that the Gran Compe 610 reach is adjustable between 47 to 61mm (14mm).

If the GC 610 brakes work on 700c wheels, they won't be able to work on 650b wheels. You can probably make this work with a real long reach brake. Tektro 559's are a great choice and commonly used on 650b conversions. For those that want to run wide tires with fenders, MAFAC RAID's can work. No guarantee that either brake will work on your frame, so take some measurements before you jump into this.

I will add that if the 610 doesn't reach, a Weinmann 750 or "Schwinn approved LC 2.7" (rebranded Weinmann 750, often found cheaper) will give you another 14mm beyond that. These are the poor man's MAFAC RAID.

If you are considering new 610s for a resto-mod, you can do better. I tried the new and very nice looking GranComp 610s (albeit with the supplied dia compe pads) and was unimpressed--went back to my BR650s. I have no experience with vintage 610s.
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The new dia compe 750s might do the trick and they would look "right" on an old Fuji; the reach is 62-78.

https://www.modernbike.com/product-2...xoClYAQAvD_BwE

Thanks, did not do math first. The look is nice but not imperitive. Tektro seems the best route to go. I have Tektro on my turd Fuji Gran Tourer for Cino with Eagle pads. Great braking power. No complaints there. Just want a decent frame with good stopping power.

I have the tektro 559’s on my 650b conversion and I am able to lock up the rear wheel. Though I’ve read some find them just adequate, in my experience they’re fantastic. This is with the stock pads set at just a hair above the bottom of the slot. I am using the tektro trp rrl levers, so I’m not sure if it’s the combination of tektro products that gives such positive results. If you’re not hung up on period correctness I’d give them serious consideration.

My Tektro 556 (predecessor to 559) with Campy Ergo levers work great on the 650B setup, especially with Kool Stop salmon pads. No discernible differences from the same bike with Dura-Ace 7402’s and KS salmons in 700C form.

FWIW, on the other bike with Gugificazione’d brazed-post centerpull in front and a Shimano dual pivot standard reach rear (both with KS salmons), also Campy Ergo levers, brake feel is also the same.

No braking complaints on the many steep hills around Seattle, other than the squeal that developed after years of use from the rear 7402 that no amount of toe adjustments, pad changes or rim cleaning could cure. Now using a Tektro R540+KS salmon in back with no squeal, until I go back to the 556’s when [MENTION=401497]RiddleOfSteel[/MENTION] is done evaluating with my 650B wheels.