Dual Disk Upgrade

[3DIronman]

First post from a new member here. I thought this may be the obvious, best place to get advice on a first time Cafe build I am starting with a 1980 CB750 Custom.

This came with a single disk front brake. I prefer the look of the dual disk, so I went on eBay and bought a set of forks, c/w triple tree and axel. Then I bought a second front disk for the other side. This is where I am seeing the first issues already. The disks are not the same size, original is the 6 mm, and new is 4 mm. My wheel spacing off centre as well. I need to get more familiar with the differences of the dual front brake setup, so I can be a better buyer, and complete this conversion.
Any suggestions where I can get a history or lineage of the 750C. I am a big believer in using the shop manuals for my bikes, but this one only shows my single disk setup.
According to the add/seller on the forks, they are off a 1982 CB750C.

Thoughts & recommendations greatly appreciated!

 

[amc49]

'82 forks spread wider tube to tube by about 3/10 of an inch over '80 stuff..............they also use the slotted dual rotors and dual piston calipers. The dual rotor master cylinder is 5/8" diameter instead of 9/16" for the single too. '80 used single piston caliper.

 

[3DIronman]

Thanks Guru.
I've spent the day looking for as much info as I can, and all I have come up with, is that I may have to get ahold of pretty much everything ahead of the steering head, including the wheel? The reason I say the wheel is, by the look of it, the speedometer pick up assy appears to be too wide. Otherwise I would be able to just make up some spacers to center the wheel.

 

[amc49]

I had always assumed the wheels were same width at bearings but that may well be wrong. For sure the '82 front wheel is 2.50" wide at the tire vs. 2.15" for the earlier ones.

 

[3DIronman]

I am going to take some pics of this mock up today, then reassemble stock fork setup to do some more comparisons. As it looks to me right now, the speedometer pick up is the issue.
I'll keep you posted.
Thanks

 

[Medyo Bastos]

A speedo pick up for a different year or model might be the ticket.

Try flipping your axle over. I am curious to see if it fixes your wheel centering issues

 

[3DIronman]

This fork set is threaded on one side, as opposed to the U Clamp style on the '80 fork set. Can't flip

 

[amc49]

X2.........

 

[3DIronman]

HaHa, If that means I should scrap the idea, you may be right!
Thanks for the input Guru

 

[amc49]

Go to like Partzilla and plug in '80 and '82 CB750C bikes and compare the part numbers on like spacers, axles, and speedo boxes, you will see that some of them are different for the difference in fork width. Single disc may be different OD and/or thickness but thinking the twin discs are the same for slotted and earlier unslotted. They interchange.

 

[ShopMosesBC]

I'm on a 1980 CB750k , what I am doing is using all the oem parts: wheels lower forks calipers rotors etc off a 1980 cb750F

 

[3DIronman]

I will definitely look into that.
Thanks