Topic: 550 rear disc conversion, 750 rear disc pic request

[nicks2319]

Hey everyone, thanks for looking. Going all out on my 550 project, considering dual disc front and thinking about doing a rear disc conversion. I've only seen a couple threads on this, but it seems like the easiest way to do this would be to grab it all off the later model 750's that had the stock rear disc setup.

Couple questions:

- Anyone have some good pics of the 750s with rear discs? I need some detail views to see what all I need. Assuming rear wheel, disc, caliper, master cylinder, brake pedal.
- Anyone have an exploded diagram?
- Will I need to use the 750 swingarm, and is there a special one for the rear disc? I can't tell from photos how the rear caliper actually mounts.

Really hoping I can just use my existing 550 swingarm, as I already have it powdercoated.

Thanks!

[Restoration Fan]

Only the F model 750's had rear disc brakes.  And the F2 and F3's had mag wheels, not spoked.

[Retro Rocket]

Only the F model 750's had rear disc brakes.  And the F2 and F3's had mag wheels, not spoked.

Some K7/8's had rear discs too Ron, member Wobbly has one...

You can use your stock swingarm but you may need to play with the spacers a bit to get it all lined up with the 750 parts, you'll need a FO/F1 rear hub and disc as well as the disc carrier/bracket and lines and stuff, you'll  have to add a mount for the slave cylinder and master cylinder to the frame, you'll also have to modify your brake pedal....  Hmmm, you may be better off using the 750 swingarm as well as the disc brake model has the brake arm on the top....

[nicks2319]

Only the F model 750's had rear disc brakes.  And the F2 and F3's had mag wheels, not spoked.

Some K7/8's had rear discs too Ron, member Wobbly has one...

You can use your stock swingarm but you may need to play with the spacers a bit to get it all lined up with the 750 parts, you'll need a FO/F1 rear hub and disc as well as the disc carrier/bracket and lines and stuff, you'll  have to add a mount for the slave cylinder and master cylinder to the frame, you'll also have to modify your brake pedal....  Hmmm, you may be better off using the 750 swingarm as well as the disc brake model has the brake arm on the top....

Do you know what exactly mounts to the swingarm?

[Jerry Rxman Griffin aka MuthaF'er]

The brake stay/arm mounts to the top right side in the front.

Go to a site like Motogrid.com and look up the F models for your diagram and parts list.

[SOHC4 Cafe Racer Fan]

You'll want to source a 1975-76 CB750F wheel hub/disc/caliper carrier/sprocket assembly. 

[Retro Rocket]

The brake stay/arm mounts to the top right side in the front.

Yep, thats the one...

[nicks2319]

Thanks everyone for all of this info! Guess I should start hunting for parts.

[GV1390]

I did it a few years back on my old 77' 550....

It was entirely bolt-in from a 76' CB750F. Pulled pivot bolt and everything else was left attached to the swingarm. The wheel, master, axle, spacers were all CB750. The pivot bolt is the same diameter and width of the arm. I used a dirtbike master but, it could've been any master.

I also did the dual 750 fronts...

[nicks2319]

I did it a few years back on my old 77' 550....

It was entirely bolt-in from a 76' CB750F. Pulled pivot bolt and everything else was left attached to the swingarm. The wheel, master, axle, spacers were all CB750. The pivot bolt is the same diameter and width of the arm. I used a dirtbike master but, it could've been any master.

I also did the dual 750 fronts...

Exactly what i'm looking for! Awesome. If you have any closer up pics of the rear that would be appreciated. Thank you

[slikwilli420]

Really you could get away with just a hub and disc for this. The swingarm is only different due to the tab for the torque arm which is easy to remedy and you could pick any caliper and build a mount/torque arm to do it over the top like stock or underslung, eliminating the need to weld the swingarm. Then pick up a Nissin master and fit that to the frame by the brake pedal and you are good to go.

[GV1390]

I believe @Fastbroshi is doing this as well...

[Retro Rocket]

I did it a few years back on my old 77' 550....

It was entirely bolt-in from a 76' CB750F. Pulled pivot bolt and everything else was left attached to the swingarm. The wheel, master, axle, spacers were all CB750. The pivot bolt is the same diameter and width of the arm. I used a dirtbike master but, it could've been any master.

I also did the dual 750 fronts...

Exactly what i'm looking for! Awesome. If you have any closer up pics of the rear that would be appreciated. Thank you

Nick, if you want to retain all the stock geometry, the early 750F swingarm isn't as long as the 550 arm,  its all bolt in but there are differences in geometry due to the different swingarm length, if that's not important to you then go right ahead... I'm doing a disc conversion on a K model 750, I'm using the K model arm and putting the brake caliper on the bottom like slikwood420 mentioned above, thats another option for you.. There's a little bit of fabbing but its all pretty straight forward..

[GV1390]

Exactly what i'm looking for! Awesome.

[nicks2319]

I did it a few years back on my old 77' 550....

It was entirely bolt-in from a 76' CB750F. Pulled pivot bolt and everything else was left attached to the swingarm. The wheel, master, axle, spacers were all CB750. The pivot bolt is the same diameter and width of the arm. I used a dirtbike master but, it could've been any master.

I also did the dual 750 fronts...

Exactly what i'm looking for! Awesome. If you have any closer up pics of the rear that would be appreciated. Thank you

Nick, if you want to retain all the stock geometry, the early 750F swingarm isn't as long as the 550 arm,  its all bolt in but there are differences in geometry due to the different swingarm length, if that's not important to you then go right ahead... I'm doing a disc conversion on a K model 750, I'm using the K model arm and putting the brake caliper on the bottom like slikwood420 mentioned above, thats another option for you.. There's a little bit of fabbing but its all pretty straight forward..

I think i'm going to hunt down everything but the swingarm, and see if I can make it work with the 550 arm. Are the 550 and 750 rear axles the same size? I feel like I remember reading that they were different. Aka can I bolt the 750 wheel/hub to the 550 swingarm without modification.

[Retro Rocket]

I'm prety sure they are the same size, A long time ago i had problems with an old dual sprocket set up, my friend had a 500 that wasn't running in the garage, From memory {it was around 1980},  I swapped out the complete wheel so i could go for a ride, and replaced the sprocket later and went back to my 750 wheel. Some 500/550 racers use the shorter 750 swingarm on their race bikes to make the bike turn in faster on the track, not needed on the road...

[Jerry Rxman Griffin aka MuthaF'er]

77/78 K and F have a 10mm longer axle for their wider swingarm

[Rookster]

This may not be what you are looking for but there are ways to do what you want by not swapping any swingarms, wheels, axles, or welding tabs.  There was a kit in the 70s that used a disk carrier to mount in place of the drum brake plate.  The caliper was underslung so you just used your drum brake stay to brace the caliper bracket.  CycleX recently sold a kit that was an updated version. 

The original kit from Hallcraft/Hurst Airheart:


My version using period parts: Hurst Airheart caliper, AP Lockheed disk, newly made disk carrier and assorted brackets and spacers.




Scott

[nicks2319]

That's awesome, I would have never thought about going that route.

[seanbarney41]

Rookster, did you buy that caliper brand new?  I have seen that at least some versions of the Hurst airheart caliper are still available.  I want to use a similar set up on a bike with Kimtab wheels that have no provisions for drum rear.

[Rookster]

The caliper I have with the integral mount is no longer available.  They come up on ebay every once in a while.  It uses the same piston and seals as the Airheart 175H which is still available.  I wanted something that was available in the 70s.  If I was going for a new caliper with a vintage feel the Lockheed aluminum caliper is hard to beat. 

Scott

[seanbarney41]

thanks for the info...your bike is awesome

[Rookster]

Thanks.  Much appreciated.

Scott

[strynboen]

this disc cariier can safe a lot of bikes vith cracked drums.
..mine 550 have problems,,and lives not for long.
..so only vay to safe the rear hveel/hub.. is to get turned a disc adapter..and eventuel veldet in to the drum..after  have remover the cast iron drum insert....in a lathe..veld it inn and make a finaly spoontaking to line perfekt up

[christopherk]

Hey dood, I have my 750f rear torn apart right now. Rebuilding the rear caliper & master cylinder. Shoot me a text & I'll get ya all the pictures you need.