Topic: my cb750k rebuild

[jules]

Hi everyone, I've inherited my late fathers 750/4. it's a 77 chassis with 75 motor, tank and if i remember correctly from when i was younger it has a k2 front end. Dad dismantled the bike about 18 months ago to do the timing chain and restore it to some extent when he died suddenly late last year. I've been slowly piecing it together from the shed full of spares.
I've run into a wall with the front brakes recently, I've got two types of brake calipers but both hit the rotor. Has anyone got any clues as to what could be the issue?

[Stev-o]

Welcome to the site, nice looking bike. Sorry to hear about your father, lost mine a couple years back too. 

The early K models used a different caliper mount, the lower forks are different too. Is it possible you have the early fork lower with the later caliper mount?

[jules]

I've got two different caliper mounting arms as well as the large and small calipers but haven't found a winning combination as of yet. I've been told there were two sizes of rotors?

[Retro Rocket]

Hi Jules and welcome. You say its a 77 frame with a 74 motor, just check the chain line, the 77's had a 10 mm wider chain line, the 77 engine had a wider chain line because the counter shaft was 10mm wider than the earlier 69/76 750's, so the sprocket carrier was also 10mm wider, if it still has all the 77 rear wheel and spacers and sprocket carrier and swingarm then the chain will be out of alignment 10mm, this can be corrected by using an earlier rear sprocket carrier and adjusting the rear spacers on the left hand side.... Just something worth checking...

[jules]

Thanks for the helpful info guys, I've checked the sprockets and they line up spot on.
I've measured up the 3 rotors I've got here and they're all 296mm, so I think I may need to find myself a 290mm rotor to fix the front brake issue.

[70CB750]

Welcome, Jules.

For brake parts you can post in the Wanted section or PM Bill Benton:

http://forums.sohc4.net/index.php?action=profile;u=11731

and ask him, chances are he will have what you need.

Edit:  my bad, didn't know you are Aussie

[jules]

Well I've had success with the brake situation, I found a 280mm rotor and it all fits spot on. My question is though, what cb750 ran a single 280mm rotor?

Excuse the caliper bolts, they're just there to hold it all in place.

[jules]

Sadly this disc is too small... The pad hangs off the edge of the disc so it looks like ill just get one of the 296mm rotor's taken down to 290mm

[Retro Rocket]

You shouldn't have to do anything to the rotors, what model front end is that..? 

[jules]

As mentioned above it's a k7 with a k2 front end but for some reason I've only got 296mm rotors which are too big to let either of the brake calipers  mount up properly.
The only other rotor I have here is a 280mm which is too small.

[Retro Rocket]

That caliper is not a K2 caliper, thats probably your problem, not the disc....   It seems you have a K7 caliper and need a K2 one..

https://www.google.com.au/search?q=CB750+front+brake+caliper&biw=1366&bih=663&source=lnms&tbm=isch&sa=X&ei=fWEkVYP9KMK7mQXv-4Bo&ved=0CAYQ_AUoAQ#tbm=isch&q=CB750+K2+front+brake+caliper&imgrc=0KGVZr68LNzWpM%253A%3B71zA9Udj4VSInM%3Bhttp%253A%252F%252Fimages.cmsnl.com%252Fimg%252Fproducts%252Fcaliper-a_medium45101300053-01_173b.jpg%3Bhttp%253A%252F%252Fwww.cmsnl.com%252Fhonda-cb750k2-four-european-direct-sales_model14369%252Fpartslist%252FF06.html%3B446%3B334

[jules]

I've got both calipers and mounting arms but only have the bigger rotor.

[Retro Rocket]

I've got both calipers and mounting arms but only have the bigger rotor.

I have a 750 rotor sitting right next to me and its 296mm, thats the correct diameter for a K2 disc...?  You must have mismatched parts or something, something isn't adding up...?  Sorry I can't do any measurements, I have non stock front ends for my bikes, I don't have a complete 750 front end but I have a disc, actually 2 discs, and they are both 296mm...

[Stev-o]

Mick's right, something just doesn't add up.  Do you have two differant caliper arms? Is it possible you have parts from a non-750?

[jules]

After much stuffing around I've found the 280mm rotor is from a 75 gl1000, this leads me to think somehow the k7 we pulled the spare front end off had a gl1000 front end aswell as the brake mounting points on the forks are in an identical spot. I'll be on the lookout for another 280mm rotor so it has a twin disc front.

[Stev-o]

There ya go.  Installation of the GL front end is fairly common on these bikes, good way to get the twin front disc setup. 

[SOHC4 Cafe Racer Fan]

After much stuffing around I've found the 280mm rotor is from a 75 gl1000, this leads me to think somehow the k7 we pulled the spare front end off had a gl1000 front end aswell as the brake mounting points on the forks are in an identical spot. I'll be on the lookout for another 280mm rotor so it has a twin disc front.

Check out the build thread for my Nine Lives CB750K7. I swapped a 1976 GL 1000 directly on. It's a direct fit - better forks and brakes.

[jules]

Direct fit sounds amazing haha. Will definitely be keeping an eye out for one that's being wrecked here in Australia.