DIa-Compe 750 nutted to recessed- help?

[Will Vautrain]

I bought this Dia-Compe 750 rear caliper that included a couple of aluminum spacers of different thicknesses and a recessed nut, as well as regular nut and washer for a vintage brake bridge.

Problem is, even with the two spacers stacked, I'm sure distance from the spacers to the recessed nut is too long. The only way I can think of to remedy that would be to cut the bolt to size with a hacksaw. Is that the idea, or am I missing something?

[jack.k...@gmail.com]

Hey Will,

Are you sure the caliper is intended as a rear? If you hacksaw the bolt short enough for recessed install will you have enough threads left for a secure mounting?

It sounds like you're wanting to use the recessed nut, which means the stack of spacers is between caliper and bridge?

-Jack

[Will Vautrain]

The box is definitely labeled "rear". I took a couple of photos. With the spacers on between the caliper and bridge, and the included recessed nut screwed all the way in, the bolt is obviously still too long. There are not enough threads unless I add more spacers. Another option would be to cut more threads with a die, but I don't have the tools or the expertise. Or I could just try to install this with the traditional nut somehow.

My inclination is to find a slightly longer spacer (looks like 14mm would be ideal, but unlikely), and cut down the bolt, so I can use the recessed nut with a single spacer. Thoughts?

[Brad]

The trick I have used is to  take a pointy serrated washer from high end side pull brake and then clamp on it with an allen nut.  One source of nice looking allen nuts are threaded stud kool stop pads.   There is another trick involving using a plastic sleeve in the recessed brake hole.  Riv sells them, but they are also available at your local hardware store for less.

[jack.k...@gmail.com]

Despite the "Rear" box label, I'm pretty sure that's a front. That bolt looks huge for the rear of even a chunky CF or aluminum frame.

-Jack

On Tuesday, February 5, 2019 at 8:04:12 PM UTC-5, Will Vautrain wrote:

[Will Vautrain]

Thanks, I’m trying to use the recessed brake bridge as it was intended and avoid kludging it, if I can.

[Will Vautrain]

After comparing it with the mounting bolt on a front Tektro 559, I think you're right on the money. Seems like with a regular serrated brake washer or two, it would work perfectly in a recessed front mounting. It's not long enough for a nutted front mounting, though.

I believe M6x20mm or M6x22mm bolts with a 4mm thick 10mm flat hex head would work (assuming 5-8mm of spacers, needed for some seat stay clearance anyway, and a 9-10mm length recessed nut). I'd just have to disassemble the brake to insert one. I found some nice stainless ones on eBay, but I have to buy a pack of 10 each size. I might try that out and report back.

[satanas]

That sure looks like a front to me. One way around this is to run an M6x1.0 tap further down the centre bolt, then cut it to a suitable length; we used to do this when recessed frames were new and matching brakes were rare.

Later,
Stephen

[Will Vautrain]

The irony is, I bought this one cheap from eBay and ordered an expensive recessed front version from Soma the same day. If I’d waited to order the Soma, I would have purchased their recessed rear version instead, and I would have been all set.

Yeah, if I had a die, I’d run it down the bolt and cut it to fit. As it is, I think finding a replacement M6x1 20mm will be cheaper and probably easier.

[satanas]

^ Of course, I mean to say die, not tap. :-(

The heads on the DC centrepull brake bolts aren't exactly normal, so finding something that won't spin around might not be so simple. M6 dies shouldn't be expensive, or hard to come by, although I guess things might be different in Trumpville; has the Metric system been banned over there yet?

Later,
Stephen

[Will Vautrain]

Haha, I think it’s possible but I probably need more tools as well. I’m not a great DIY metalworker and don’t even have a vise. I do have a small hacksaw though.

I measured the caliper last night and an M6 bolt with a 10mm wide, 4mm thick hexagonal head should work fine and not spin around. The original bolt is a little odd but has flat sides 10mm apart that sit against the plate on the caliper.

[Will Vautrain]

I also meant to say that an M6 with a 10mm hexagonal head 4mm thick seems to be a commonly available bolt. I found a bunch in different lengths in stainless steel on eBay, but I’ll try a local hardware store first.

[Will Vautrain]

So I bought the bolts from the hardware store, and I'm pretty sure they'll do the trick. I bought one 20mm, one 25mm (depending on how many spacers I want to use between the caliper and the bridge), and one 40mm (in case I want to swap out the OEM bolt on the front caliper as well). 

Any ideas on the easiest way to swap out the center mount bolt on the 750? I'd rather not remove a spring and have the whole caliper fall apart, if I can avoid it. The bolt is loose in the hole, so if I can just clear the arms out of the way, it will fall out.

[satanas]

I haven't set eyes on a DC 750 (or 610) caliper for decades so my memory may be faulty, but can't you just squeeze the pads together to make room to pop the centre bolt out?

[Will Vautrain]

You're right. I did have to remove one of the pads, but then I got the arms close enough together to let the bolt slip out.

Unfortunately, I took a lot of measurements last night, and my memory was faulty. The original bolt head is 15mm wide, not 10mm wide. I inserted the 20mm long bolt and mounted it with a 5mm spacer and a 2mm serrated brake washer, and torqued it down perfectly using the recessed nut. I stopped the bolt from spinning by wedging a 2 or 2.5mm allen key on one side, between the 10mm bolt head and the recess in the plate. 

I think it's actually fine like that, but my friend who started building frames in the past couple of years told me tonight that he just got a die set, so I may have him cut new threads and cut the length of the original bolt to make it perfect.

[Will Vautrain]

I actually confirmed that the bolt is intended for a rear mounting. If you look at the listing for these brakes on the Rivendell site, it clearly states the rear bolt is 45mm long, which is how long this one is. The difference is that my kit also included a recessed mounting nut (in addition to the regular nylon locking nut), but clearly, give the length of the bolt, the addition of the recessed nut can only be seen as a joke. Or, perhaps it's meant for the serious DIY'er who would just automatically know cut down the bolt and cut more threads with a die. 

On Tuesday, February 5, 2019 at 8:22:52 PM UTC-6, jack.k...@gmail.com wrote:

[wg...@yahoo.com]

Will, if you get a chance, could you post a picture with the new bolt in?  I have a DC 750 that I may want to do this with.

Thanks!

geoff s

arlington, ma

[Will Vautrain]

Sure, here you go, and a photo of how I wedged the allen key between the head of the bolt and the recess in the plate to keep the bolt from spinning while I tightened it. I did have to wiggle it free once everything was tight. That's an M6x1 20mm long stainless bolt, a 5mm aluminum spacer (I thought about using a stainless nut or washers) and a standard brake washer, and a regular/short recessed brake nut. I got a 40mm long one for the front just in case I want to do the same thing there. Haven't tried fitting the front caliper yet.

The bike is my 2012-ish Soma ES. I'm putting it back on the road with a Whisky No 7 Road+ fork for increased clearance up front, I should be able to run 42mm tires front and back that way. The Dia Compe 750 was really my only viable option, the Tektro 559 I had on before were only long enough in back until brake pads started to wear down. Not ideal. Maybe the Dia Compe BRS 202 would work better (75mm reach).

[wg...@yahoo.com]

Thanks Will, nifty trick with the allen key!

geoff

[Will Vautrain]

Interesting arrangement with the front recessed brake from Soma. The internally threaded nut is anchored to the backplate, like the bolt is on the regular version, and the bolt screws in with the recessed allen key head. Main disadvantage here is that it could not be exchanged with a Sheldon nut, or even a standard recessed nut.

On Tuesday, February 5, 2019 at 2:46:42 PM UTC-6, Will Vautrain wrote:

[Mark Bulgier]

Doesn't that require the front of the crown/bridge to be drilled out to 8 mm?  I don't like that idea, because then if you switch back to a brake with a 6 mm bolt, it will be swimming in that 8 mm hole, not held on-center.  Or am I misunderstanding how this works?

Mark Bulgier

Seattle

[Will Vautrain]

It just arrived, and I haven't taken any measurements, but you might be right. There are a bunch of spacers in the kit, perhaps they provide a way to install this on a fork without drilling it out. 

[Will Vautrain]

It looks like the fork doesn't need to be drilled out, but you need to use 7 or 8mm of the included 8mm ID spacers, which I'm not sure I want to do.

On Thursday, February 7, 2019 at 3:18:38 PM UTC-6, Mark Bulgier wrote: