Topic: CB750K Dual Disk Conversion

[SuperDave]

I just finished converting my CB750F/K to dual front brakes and would like to share what I've learned. I know I'm not the first to do this and write up a how to, but maybe my method is different enough to supplement other posts with a few new ideas. I'll do this in multiple post over the next few days with pictures added later. I'll be editing the write-up as I think of new info. Please hold any questions until the last post with the results. Also, I haven't figured out how to imbed the photos so check the attachments! Thanks!

First off I should detail what bike and components I did this to. Some readers will have a different experience with this project if their components are a different model. I'm not a CB750 history guru so I don't know every change they've made over the years and thus I'm unsure of what exact year some of my parts are but I have a pretty good idea.

1978 CB750F bike with original F3 upper and lower fork clamps, and brake master cylinder.
1972-1976? fork sliders over Cycle-X fork tubes. I forgot the year of the sliders.
1972-1976? original CB750K caliper & holder assembly on the left side (these came on the fork sliders)
Same style oem caliper holder on the right side but with repro caliper assembly (CA-02) and adjustment screw (BCSS-01) from 4into1.com
2 original 296mm CB750K disks.
CB550(?) hub laced to oem CB750 rim with Buchanans spokes (This hub came with a 276mm Disc so I thought it was a smaller cc CB. I think the CB750 hub is the same.)
Some CB style fender I found at the salvage yard. I assumed this was a late 70's CB750K fender.

GATHER PARTS AND PLAN
I made sure I had all the parts on hand before I started. Just look at your left side brake assembly and copy everything over. You'll need to figure out your own way of running the second brake line. I had a dual banjo bolt and 35 degree banjos for the master cylinder from when I still had the dual disk F3 front end. The right side caliper will require a longer brake line since the pressure port (and bleed port) will be flipped and facing forward. More on that later. You'll need bolts for mounting the caliper holder. I bought the bolt sizes and lengths listed in the Honda parts manuals. Longer brake disk to wheel bolts will be needed. More on those below. I recommend trying the 4into1.com caliper kit but there were some issues with it I'll detail later, This could have been because of this "unique" application rather than the quality. The quality of that caliper looked excellent. 

REMOVE THE WHEEL
First thing to do is to get the wheel off and remove the speedo gear, disk bolts, disk, the "cover, gear box retainer " and the "gear box retainer". I don't like that term so I'm going to call the thing that is keyed to the wheel and turns the speedo gear the "speedo gear driver" or just "speedo driver"(or just "driver" because I got lazy later on). The retainer will go in the spare parts box never to be used again. There is also supposed to be and oring under the speedo driver but mine didn't have it(oops). That is useless now as well.

THE CONVENIENT FEATURES
Now, right in front of you on the right side of the hub is one of the great mysteries of the CB750 (at least to me). A perfectly machined hub shoulder that a second disk will fit to with flats for some reason hmm. Another mystery is the pre-tapped bolt holes in the right side front slider. Those hole features have bugged me for years. It's as if Honda wanted us to do this modification! Go ahead and drop your second disk on there and have a look. You know you want to. Does it look cool? If you want it to stay there, you'll need to get longer bolts.

Tip: Now is the time to copy you fancy disk hole pattern to the new disk(should you have holes). I just bolted the two disks together and used the original as a drill guide for the second. A cobalt drill and a lot of WD-40 will get the job done.

LONGER BOLTS
The parts manuals say the K0 oem bolts should be 8x106mm and the K1-K5 should be 8x102mm. I don't know about the other models or years. I measured my oem bolts at 4.004"(101.7mm). The disk base/flange thickness measured 0.396"(10.05mm). So when I added that to the factory bolt length I got 4.400"(or 111.76mm). This would be the ideal bolt length but with metric the closest is a 110mm or a 120mm. If the threaded end of the bolt is assembled facing the left side of the wheel (which you MUST do), then there is plenty of clearance for a few mm extra length but 120mm is just too long im my opinion. So, get [6] M8-1.25mmx110mm hex head bolts. MY hardware store only had 5 so I got one 120mm and cut it to 110mm. Also, do NOT get flange bolts, the head is too tall. You'll see how little clearance there is between these bolt heads and the speedo gear later. I noticed that 5/16-18x4.5" bolts are also the right diameter and close to the right length but I wanted to reuse my M8 nuts and not enrage the next person who tries to remove the disks with metric tools.

To be continued...

[SuperDave]

THE "SPEEDO DRIVER"
Now that there is a disk where the speedo driver and retainer used to be, the driver will need to be modified to fit. I felt that the driver should be held on center by the hub or disk and not rely on the speedo gear seal to remain centered. Since the brake disk flange fits slightly higher than the hub, the ID of the disk flange can be used to center the driver. I measured the ID of the disk flange at about 2.358" or so. I also noticed that the speedo driver back side should fit nearly flush with the bearing behind it.

My solution was to chuck the driver in the lathe and cut off the cup shaped portion, then turn the outside to a few thousandths under what I measured the disk ID. Then I cleaned up the back side so that it would lay flat on the hub. My final driver OD was 2.356" and it dropped inside the disk perfectly. I placed the driver in the speedo gear to see how far out the speedo gear bushing/spacer stuck out to make sure there will be clearance.

The next step if to figure out how to keep the driver spinning with the wheel. Honda left some convenient flats on he hub. Hmm I wonder what those are for? Well the most obvious solution I saw was to bend to edges of the driver down into the flats. This could have been that simple with the right tools but I was worried I wouldn't be very successful bending such a wide and short bend so I decided to cut out small tabs to bend. My biggest mistake was to grind the sides of the tabs shorter before I bent them. My first attempt left the tabs too short and when assembling and testing I noticed the tabs sat on the chamfer of the hub flats and not the flats themselves. This caused my driver to push away from the hub and rub against the speedo gear. I ended up beating the tabs with a hammer to stretch them out so they reached the flats of the hub.

I feel I did a terrible job relative to my craftsmanship skill but in the end the speedo driver and the speedo gear fit and functioned perfectly! This is when I noticed I got a little lucky with the short head bolts. I almost bought the flange head bolts. The regular hex ones just barely clear the speedo gear!

Afterward I went back to my model and drew up the proper way to do this. Bend the edges of the machined driver so that it just fits the width of the hub flats, then just grind the tips of them if they are touching the bottom of the flats and preventing the driver from sitting flat on the wheel bearing surface. See the attached sketch.

Now to fit the wheel and give the wheel a good spin. I looked to make sure the driver wasn't rubbing the speedo gear anymore. All was good. On to the caliper...

[SuperDave]

CALIPER ALIGNMENT
I knew the caliper wasn't going to mount perfectly without some shimming. So the first thing I did was take some measurements to see how offset the new disk and caliper will be relative to the oem disk setup. I measured the distance between the fork and the disk on both sides using a scale and precision eyeballing. The oem side (left side) measured about 5/32". The new disk side (right)measured about 1/4". This is a difference of 3/32". So the new disk sits 3/32" farther inward than the oem disk does. This is mostly due to the speedo side machined surface on the hub where the new disk is mounted. I determined the distance from this surface to the spoke flange differed from the opposite side by about 3.5mm or 9/64". The bushings on the axle positioning the hub may play a part as well.

The next obvious difference is the caliper holder. It is designed with an offset between the center of the caliper mount surface and the axis hole. I took a quick measurement off the outer radius of the axis area vs the caliper mount surfaces. This isn't as ideal as measuring to the pin hole itself but I knew any numbers I came up with didn't need to be critical. I found a difference of about 5/32". What this means is the oem setup has the caliper holder offset inwards 5/32" relative to the mount axis hole. When this same holder style is mounted to the right side of the wheel for the new disk, it will offset the caliper outward 5/32". Well crap! That makes the offset-inward disk situation worse! So combine those offsets and that is a 1/4". So the caliper is going to be offset 1/4" outward relative to the new disk!

TO SHIM OR NOT TO SHIM?
I knew that this cb750 style caliper and pad had a bit of alignment compensation designed into it. The design allows the pads to remain flat on the disk as they wear while the caliper holder pivots outward to compensate. Still, I was dreading 1/4" shims under my caliper holder mounts. It would mean I'd need longer mount bolts and they would carry more bending forces. It would mean that the caliper adjusting screw would likely not engage enough threads in the fork and possibly pull out. So this is why I'll be avoiding as much shimming as I can.

I knew that the fender mount flange would be happier if it remained against the fork rather than being bent inward and bolted to the inside of the caliper holder mount like the oem setup. I measured the thickness of the fender at 0.088". It was to be subtracted from my required shim size on the upper mount. I found some tire stem nuts at about 0.160" thickness for the upper mount and a 1/4" shim for the lower. But first, I decided to just bolt the caliper holder up directly against the fork and see how far off it would be as is and see how little shimming I could get away with.

[SuperDave]

GETTING IT STRAIGHT
I noticed when the lower mount bolt was tight, the upper mount didn't want to lay flat on the upper hole of the fork. Tightening all three bolts would bend the holder pivot shaft! I loosened the lower bolt and just finger tightened the upper an lower bolts so the mounts were as flush with the fork holes as possible. Looking from the front view of the bike I could see that the caliper and the disk were not parallel relative to a vertical line. The upper part of the caliper was tilting outward. I suspect the upper caliper holder bolt holes in the fork may have been machined shorter to accommodate the rubber bushings and spacers in the fender. I couldn't see much a difference between the hole height between the left and right forks but I guess there possibly is a difference. It could also be something about the caliper holder and mounts themselves.

I then loosened the upper mount bolts and tightened the lower bolt. Now the caliper was parallel with the disk from the front view. So maybe I just needed to shim the upper mounts? I measured the gap between the upper mount and the forks at about 0.090". Wow, how convenient it would be if the only shim I needed was the 0.088" fender! From the top view I could see how the caliper holder was rotated inward quite a bit since I didn't have any shims. A little is ok but it was so far inward that the caliper was hitting the spokes. The brake pads were also jamming and preventing wheel rotation as well.

CALIPER/PAD ISSUES
I played with moving the mount end of the holder inward a bit as if adding shims but that only helped a little. It was then that I started suspecting something was off with the caliper/pad assembly itself. Upon inspection, I noticed that the inner pad sat far out of the caliper and the cotter pin that retains the pad was pulling it back too far and it was loosing it's ability to tilt and compensate for misalignment. I could have drilled the hole for the pin bigger at the risk of breaking through the rod but since I have a lathe I just chucked it up and re-machined the back round portion of the pad. I left a bit of an angled surface so it can still tilt. Maybe this would not have been necessary had I shimmed the mount enough? That seemed to help with the jamming but the caliper was still way too close to the spokes for comfort. I noticed the pad (which came with the caliper) was thicker than my other pads so I said "F- it" and "pre-wore" the pad on the belt sander. This again may or may not have been necessary had I had some large shims. That seemed to do the trick and gave me enough to clear the spokes. Hopefully the next time I replace the pads I won't need to do this because of the final shimming I did later!

[SuperDave]

THE FENDER
Now to try the fender as the only shim. The rubber bushings and spacers in the fender had to be removed. The fend holes left are huge. I wasn't sure if that would cause the fender to move around so I pressed in some washers I had to reduce the hole size. I don't think this was necessary now and it actually made installation harder. I also had to press the fender flange flat so that the caliper mount will lay flat on it. This causes the fender holes to spread apart slightly and with the reduced holes, less room for error.

I installed the fender with its flange as the only shim. I installed the speedo cable and zip-tied it so it wouldn't touch the disk. I installed the caliper adjustment screw with the nut on the inside of the fork. This is necessary because the caliper is so much further in. Setup like this, the spring is compressed just as much as the oem one and it works fine. With only the fender as a shim, the adjustment screw just reached the far end of the threads in the fork. I finished assembly of the second brake line with the double banjo bolt at the master cylinder with two 35 degree fittings and bled the brakes. My right side brake line was a bit short since I originally planned to connect it to the rearward facing caliper hole. I just zip-tied it to the fender until I get a longer one.

FIRST TEST RIDE AND FINAL ADJUSTMENT
I took the bike out for a quick test ride and everything felt great until I got back to the garage. I noticed when I rolled backward the brakes were causing noticeable drag. I attempted to re-adjust the caliper adjustment screw and noticed no matter how loose the screw got I could not feel play in the caliper when I wiggled it. This means it was still jamming the pad at the end of it's angular compensation limit. Fine, I'll give in and use shims! I grabbed three washers, one 8mm washer and two 6mm. I didn't measure what thickness they were but they were typical sized washers at around 0.050"-0.070". I placed these under the caliper holder mounts and that did the trick. Now there was no more dragging, and the adjustment screw works as it should!

EAR TO EAR GRINS
I took it for a ride and it stops so fast I feel like I could bend those wimpy forks! I feel I could possibly get the rear wheel off the ground! It stops like a modern bike now!

AFTERTHOUGHTS
If I were to do this again I would make these modifications to the procedure:
*Bend the speedo driver without cutting smaller tabs and with proper bending tools, just like in the drawing.
*Start with standard washers and the fender as shims under the upper caliper holder mount, and one washer as a shim under the bottom mount.
*Do not reduce the fender hole size.
*Only flatten fender if required after shims are added.

I will add info as I remember more details. I hope this helps someone. Thanks for reading and good luck!

[newday777]

Nice. Thanks 
I've saved this as I'm getting ready to do the same.
I had dual drilled disc's on my K6 back in the 70s and yes they made a huge difference in stopping.

[Don R]

   Your experience was similar to mine, I did a 76F and used two of the center outlet calipers. I drilled the rotors back to back but think it's an incomplete pattern since it leaves stripes in the undrilled swept area. I also cut down a stock speedo drive plate and bent the flanges.
   I tried to order double Speigler lines but my local shop just agreed with what they asked over the phone and they ended up too short and still used the steel tubes. I was able to order new hardlines and re-bend one to fit like original.
  I scalloped the old front tire that was on it practicing emergency stops. It was great.

  Update: I just did another dual disc setup, I used 76F forks, a cut down late fender that came on the bike and two center feed calipers that I stripped and polished a bit. I had a rotor with two tab notches for the speedo drive. I did mine like above except my 750A hub had no flats on it. I welded two tabs on the drive that fit flat into the rotor notches.