I finished installing the Yamaha blue spot 4-piston caliper, and I’m pretty happy with how it turned out:
Regular followers will recall that I originally put this front end together for the aborted Aermacchi project, and it consists of mismatched Suzuki components typical of my builds: ’83 GS750ES forks, Bandit 400 wheel, and RF600R brake disk. As my initial test-fit a few years ago showed, the Yamaha caliper is an appropriate choice and closely matches the diameter of the rotor, but it doesn’t come close to fitting the fork mounts in any plane.
So, the first step was to “dummy up” a temporary bracket that would allow me to accurately measure the relationship between the various mounting holes. I mounted a piece of 1/8″ alloy to the fork, and another to the caliper. Then, with the caliper properly positioned on the rotor (thanks to a couple of thin brass shims), I drilled holes into a bridge between the two sections to lock them together that way.
Because the caliper adapter bracket had to be positioned so far outboard from both the caliper mounting bosses and the brake rotor, I could see the possibility of the caliper deflecting under braking. I decided to over-engineer things and install an additional inboard bracket that would tie the lower caliper mount to the inside of the fork bosses, since there was room available. Mostly as a way to get past my own procrastination, I chose to design and order custom hardware from SendCutSend, rather than fabricate something from scratch myself. One of the nice things about SCS is that you don’t need to use CAD software. I upload 2D EPS files I create in Adobe Illustrator, and they work fine.
I added some fancy lightening/accent slots, just because I could.
The inner bracket is 1/4″ thick 7075 alloy, the outer one is 3/8″ thick 6061. I splurged and had the outer one blue anodized, hoping it would match the blue spots on the caliper (it turned out to be a bit off).
For the fork mounts and the lower caliper mount, I had to use countersunk M10 bolts that insert from the inboard side to avoid hitting the bobbins on the disk rotor. I found out after the fact that this would be slightly problematic. The holes in the Yamaha caliper have M10x1.25 thread. I found out after getting the brackets that you can’t get M10x1.25 countersunk bolts anywhere. Even McMaster-Carr doesn’t offer them. I briefly considered drilling out the lower caliper mounting hole and putting in an M10x1.5 helicoil. However, with the lower caliper mount now sandwiched between the two alloy plates, it didn’t really need to be threaded at all—as long as the hole isn’t crooked or sloppy.
I put the caliper in my drill press vice and used a headless, partially threaded M10x125 bolt as a guide to ensure that it was flat and properly centered. Then I replaced the bolt with a 10mm drill bit and removed the threads. I was pleased to see slight remnants of thread around the entire hole, which shows I got it right.
I also had to make spacers to center the caliper on the brake disk. Once it all went together, it looks like this:
Here’s a view through the wheel. You can see one of the countersunk bolt heads on the other side of the disc carrier.
I hooked up a spare braided brake line this morning between the caliper and the Grimeca master cylinder. I still need to get a bleeder bolt for the caliper before I can add fluid and see how it works. Unfortunately, this chassis needs to get shuffled to the storage unit for the summer to make room in our basement, so I won’t be taking this any further for at least a little while.