I’m posting this so I have my own record of it. I always have to look it up and this is a great place to store my ‘notes’.
Typically, tire sizes for a R75/5 R90/6 R90s R100S, etc will have a:
3.25 x 19 Front
and
4.00 x 18 Rear
The metric metric equivalents would be 90/90-19 for the front and 100/90-18 or possibly 110/90-18 for the rear.
Here is a photo essay of sorts detailing the disassembly of a 1973 BMW Final drive. The same applies to all 1970’s era BMW’s (R90 R100 R75 R80 /5 /6 /7 S etc). The seals were shot inside causing leaks. The splines are decent so no need to weld on a new set of splines. I plan to have the drive blasted and powder coated to match the rest of the bike for an upcoming build.
The image descriptions have notes on the disassembly.
This is one of those things I try to do to every BMW motorcycle I rebuild.
The Motorcycles Cushman Truckster is one of the more popular models of bikes in the United States. It has been around since the mid nineteen-hundreds and has always been a top seller among collectors and bike lovers. While it doesn’t come with a warranty, you can get around that by getting a Cushman Tribute package, which pays for repairs or parts that might need them in the future. There are many sources online for Cushman bikes, including parts selection, prices, and even complete restoration packages for your bike if you’re not sure what you want to do with it. When you buy this bike, be prepared to spend a little extra money up front to purchase the bike you want. But remember, it will end up being cheaper in the long run because of the extra care you’ll be taking with it.
It is almost never the engines in BMW`s that are the problem; properly serviced with clean oil at regular intervals, and no abusive riding, they should easily see 200,000 miles
This area is cracked on nearly every frame I tear down. It is the rear foot brake area which also is the muffler / silencer hanger, and where the passenger foot pegs attach. This frame is a 1978 BMW R100s. If you are considering rear sets on your bike that attach to this section, then I HIGHLY RECOMMEND doing this procedure.
cracked tab near rear foot brake pivot hole on BMW airhead framecracked tab near rear foot brake pivot hole
cracked tab near rear foot brake pivot hole on BMW airhead frame
New piece of metal to weld onto the old cracked section to reinforce the area making it MUCH stronger and no longer prone to cracking.
Welded metal onto the rear foot brake section of a BMW R100S
Finished powder coated frame with the welded metal. This area is out of sight to most people unless they take the rear tire off.
When I first got this bike, I had to use all my muscle to rock it off of the center stand. It made me wonder about the R65 model… but then I realized that both tires were still touching the ground while it was on the center stand. I still wasn’t sure if this was correct or not. I thought it could have been on of the R65 quirks until a local mechanic friend told me it was due to the center stand being slightly bent.
1980 BMW R65
So the bike went back up on the lift…
1980 BMW bent center stand tabs where it touches the frame
The bent area was filled in with a weld on both sides
Ground the weld down to be flat again. I could have kept a bit more material in hind sight.
Painted and ready for re-install thanks to my mini garage helper.
The front wheel no longer touches the ground and it is much easier to rock off the center stand. I could have built up even more material so it would sit a bit higher but I was being conservative. I wasn’t really sure how much was too much. But for now, the problem is solved.
1980 BMW R65 red
1980 Red R65 BMW
Although the directions are fairly clear, I went looking online for some installation help while installing a Boyer Brandsen electronic ignition on a R65 BMW that I am fixing up for a friend.
So… here is my install procedure. I am open to suggestions or tips but this is how my install went.
The carbs were tuned nicely before doing this install. The idle never seemed to get low enough which might indicate ignition problems, or tight valves but the valves were adjusted to spec.
I might suggest you get a spare bean can to perform this on. You end up modifying the original can somewhat and it may be difficult if you ever want to go back to stock.
Boyer Brandsen Micro Digital ignition system.
I purchased this from Moto Bins in the UK and it arrived within a week.
Of course, you want to disconnect your negative cable from your battery before starting the procedure.
Gastank Removal is necessary for the wiring.
And remove the front engine timing cover.
I took the bean can cover off and front bearing before I started shooting photos. I was too eager to peek inside the bean can and see what is in there (It is my first time with a points in a can bike).
I then removed the bean can from the bike.
R65 Timing cover removal and removal of the ignition can.
Removed points in can 1979 – 80 BMW ignition system
The large circlip needs to come out to disassemble the internals of the can.
The points come out easily and then you need to remove the points base plate.
Notice the ‘e’ circlip on the center shaft. Put a rag over the can when you remove that clip just in case it tries to go flying across the garage. The rag might catch it.
Points base plate removed.
Circlip removed from the center shaft. The cam has to come off the shaft and you then need to remove the springs to slide off the contact breaker unit.
After you get the center cam removed, be careful with the very small circlips that hold the bob weights. Those need to come off to get the bob weights off.
Bob weights and center cam removed.
Now it is time for assembly of the electronic ignition!
This was the first hiccup. The magnetic rotor does not fit into the housing properly without bending the spring mounting tabs. They are not easy to bend so it took some work.
Prepping for magnetic rotor install
Note the next photo. The tabs have to align with the notches where the tiny circlips were. The new rotor should sit flat, spin freely and the large clip should secure it to the center shaft.
Magnetic rotor installed
Another small snag. When the stator plate was installed with the cord going out the square hole, and the mounting screws were tightened, the magnetic rotor would not spin freely. It was a head scratcher in which everything was taken apart, put together again a few times. Finally by rotating the stator plate 180 degrees and making sure everything spins freely before securing the mounting screws, it happily went together.
Install of the stator plate
Stator plate with cable going out rubber grommit hole vs square hole.
Now it is time to install the can back on the bike. 
The very large circlip went in and the center bearing plate installed.
I removed the diode board so I could route the wires behind it.
Removed Diode Board for wire routing
Control unit tucked and strapped to the frame components. It seemed like a tidy place to put it between the two coils.
The green wire connects with the existing green wire terminal on the rear ignition coil.
I also connected the ground wire to the coil mount bolt as well.
Not photographed but I made about a 12inch extension wire to connect the black wire from the ignition unit all the way to the other coil to replace the wire on the terminal that was connected to the old points condensor. For some reason, they put shrink tubing on the wiring as if they all connect in the same vicinity. It was either make an extension wire, or cut up the clean tubing so the black wire could reach over to the other coil.
It is all wired up and now time to set the timing.
I am not used to these flywheels so timing it took a bit of back and fourth. There is no ‘F’ mark on the flywheel. But there is the OT and a Z-dot.
Once I got it timed, and I fussed with the idle a bit, it ran great. Really smooth and more responsive then before.