Topic: Twin disc conversion

[38rudge]

I am looking at doing a twin front disc conversion to my 72 CB750K2.

What modifications have to be made to the std LH caliper holder brk, part that bolts to the fork leg, in order for it to fit to the RH fork leg so that it will align correctly with the RH mounted disc?

I understand that that the early, up to K2, front hubs were not symmetrical and that the RH disc when mounted is some 3mm closer to the spokes.

Thanks

[low-side]

None.  Remove the caliper and reinstall in the correct orientation.  Use spacers to get the caliper bracket where it needs to be in relation to the rotor.  There's some pretty extensive threads on here about mods required for the speedo drive, facing the calipers backwards, etc. that you may want to review as well.

[PeWe]

Brake line connection and bleeder to swap places.
I use 5mm spacers for the 3 bolts holding bracket to fork. The M8 need 7mm, I use 5mm + a washer (same thickness as fender bracket)

Different order to mount vs left side to fork.
1. Fender bracket
2. Caliper bracket holder.
Left side opposite.

There are several threads describing dual brakes with CB750 stock calipers.

Wheel must go on before right brake caliper. Opposite when taking off the wheel when changing tire.
Abdy Cepok eBay.de sell a kit with longer bolts and speedo gear adapter.

Yamiya sell caliper and bracket prepared for right mount.
You beed a longer banjo bolt where lower lines connect as stock does.

[MCRider]

You will need to cut down, or buy a cut down speedo drive. There are threads here about that and someone who used to make them.

[38rudge]

Thanks for the replies.

Have done a site search but it has not come up with much.

I am aware of the speedo drive mods required but it's what is required to be able to fit the std LH caliper brk to the right fork leg that I am trying to find specific detail about.

I know that Yamiya do a RH caliper brk kit but here in the UK it works out at around £250/$325 when postage and impots cost are added so looking to see if it is a relatively easy mod to be able to fit the LH brk to the RH fork leg to give the correct alignment of the caliper and disc.

[MCRider]

Thanks for the replies.

Have done a site search but it has not come up with much.

I am aware of the speedo drive mods required but it's what is required to be able to fit the std LH caliper brk to the right fork leg that I am trying to find specific detail about.

I know that Yamiya do a RH caliper brk kit but here in the UK it works out at around £250/$325 when postage and impots cost are added so looking to see if it is a relatively easy mod to be able to fit the LH brk to the RH fork leg to give the correct alignment of the caliper and disc.

I agree with low-side that you don't need anymore than another serving of the original brackets/hangers. Flip everything around. 1 of the 3 bolts that fastens to the leg, is proper as is. Then use spacers, I used 8 mm washers to get it close, then a thinner one to finish off. That's the poor mans twin disk kit.

You can buy a billet bracket but that's overkill $$.

[Don R]

 Early K2 may have the K0/K1 style caliper bracket. It's a little more complicated to mount, I haven't done one myself. The later K3-up style just flip around.

[PeWe]

I use std speedo gear no machining needed.

 I use this that is fastened by 2 small screws in the hub.
Here a set with longer bolts and nuts.
The earlier had not flanged nuts, std nuts with plates locking them, so this kit fits early K1- K2 too. I think early K0 has different width, different bolt lenghts?

https://www.ebay.de/itm/292307163031

I use the aftermarket MC from DSS UK that has the best price on that one. Their brake disc too, 2 of them on my K6.
My K2 got one a few weeks ago since it had the old Honda disc from my K6 that really sucks.

Too hard stainless steel that did not work in higher speeds.

[bryanj]

Thats an expensive kit when it takes less than 30 mins to modify the drive

[38rudge]

Early K2 may have the K0/K1 style caliper bracket. It's a little more complicated to mount, I haven't done one myself. The later K3-up style just flip around.

Yes I have the early K0/K1 mounting brk on my K2. So as i understand it not just a case of flipping over the mounting brk?

[Rookster]

Here is how I did mine on my K1. I filed the lower mounting boss on the caliper bracket so it would sit closer to the fork leg.  The upper mounting bosses need to be shimmed.  In my case 1/2 spacers worked perfectly.  Here is a few pictures of the bracket so you can see what needs to be done.





You will also need to remove the brake line fitting from the caliper and move it to the bleed hole.  You can see it in the first picture laying next to the caliper.

Next you have to either modify the 2nd disk or cut down a Speedo gearbox retainer.  I modified my 2nd disk so the retainer fit inside it.





The last step is the brake line.  I modified a stock lower hard line by bending it the opposite way with a brake line bender.





Everything is a straight forward swap with just a few modifications. Once you get the extra caliper bracket and position it inside the right fork leg you will see what has to be done.

Scott

[38rudge]

Here is how I did mine on my K1. I filed the lower mounting boss on the caliper bracket so it would sit closer to the fork leg.  The upper mounting bosses need to be shimmed.  In my case 1/2 spacers worked perfectly.  Here is a few pictures of the bracket so you can see what needs to be done.





You will also need to remove the brake line fitting from the caliper and move it to the bleed hole.  You can see it in the first picture laying next to the caliper.

Next you have to either modify the 2nd disk or cut down a Speedo gearbox retainer.  I modified my 2nd disk so the retainer fit inside it.





The last step is the brake line.  I modified a stock lower hard line by bending it the opposite way with a brake line bender.





Everything is a straight forward swap with just a few modifications. Once you get the extra caliper bracket and position it inside the right fork leg you will see what has to be done.

Scott

Hi Scott

Thank you for your response and photos which are extremely helpful.

1. Looking at your photos and description it looks like you have had to remove some metal from the inside of the bottom mounting boss? I am assuming that you had a std RH CB750K fork leg fitted?

Is this done such that the face of the inner brake pad, with the caliper fitted to the caliper arm assy, aligns parallel to the inner face of the disc when fitted?

 2. In order to provide the same brake line and bleed nipple fittings as on the LH side what have you physically had to do to the caliper? I understand that there is a removable 'olive' on the brake line side of the caliper that provides the raised conical seat. Is it just a question of re-inserting this into the bleed nipple side or can you explain how you were able to provide the appropriate seats for the brake line and bleed nipples in the opposite positions?

3. Looks like you have machined the disc mounting ring to accommodate the std speed drive flange. I take it that this has required opening the inner diameter of the mounting ring and machining slots to accommodate the drive flanges?

[MauiK3]

Inside the caliper there is a seat that must be removed and swapped to the other port. Mine was so corroded I ended up purchasing a ready-to-use caliper from Yamiya. Worked great.

[bryanj]

If you send me a pm i will send you a word document on twin discing a 500 that has pictures of thr two speedo drives and how to modify the later one with just a hacksaw, hammer and file

[38rudge]

If you send me a pm i will send you a word document on twin discing a 500 that has pictures of thr two speedo drives and how to modify the later one with just a hacksaw, hammer and file

PM sent

Thanks

[Rookster]

1.  I am using the standard right side fork leg on the K1.  I removed material on the caliper brackets lower mounting bolt to get the bracket in the correct position.  It is more than just getting the pads parallel to the disk.  The bracket has to sit closer to the fork leg so the caliper is positioned correctly over the disk.

2. I had to remove the fitting / olive from the caliper.  I used a finishing nail inserted from the caliper bore.  I then clamped the nail in a vice.  I used a brass drift and hammer to force the caliper off the fitting.  It took a few solid hits but came out fine.  You can see it in the first picture of the caliper bracket.  To insert it into the bleed hole just drop it in and tighten the brake line.  It will force the fitting into position.

3. Yes.  It is exactly as you described it.  There is a much easier way to do it by modifying the Speedo retainer instead.

In fact there are different ways to do most of this.  You can remove material from the right side fork leg instead of the bracket. You can also shim the disk so they are both the same distance from the fork legs.  You can cut down the Speedo retainer instead of modifying the disk center.  I was using Lester wheels so I had to modify the disk center because there are no flats on the hub for the Speedo retainer to catch on.   

Scott

[38rudge]

2. I had to remove the fitting / olive from the caliper.  I used a finishing nail inserted from the caliper bore.  I then clamped the nail in a vice.  I used a brass drift and hammer to force the caliper off the fitting.  It took a few solid hits but came out fine.  You can see it in the first picture of the caliper bracket.  To insert it into the bleed hole just drop it in and tighten the brake line.  It will force the fitting into position.

Scott

Thanks for the reply Scott.

Would appreciate a bit more clarity on #2.

Are saying that:

1. With the 'olive' removed the seat that is left is the correct conical face that will seal correctly against bleed nipple?

2. That you simply then fit the removed 'olive' into the original bleed nipple hole and that it will then seat when you tighten up the brake line? The reason I ask is that the olive looks to have a stepped bottom with flat surfaces but the original bleed nipple position would have had an inverted conical face to seat against the bleed nipple?

[Rookster]

1. Yes
2. Yes.  It will seal.  The bleed screw is steel and just needs a small ring to tighten against the aluminum caliper in order to seal.  The fitting works the same way. It is an interference fit into the caliper and will seal once it has been pushed tightly in place by tightening the brake line.

Scott

[scottly]

There was an aftermarket right-side "C" style bracket, made by Checkered Flag Customs, or CFC. These were made from magnesium, and are a bit rare. The one in the pics is now owned by 754..
 

[754]

If only I could find the magnesium 4 bolt lower legs they made as well.

[38rudge]

There was an aftermarket right-side "C" style bracket, made by Checkered Flag Customs, or CFC. These were made from magnesium, and are a bit rare. The one in the pics is now owned by 754..

Thanks for the photos.

Yamiya do, or have, some in aluminium for the RH side. But at around £250/$330 its a rather expensive route to go if the std LH brk can be modified. Would be nice to have an engineering drawing of the RH brk that gave the dimensions of the boss faces relative to the pin centerline and it would make it easier to know how much to remove from the bottom boss and then what top spacers are required. But that take the fun out of it!

[38rudge]

1. Yes
2. Yes.  It will seal.  The bleed screw is steel and just needs a small ring to tighten against the aluminum caliper in order to seal.  The fitting works the same way. It is an interference fit into the caliper and will seal once it has been pushed tightly in place by tightening the brake line.

Scott

Thanks Scott

[Doobie]

Have done a site search but it has not come up with much.

Try different search terms that whatever you used. Dual disk conversion may not be exactly a hot topic but it has been discussed quite a bit here. It's also been discussed on the caferacer forums like dotheton.com. I did this years ago using the info I found here.

[PeWe]

Google search
sohc forums cb750 dual brakes site:forums.sohc4.net

[38rudge]

Found the following detail attached originally done by Fishhead in June 2009. Not sure if he is still active on the forum?

If not perhaps somebody else may be able to advise as I was considering the GL1000 speedo drive flange as an option. However, he clearly states that can't be used as it is too wide for the CB series?

Can somebody please advise what part is too wide? Is it the raised slotted section that fits into the speedo gearbox? Is that because the GL1000 speedo gearbox is different to the CB ones?

I have sourced some 2nd hand GL1000 bolts and chrome cover with the 'bump outs' for the speedo drive flange tabs and that would be the easiest option as my wheel hub does not currently have the machined faces to take the folded down tabs flange version.

Also I do not fully follow where it is stated:

The chrome cover off a GL1000 would fit (or a cut down chrome cover for a single disc) would cover up the "handy work" but it is not really needed as it would be driven by the heads of the bolts (bolts go in from the speedo side for speedo clearance).

Don't follow: "as it would be driven by the heads of the bolts", surely in this arrangement the tabs of the flange will be sitting on top the disc carrier and you would have to fit the GL1000 chrome cover to retain it in place? Or is it envisaged that the tabs will have to be long enough to rest up against the the hds of the disc bolts which will drive the flange?

Will probably have to modify a std flange and modify the wheel hub but just just want to consider all the options first.