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Old 04-21-2014, 03:29 PM   #1
hoodun OP
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Joined: Jun 2012
Location: San Francisco, CA
Oddometer: 128
Undertank Master Cylinder Rebuild Kit

I noticed there is a 14mm single kit for the 74-75 and one for 76-80.
http://www.motobins.co.uk/displayfin...UL+KITS&model=

I have a 75 bike but I have the kit for the 76-80. Is this going to be a problem? Does anyone know the difference. The bore size is the same.
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Old 04-21-2014, 03:35 PM   #2
bmweuro
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Quote:
Originally Posted by hoodun View Post
I noticed there is a 14mm single kit for the 74-75 and one for 76-80.
http://www.motobins.co.uk/displayfin...UL+KITS&model=

I have a 75 bike but I have the kit for the 76-80. Is this going to be a problem? Does anyone know the difference. The bore size is the same.
The way the reservoir attaches is the biggest difference.
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Old 04-23-2014, 12:07 AM   #3
RagerToo
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76-80 is supposed to be 17mm. Dual discs.

According to Max's http://www.maxbmwmotorcycles.com/fic...7&rnd=04302013

Another question for mine. I need a new reservoir gasket for my undertank. Should I just cut one from some neoprene roofing material? Cripes, they want US$40.00 for the gasket that comes with a new cap + accouterments. ;-)
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Old 04-28-2014, 02:10 AM   #4
_cy_
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tutorial for undertank brake master cylinder here .. scroll down to #897
http://www.advrider.com/forums/showt...169507&page=60

best not to buy any BMC kit until you've measured internal diameter of bore and inspected bore to make sure it doesn't have pits too deep to sand out.

you can go to any Oring seal company and match up seal between tank and master cylinder. but it's got to be a specific EPDM Oring or it will desolve and puke fluid all over your frame.

Quote:
Originally Posted by _cy_ View Post
here's the correct EPDM Orings for Tank to Cylinder.
then for a backup I've got EPDM flat material in 1/16in and 1/8in to custom cut out flat washers to match original equipment seals. edit: to do flat washers correctly needs .1015in EPDM flat stock.

going to run the two EPDM orings below first. then if that doesn't hold. will custom cut out flat washer seals like OEM. first preference to come up with a solution easily duplicated anywhere.

go to any O ring supplier and ask for:

EPDM 2-106
EPDM 2-111

picture below show flat seal punched out with common leather punch using 1/8in EDPM flat stock.
note seal is not round, but that's not critical, like flat surface. advantage of this type seal is larger surface area. it's really hard to punch out a perfect seal.

after fitting both custom cut 1/8in EPDM seals and stock EPDM .1015 seals.
above two stock orings seal better due to correct thickness.

this picture shows the clear winner. we'll see how it how after a few months.









it's hard to tell thickness between custom cut gasket punched from 1/8in EPDM
gap is too large ... it would probably work, but seal is suspect.



this is using EDPM orings .1015 in thickness, much better fit!
only better seal is if 3/16 EPDM flat stock is available.


_cy_ screwed with this post 04-28-2014 at 02:19 AM
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Old 04-28-2014, 04:00 AM   #5
RagerToo
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Thanks for the master cyl info cy!
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