Bringing 74 R90S back to life after sitting 30 years

Keep an eye on your oil level for the first thousands of miles. If airheads are using oil, they can drink it! Don't count on the oil use stopping after the oil gets half way down the stick. I have seen too many run completely out of oil!

Take it from me, don't torque that more than 5lbs.
I sheered mine off and had to drill a hole through the cam then thread that to accept a bolt in order to use it again...it was crappy.

that was the sweetest 2 miles that I have ever ridden!!!

just yanked the tank back off, brake fluid all over frame.
BMW doesn't offer replacement rubber grommets that seal tank to cylinder.

looks like it's parts waiting time...

You can replace the complete right hand switch gear with a the later version that incorporates the handlebar master cylinder......

That's the best way to fix the ATE master cylinders. Replace them with something else.

I think we discussed the handle bar fix earlier. It's still a good idea but when I park my R90/6 next to yours at the concours review, I will get extra points for still having the MC under the tank. Even though your bike is nicer than mine.

All you need is the O-ring for the reservoir? A generic might fit. Oh, you say it's a grommet? Maybe an O-ring won't work. I think I have some extra MCes around here. I might be able to mail you a used one. Let me know, I'll hunt them up and see if I can get the grommet out.

yes the best fix is changing out to handlebar mounted BMC.... but really want to stay original.
thanks for very much for your generous offer. but solution is either new correct seals or a new brake master cylinder.

logic says hydraulic seals should survive brake fluid. if that's the case I can match up new seals at a local hydraulic repair supply shop. if hydraulic seals will not work, then I'll dig up new natural rubber material and cut out my own seals. or maybe I should stump up and pay the $400 for a new brake master :eek1

since a solution is needed for anyone with a leaky under tank BMC... should would be nice to figure one out. really gripes me to be forced to buy a $400 cylinder over a $5 seal.

edit: after talking with a seal place... what's needed to survive brake fluid is PBR or Poly Butadiene Rubber

Just as an FYI, Motobins in the UK has the complete MS for less than $400 before shipping and for the same price, a complete MS update kit..

http://www.motobins.co.uk/bmw-parts.php?model=R Series 2 valve Twin

ditto that, good upgrade, did to my bike. Know that the brake lever will not match your clutch lever in appearance though...

Well then. It's definitely OUT. The finger grip levers are what the bike is all about.

I found a part at JC Whitney, of all places, for ATE master cylinders. It's called a grommet.

http://www.jcwhitney.com/master-cylinders/ebc-clutches/b2084c15929j1s25.jcwx

I think that link should work. It's towards the bottom of the page.

Now I'm editing this post. JC Whitney has a really nice feature. After you find the part you want, then you can enter the info for your vehicle and it will tell you if the part will fit. According to them this part doesn't fit. Sorry. It must be a similar, looks the same, but different size.

If you want to keep a bike like the 90s stock, I think that is a good thing.
The best way to fix the master cylinder is to have it sleeved with stainless or brass and put new seals in it. There are anumber of shops in this country that have done this work to quite a few airhead cylinders. If you google R90S World Net, they have a guy on the East Coast that everyone there likes. Costs $100-125. I had it done on one of my 90S bikes by a shop in Seattle but don't recall the name of the shop.
With all those tools and shop equpment why not do it yourself?.

thanks my cylinder bore is in great condition, thank goodness doesn't need re-sleeved. no problems lathing out cylinder, but sourcing material for one liner would be all but impossible. you'd have to purchase several 10ft sticks to get supplier to sell to you.

hydraulics works great, just that seals between plastic tank and cylinder are not available.
nitrite orings installed didn't survive brake fluid for long resulting in brake fluid dumped all over frame. what a mess!

did a bit of digging... hopefully sealco can match up PBR rubber seals that should survive brake fluid. then I'll be back in business.

getting a 37 years old bike reliable enough to tour is going to take some effort...
spare parts along with a fairly complete tool kit will be carried.

I'm sure the seal supply here in town could get the proper size. Let me know if you need any help.

I don't have the application chart handy, but think Viton should work.

If it was me, I would convert to the later (and better) style, but keep the original to go with the bike at a later date....

thanks for all the help everyone!

really want to stay stock so are willing to spend extra time to achieve reliability without changing original equipment. now that I know PBR rubber is the key to holding up under brake fluid. matching up correct seals should not be a big deal. I've got precision punch dies and can make correct seals if need be.

now my starter stopped working ..have not tracked down problems yet. but a new relay, voltage regulator, new brushes, etc will be replaced regardless. plan is to carry most of the replaced parts for spares.

speaking of reliability... is my goal of getting a 37 year old BMW reliable enough to tour anywhere realistic?

Very definitely! Emphatically YES!!

Absolutely. Many of us on here rack up big miles on our airheads. They're very reliable once you know them well and stay on top of your maintenance. When they do break down, they're incredibly easy to work on.

And normally give you gobs of warning. They rarely leave you stranded.

Quite.