Modern front brake caliper.

[vmx51]

Has anyone fitted a modern caliper , but kept the stock RD250/350 disc. Any brackets out there ?

[Dvsrd]

Has anyone fitted a modern caliper , but kept the stock RD250/350 disc. Any brackets out there ?

Not quite. But I went from the OEM (Euro model) dual disc setup on my 77 XS 650, to a single 320 mm floating ISR disc and a 4 pot Brembo caliper/ 13 mm Brembo MC. I made the caliper bracket myself, from 12mm 6082T6.  This setup gave a massive weight reduction, massive brake and suspension improvement.
Now, regarding using a modern caliper with a stock RD disc really doesn't exploit the advantages of a 4 pot caliper. The swept area is way too wide for a modern caliper, and the RD disc is 7 mm thick. These factors make it  heavy for its size. A 300-320 mm floating disc, made for 4 pot calipers, will be a lot lighter, and give better brakes as well.

[pdxjim]

Blue/gold spot caliper off the R6/R1 is the most popular choice.

Garrett GRA2STROKES on here makes a bracket to fit to the RD350/400 forks using an FZR/Ducati disc.

No sure if he has any in stock, but it's worth an ask.

[Dvsrd]

Deleted. Dual post. Sorry.

[Dvsrd]

Blue/gold spot caliper off the R6/R1 is the most popular choice.

Garrett GRA2STROKES on here makes a bracket to fit to the RD350/400 forks using an FZR/Ducati disc.

No sure if he has any in stock, but it's worth an ask.

At least IMHO, both the "40 mm" and "65 mm" 4 pot Brembo calipers look better and have a more "bracket friendly" mounting lug location. If using FZR or R6 forks, eliminating the need for adapter brackets, it is a different story.
Also, be aware that the very common Ducati discs with round holes in the (steel  !  ) carrier are quite a bit heavier than the alloy hub/carrier versions. On a lightweight RD, keeping unsprung weight low is sort of important.......

[rodneya]

Would something like a blue dot caliper even fit over the thick stock disk?

[oxford]

A stock disc can be thinned down to 5mm.

[rodneya]

A stock disc can be thinned down to 5mm.

If its been drilled, then you are hooped.

[pdxjim]

Would something like a blue dot caliper even fit over the thick stock disk?

I have them on my LC with stock 5mm discs.

Not much more room than that.

[Kawtriplefreak]

We used an R6 disc

[m in sc]

you can us e a1 pc daytona or xs disc, notably thinner.

[vmx51]

I want to keep the RD disc as I already had it thinned and drilled by joe spooner.

[Dvsrd]

 

I want to keep the RD disc as I already had it thinned and drilled by joe spooner.

As the stock RD caliper is actually quite good (has larger pistons than most), you will get a significant improvement from a smaller master cylinder. Between 12 and 14 mm bore, depending on personal taste. Recently, someone recommended the 14 mm MC from an SR400. I use a 13 mm Brembo with the stock type caliper, and really like it. A buddy had a 1/2" Grimeca on his RD, and it was good too. With a good, braided line, careful bleeding and good pads, you'll squeal the front tyre with 2 fingers. For a lighter setup, EC have reproduction RD calipers in aluminum alloy. These will actually be lighter than a R1/R6 blue spot caliper, plus bracket and extra bolts.
Also, at least IMHO, mounting a 4 pot caliper with a bracket, in the stock location, just looks weird ( sorry, Kawatriplefreak  ) If anything, it will increase the risk of tank slappers, as it lowers the resonant frequency of the fork by moving the caliper away from the steering axis. This can be mitigated by swapping the fork legs, having the brake on the LH side.
Again, using a modern 4 pot caliper on an old school disc, like an RD, makes little sense. Certain calipers are designed to work with a certain type of discs.
If someone, really, really wants to use a 4 piston caliper on stock size disc, the disc could be drilled out more where the pads do not touch. Or use a disc hub from a 73-76 XS650, and machine the RD rotor to fit. The XS disc hub looks just like an RD hub, only 30 or 31 mm larger diameter. (The XS disc is 298 mm)

[Dvsrd]

For reference, post#22 in this thread has a pic of my XS front end:
http://www.xs650.com/threads/77-xs650-dual-disc-upgrade.54521/page-2#post-590792

[oxford]

If someone, really, really wants to use a 4 piston caliper on stock size disc, the disc could be drilled out more where the pads do not touch.

I think on a rd there are far better (easier) options but this would be possible for a stock rotor.  These are pictures of a stock Kawi triple rotor and one that has been modified.  The same could be done to a stock rd rotor, thin it down to 5mm, cut the sections out where the pads don't contact and drill it.

Stock rotor



Modified rotors.