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If you can get a good pair of the large piston calipers and get your master sleeved down to 14mm, I bet you would have the best setup possible without spending big money and making a bunch of modifications. Like any mod, it can turn into a black hole pretty quickly. A Brembo radial master with a pair of Brembo radial calipers and some custom hydraulic lines with some lightweight rotors would be fabulous. Might cost less than a new car…….

But based on that article I linked the target ratio would be 12:1 to 14:1, with that author's preference of around 13.5 : 1 (I think).

It seems like I can get a better ratio by changing one or the other (master or calipers).  But if that article is correct, It seem like doing both would be a bit too far. 

Here are the numbers:
5/8 master, 38mm pistons : 11.54 : 1
5/8 master, 42.8mm pistons:  14.54: 1
14mm master, 38mm pistons: 14.84 : 1
14mm master, 42.8mm pistons: 18.70 : 1

But wait!  I have been thinking about an entirely different way to skin this cat.  I've been thinking about making a lever for the master cylinder, where the cable mounting point is moved out to increase the leverage. It may be unreasonably expensive to have one machined.  Also, the cable might not appreciate coming out of the housing at an angle.  And other possible issues.  But I figured I'd look into it.  This could allow me to try different ratios quickly.   



 

You're spot on with messing with the mechanical ratio. Most modern radial MCs trade hydraulic advantage for mechanical advantage anyway. For example; a standard 16mm MC with a 22mm pivot offset will have similar feel to a radial 17mm MC with a variable 16-18mm pivot offset.

Can you bodge together a new lever with some hand tools to see if this will solve your problem before spending money on machining a part? perhaps 3d print the part first? I also agree with Scirocco that the more components in the system the less "feel" you will have. Alternatively you could probably fit a standard MC with a remote reservoir on there.

Another update.  After a ride, as the bike was rolling while pushing it around, I noticed a clicking from the front sprocket area. I was worried that the front sprocket came loose again, but I pulled the chain cover and it was still tight. I then figured maybe it really was time to replace the chain and sprockets.

Looking at chains, it is true that the selection of 630 chains is pretty limited. But I wasn’t really ready for the 530 swap. I ended up with an EK o-ring chain and JT sprockets. I also considered going to the earlier and later ratio of 15/35, but I decided to just stick with the stock ‘78 15/33. 





Unfortunately, I lost my sweet workshop and I’m back to working in my little garage with no power. Anyway, it’s still better than no garage. It’s just real disappointing after working in such a nice space. 


 

The EK chain is made in Japan, which is nice. The JT sprockets are made in Thailand. They seem fine. The only thing I didn’t like was the JT rear sprocket has round cutouts, while the stock rear has oval cutouts. The round cutouts expose a part of the hub that was previous covered by the sprocket. So that bit of unpainted aluminum and the pockets below the sprocket will now be exposed to the elements. A new place to collect gunk and debris.

The other thing I’ll mention is that those dog-bone lock-tabs are really a big pain in the neck. The tabs were bent up on 2 sides of each nut. That’s 16 tabs to bend flat and then bend back up for new sprocket. It took forever. I’m guessing they went to flange or lock nuts not longer after ‘78? If so maybe I should switch to those newer fasteners next time. 

EK have the best chains in the market...

One other random detail.  While I was poking around I noticed I had B9ES plugs installed.  There has been a lot of discussion here about plugs recently, so I was aware the manual calls for B8ES.

I have no memory of installing these plugs, but it was probably when I went though the bike in 2008.  I don't know why I would choose B9ES.  I even have some new B9ES spares in my toolbox, so I'm pretty sure I bought them.  One guess I had was that maybe I had some left over form my KZ650, but the 650 calls for B7ES.  So I have no idea.

It's funny the things you find after owning a bike for years and years,.  You know you did it yourself, but you don't know why.

In any case, I pulled the B9s and replaced them with a news set of B8ES.