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I'd say it's a life changer. The way you've gotten into this great BMW. Like a duck to water. Looking forward to your first ride report! Ha! And ZM is correct.

There are a number of different metal shims for under the cylinder, which will change the compression ratio.
When reassembling, be careful with the sealant at the bottom of the cylinder as the top 2 studs have grooves where oil flows up to the rockers and heads. If you get them sealed up, the top end won't get lubricated. No o-rings used.

The '75R90S that I care-take has the 38mm calipers. The 40's are stamped "40" while the 38s have no stamping. Changing to 40's changes the leverage ratio. On all the older BMW's that I've cared for (75R90S, 77RS, 77R100S), I swapped them out to the single disk master cylinder. I have a friend who's a machinist by trade and he machined the second port for the brake line. The castings are the same. BMW just didn't machine the second boss. Or you can get a splitter on the single port. Stainless braided lines significantly improves feel.

I have a complete dual disk fork (minus calipers) I may be interested in selling. It was sold to me as a '77 R100S fork, but I think it might be slightly newer as the top nut doesn't have the hole for the adjuster rod. I've had it hanging on the wall in my garage for the past 10 years as a "spare" just in case I ever needed it, but now I'm trying to de-clutter. Probably selling off the bikes as well.

Thanks y'all

As far as brakes go, i ride old bikes like old bikes and try to stay out of traffic and when I am in traffic I stay back and dint ride like a speed demon. I'm coming from riding either old Hondas with drum brakes or rigid choppers withfoot clutch/hand shift and no front brakes so the single hydraulic disc brake will be plenty for me! I will say, I've never had a bike with a remote master cylinder for the front brake. That was interesting. The lever operates a cable that actuates the master cylinder hidden under the tank. I like it. Keeps the handlebar controls slimmed down. That would be agood thing for the chopper folks who want a front brake to adopt since nice clean simple bars are typically desired.

THere are a couple of things it helps to be aware of when dealing with airhead BMWs. Knowing them will stand you in good stead when sourcing parts for your bike and can save an enormous amount of frustration and confusion.

1. The BMW model year starts at the end of September in the previous year. Thus, a bike made in November 1975 (for example) would be a 1976 model year bike.
2. BMW did not limit changes (sometimes quite big changes) to the change in model designation, changes sometimes occured during the currency of a particular model. This is probably most marked in the faired models such as the 'RS where the same designation was used on machines made between 1978 and 1994 but the only parts that weren't changed (much) during the period were the fairing panels. In the case of your R90/6, the build year is important since there was a major redesign of the electrical system between 1974 and 1975. In essence, when ordering parts it is important to know the year (and if possible the month) of manufacture as well as the model.
3. There is a lot of parts interchageability between models. A few parts are interchageable at the 'bolt on' level throughout the range, many can be fitted with a bit of 'fettling' and most can be interchanged if you're willing to do the work involved. In addition, BMW often substituted parts so that if, for example, you were buying a starter for a 1975 machine in 1995, you would probably be supplied with a Valeo starter rather than the Bosch unit the bike originally came with. This is quite useful when you are trying to keep an old machine running... but... it can be a bit of a two edged sword... If a part has been substituted by a previous owner, it's sometimes difficult to assess what exactly it is that you have and, if you are sourcing replacement parts at second hand, it's important to now exactly what model they are designed for not necessarily what they came off.
4. Beware of 'Must have' upgrades. The bike as it came off the production line is a very capable machine in just about every respect, especially in the context of a classic machine. You can find yourself paying out hundreds of dollars for things that actually do little for the vehicle and undermine it's originality. The only 'Must Have' I would recommend for your bike is a good original style stainless exhaust system. I'm not a fan of siamesed exhausts and I believe they can contribute to a range of running issues... but, if the bike runs well I suppose it's up to you...
5. Some people find BMWs very different to other bikes to ride. The comparatively compliant suspension has a distinctive 'feel' and the gearchange can take a lot of getting used to. My advice would be to make sure the bike is all 'correct' and ride it for a season before you start trying to 'tailor' handling. You will probably find there's no need. As far as gearchanges go, there's a very definite knack. Once you have the knack, you can change up and down the box cleanly and smoothly.

Enjoy the bike

Rob

Thanks Rob. Excellent, very useful advice on the "year of the bike".

I like what you said about the "must have upgrades". I must admit I did fall for that a bit with the electronic ignition. I do think the charging system upgrade was valid though. I never tested it since I haven't actually ran the bike but it looked so nasty and the bearing surface was pretty gouged up when I pulled it. Plus if they are anything like harleys the charging systems fail often and a new setup is always a good idea. Plus the EDL4 system I got eliminates the diode board and combines the regulator/rectifier.

I must say, I love wiring and reading wiring diagrams and troubleshooting but this bike has the most over complicated diagram and it's very small and faint in the manual. Cant say I'm a big fan of that circuit bus board in the headlight. What a cluster!!!

Here is a link to a supplier that sells colored large schematic on e-bay for these bikes, you can go to the site a select your particular year.

https://www.ebay.com/itm/BMW-R60-R75-R9 ... 0360666406?

Awesome! I will definitely order one today.