CB550F Double Disc's

[Trigger]

Master cylinder Confusion.
 I thought i would have a look at the masters on the bikes i have in at the moment.
1998 CB400 master states 7/8"
2011 GSX-R 750 master states 7/8"
2006 CBR 929RR master States 7/8"
Everything started to make sence until i looked at the Honda Blackbird, the master is stamped with 1/2". So had a look at the CB900F5, that stated 14mm.
Now all these bikes have twin pot, twin disc. So still no wiser.

[Johnwebley]

Just had a reply mail from Karl at DSS. The question was could they tell me the size of the Piston bore size on the aftermarket master cylinder for the GL1000 part number ect,ect.  Karl replied with: Thank you for your email inquiry. Unfortunately we are waiting for new stock to arrive so cannot confirm at this time.

 

the M/C I got from DSS,has  bore printed on the underside,  15.8 mm.
http://www.davidsilverspares.co.uk/GL1000K1-GOLDWING-1976/part_126547/

[Trigger]

Cheers John, I think it is time to hit the pub because Honda state the replacement/ service kit is 17.5mm for the OEM master cylinder. Now are they talking about bore size or piston dia. I will drink my confusion away.

[Trigger]

Yes Oddjob, Totally understand that bit. But those spec are not available unless you strip it down. Lost count of how many brake levers i have taken off to hear a couple of washer's drop on the floor. One time i found a nut wielded to the piston.   

[Trigger]

I think it is going to be a weekend of trying different Masters and using up my 5 ltr of fluid. Will try a 14mm bore and go up to a 7/8" which works out at round 22mm. Just to test the difference in travel.   

[andy_c101]

Really interesting reading about 'which master cylinder' (I've been considering the same conversion for my 550K3.)

But I also have another question.
What is the solution for the speedo drive with the additional brake disk??
do you have to machine the original to fit? or is there a speedo drive off a different model that fits the twin disk mod?

Andy C

[Trigger]

After some hours and circlip's pinging all over the place. As well as a load of emails to mate's that work in R&D for well known company's. The results were: If a master cylinder has 7/8" stamped on it. That would mean it is for a 7/8"=22mm handle bar fitting only  . Master cylinder's come in 4 standard sizes and on most modern bikes only 3 sizes are used and that excludes things like scooters.
1/2"=12.7mm  Fitting ideal for rear brake caliper (single or twin pistons). Single front caliper (single or twin piston)
9/16"=14.28mm  Fitting ideal for front twin calipers (single or twin pistons)  General rule: production bikes over 250cc ( down to weight)
5/8"=15.8mm    Fitting ideal for front twin calipers (single piston large capacity) and (multi small piston capacity)
11/16"=17.4625  Fitting ideal for front twin calipers (up to 6 pot double action caliper)

The orignal GL1000 master cylinder had a 11/16" bore but, was down sized to 5/8"
A common problem was found that not all master cylinders display a bore size.   

[Trigger]

I am working from this:
http://www.salocal.com/sohc/tech/dual_disks/dual_disks.htm

[Johnwebley]

I am working from this:
http://www.salocal.com/sohc/tech/dual_disks/dual_disks.htm

 that explanation/advice is so good,I hadn't found that,but its almost 100% what I ended up doing,

[Trigger]

John, What do you mean, nearly 100%?
 I went with this guys advice because it was straight forward advice and the rest had turned there explaining into a book.
This link also says, No mod to the speedo drive.

[Johnwebley]

when I fitted the caliper mounting,I carefully filed the top T piece,so not to use washers as spacers ,other than that,

 it was identical !!

this is my conversion
http://www.sohc.co.uk/index.php?topic=5572.0

 

[Trigger]

Cheers John, You had me worried a bit then.

[Trigger]

I was thinking the same things oddjob. Have not started it yet. Still collecting parts, Have 2 calipers and a disc from a CB500T. Was trying to work out the master.

[BlackSheep]

The theory about the fluid displacement being greater with a larger piston is valid. However, there are much more important things to consider. Like keeping the calipers' correctly adjusted, using pads with a higher coefficient of friction, using braided hoses, not leaving braking until it's more than too late, etc. etc.

And despite what logic might dictate, a narrow bore master cylinder (MC) piston (bike's stock part) will give a smoother and more  subtle action when a second front disc is fitted.

Sure, the fluid transfer will be less with an 11mm bore compared to one of 16mm, by a little less than a factor of 2.16. but you'll have twice the breaking surface available.

You'll max-out the pressure in the breaking system (and hence breaking force on discs) before you bottom-out the break lever.

Track-day racers often use a "smaller than stock" master cylinder setup on their clutch for similar reasons, but in the opposite sense, i.e. they're feeding-in energy - not dissipating it.

So by fitting a second disc set-up using stock MC, you'll get a smoother braking system - not a stronger one. If you want a sharper braking system, just fit the MC with a larger diameter (say 16mm) and forget about the second disc/caliper/pipes, save weight at the same time.


Does the single disc set-up look as cool as the twin disc? Hell no!

[Ewan 500 K1]

I am working from this:
http://www.salocal.com/sohc/tech/dual_disks/dual_disks.htm

 that explanation/advice is so good,I hadn't found that,but its almost 100% what I ended up doing,

yep he's got it wrong about the unmodified spedo drive plate. The alternative to the one in oddjobs link is to modify the drive plate as per attached photo, ie cutting off excess and then bending over two tangs to fit the flats on the hub. Original drive plate on the right, modified one on the left.  But.... this only works if have a hub with the flats on it, like the one in the second attached picture
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Also ,  like oddjob, I fitted the mudguard stay between the pivot arm bracket and the fork tube .... didnt need to use any shims or file anything to get the pads sitting parallel with the disc

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I do still need to find a longer adjuster bolt as oddjob suggests