Topic: CB750 K2 Rear brake drum size

[PicGrabber]

Hi guys,

Well I know everybody ask about the rear drum limit size, which in the manual show as 183mm. 

My question is a little different.

What is the standard factory original size of the drum ?

Thanks folks.

JV.

[754]

I think 180 ish. The service limit is marked inside.

[PeWe]

The hub show max 181 mm inside in the cast while the manual has 183 mm as service limit. I noticed this recently when I bought an old CB750 hub from a friend and measured the brake liner diam inside.
The rear brake is not needed to be used much since it will not add any much deceleration, better to save the liner diam.

[jaytee-nz]

He's not asking what the wear the limit is - the question is, what is the original size of the hub (before any wear occurs).

[PeWe]

He's not asking what the wear the limit is - the question is, what is the original size of the hub (before any wear occurs).

Missed that. A good question...

[PicGrabber]

I should have highlighted exactly what I need at the subject. 

[754]

We all forgot to measure back in the day.

[PicGrabber]

I have started this post as my CB750 is using a pair of Rick Paul Scorro aluminum rims, original accessory back in the day.

This rim is all built in one single piece all aluminum including the cube, and it have a cast iron insert as the brake drum hub.

This cast insert is far away from it's original shape, it's so thin it was showing cracks, I managed to remove it, in pieces, now I need to build a new one.

But for starter I'm looking for what should be the original internal size of a standard hub, so I can have it correctly sized as a brand new hub.

JV

[Bankerdanny]

So you need to replace the iron braking surface that runs around the inside of the rim and you need to know the inner diameter of a CB750 hub.

I know there was a thread fairly recently about relining a brake hub.

[754]

That is a very tricky costly procedure, that many machinists won't want to do.
 So I ask.. have you had any thoughts on converting to disc?

[PicGrabber]

Hi Bankerdanny ,

Yes you are correct, I'm relining the hub. I looked the forum about the subject, found a guy with pretty the same idea, using a car brake hub and adjust it to fit.

Still can't find what suppose to be the original internal hub size.

It seems for car 180 is a very common value, but I think is very close to some minimum limits for Honda hub I see around (181 or 183 )

[PicGrabber]

Hi 754,

I had some thoughts about going disc brake, but I'm doing as much as I can a faithful restoration. I'm trying to have as much as original parts restored.

I'm a machinist myself so it probably much cheaper doing it 

Well... Keeping my pursuit about the original size.....

Someone should have some NOS around, just waiting to have it measured 

[754]

If it's not 180 it's 178. If you get new shoes and measure it should tell you, I would think.
 Are you going to pin it in ?

[greenjeans]

vintagebrake.com can re-line it for you.
Or...   just find a good used one.  You will be the only person that would ever know.

[PicGrabber]

Hi GreenJeans,

Thanks for the suggestion, but it will be really difficult. I live in Brazil, in that way it would be really, say really expensive to send abroad to have it re-lined.

But looking vintagebrake site, they gave me a real good information. There is a note about doing the adjustments on the pad linings, they say to use a lathe and adjust the lining to .020" smaller than hub I.D. so now I know if I get a new original pair of brake pads, I can figure out the original size of the hub which will be .020" bigger than new brake pads YAY !!!!!

About a good used one.... well as they are full aluminum wheels including rim/spokes and hub, they are sort of rare in here, and the used ones are all as bad as mine.

Thanks !!!

JV

[PicGrabber]

Let me add some pictures to all understand.

2 pictures attached